Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
Thanks for the feedback. I updated the pattern and it passes all tests
(existing and the new ones I wrote). I added some brackets since there were
some warnings about missing brackets on the || and &&. Here's the updated
pattern:
(simplify
(minus (convert1? @0) (convert2? (minus@2 (convert3? @@0) @1)))
(if (INTEGRAL_TYPE_P (type)
&& !TYPE_OVERFLOW_SANITIZED(type)
&& INTEGRAL_TYPE_P (TREE_TYPE(@2))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@2))
&& INTEGRAL_TYPE_P (TREE_TYPE(@0))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@0))
&&
(TYPE_PRECISION (type) <= TYPE_PRECISION (TREE_TYPE (@2)) ||
(TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@2)) &&
(TYPE_PRECISION (TREE_TYPE (@0)) < TYPE_PRECISION (TREE_TYPE (@2))
||
(TYPE_PRECISION (TREE_TYPE (@0)) == TYPE_PRECISION (TREE_TYPE
(@2)) &&
!TYPE_UNSIGNED (TREE_TYPE (@0)))
(convert @1)))
>>> TYPE_PRECISION (type) <= TYPE_PRECISION (TREE_TYPE (@2))
Did you mean > instead of <=? With the condition you proposed, that would
trigger the optimization in cases where values may get truncated which I think
should be avoided for this optimization.
>>> Maybe the new transform could be about scalars, and we could restrict the
>>> old one to vectors, to simplify the code,
I tried limiting the existing pattern to vector types by changing it to the
following:
(simplify
(minus @0 (nop_convert1? (minus (nop_convert2? @0) @1)))
(if (VECTOR_TYPE_P(type))
(view_convert @1)))
I found that the new pattern doesn't cover some cases. Specifically, it doesn't
cover a case in pr92734-2.c:
unsigned
f10 (unsigned x, int y)
{
unsigned a = (int) x - y;
return x - a;
}
I think the pattern isn't triggering because of the !TYPE_UNSIGNED (TREE_TYPE
(@0)) check. I'm slightly concerned that changing the new pattern to cover the
existing cases would add complexity to the new pattern, making it difficult to
understand.
I also think the new pattern could be simplified by removing the convert on @0.
I don't think it's needed for the regression pattern that I was seeing, but I
had added it to be more thorough so the pattern covers more cases.
From: Richard Biener
Sent: Monday, August 9, 2021 2:58 AM
To: Marc Glisse
Cc: Victor Tong ; gcc-patches@gcc.gnu.org
Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division
followed by multiply [PR95176]
On Sat, Aug 7, 2021 at 12:49 AM Marc Glisse wrote:
>
> On Fri, 6 Aug 2021, Victor Tong wrote:
>
> > Thanks for the feedback. Here's the updated pattern:
> >
> > /* X - (X - Y) --> Y */
> > (simplify
> > (minus (convert1? @0) (convert2? (minus@2 (convert3? @@0) @1)))
> > (if (ANY_INTEGRAL_TYPE_P (type)
> > && TYPE_OVERFLOW_UNDEFINED(type)
> > && !TYPE_OVERFLOW_SANITIZED(type)
> > && ANY_INTEGRAL_TYPE_P (TREE_TYPE(@2))
> > && TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@2))
> > && !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@2))
> > && ANY_INTEGRAL_TYPE_P (TREE_TYPE(@0))
> > && TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@0))
> > && !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@0))
> > && TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type)
> > && TYPE_PRECISION (TREE_TYPE (@0)) <= TYPE_PRECISION (type))
> > (convert @1)))
> >
> > I kept the TYPE_OVERFLOW_SANITIZED checks because I saw other patterns that
> > leverage undefined overflows check for it. I think this new pattern
> > shouldn't be applied if overflow sanitizer checks are enabled.
> >
> >>> why is this testing TREE_TYPE (@0)?
> >
> > I'm checking the type of @0 because I'm concerned that there could be a
> > case where @0's type isn't an integer type with undefined overflow. I tried
> > creating a test case and couldn't seem to create one where this is violated
> > but I kept the checks to avoid causing a regression. If I'm being
> > overcautious and you feel that the type checks on @0 aren't needed, I can
> > remove them. I think the precision check on TREE_TYPE(@0) is needed to
> > avoid truncation cases though.
>
> It doesn't matter if @0 has undefined overflow, but it can matter that it
> be signed (yes, the 2 are correlated...) if it has the same precision as
> @2. Otherwise (int64_t)(-1u)-(int64_t)((int)(-1u)-0) is definitely not 0
> and it has type:int64_t, @2:int, @0:unsigned.
>
> Ignoring the sanitizer, the confusing double matching of constants, and
> restricting to scalars, I think the tightest condition (without vrp) that
> works is
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
On Sat, Aug 7, 2021 at 12:49 AM Marc Glisse wrote: > > On Fri, 6 Aug 2021, Victor Tong wrote: > > > Thanks for the feedback. Here's the updated pattern: > > > > /* X - (X - Y) --> Y */ > > (simplify > >(minus (convert1? @0) (convert2? (minus@2 (convert3? @@0) @1))) > >(if (ANY_INTEGRAL_TYPE_P (type) > >&& TYPE_OVERFLOW_UNDEFINED(type) > >&& !TYPE_OVERFLOW_SANITIZED(type) > >&& ANY_INTEGRAL_TYPE_P (TREE_TYPE(@2)) > >&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@2)) > >&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@2)) > >&& ANY_INTEGRAL_TYPE_P (TREE_TYPE(@0)) > >&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@0)) > >&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@0)) > >&& TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type) > >&& TYPE_PRECISION (TREE_TYPE (@0)) <= TYPE_PRECISION (type)) > >(convert @1))) > > > > I kept the TYPE_OVERFLOW_SANITIZED checks because I saw other patterns that > > leverage undefined overflows check for it. I think this new pattern > > shouldn't be applied if overflow sanitizer checks are enabled. > > > >>> why is this testing TREE_TYPE (@0)? > > > > I'm checking the type of @0 because I'm concerned that there could be a > > case where @0's type isn't an integer type with undefined overflow. I tried > > creating a test case and couldn't seem to create one where this is violated > > but I kept the checks to avoid causing a regression. If I'm being > > overcautious and you feel that the type checks on @0 aren't needed, I can > > remove them. I think the precision check on TREE_TYPE(@0) is needed to > > avoid truncation cases though. > > It doesn't matter if @0 has undefined overflow, but it can matter that it > be signed (yes, the 2 are correlated...) if it has the same precision as > @2. Otherwise (int64_t)(-1u)-(int64_t)((int)(-1u)-0) is definitely not 0 > and it has type:int64_t, @2:int, @0:unsigned. > > Ignoring the sanitizer, the confusing double matching of constants, and > restricting to scalars, I think the tightest condition (without vrp) that > works is > > TYPE_PRECISION (type) <= TYPE_PRECISION (TREE_TYPE (@2)) || > TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@2)) && >(TYPE_PRECISION (TREE_TYPE (@0)) < TYPE_PRECISION (TREE_TYPE (@2)) || > TYPE_PRECISION (TREE_TYPE (@0)) == TYPE_PRECISION (TREE_TYPE (@2)) && > !TYPE_UNSIGNED (TREE_TYPE (@0)) >) > > (where implicitly undefined => signed) but of course it is ok to handle > only a subset. It is too late for me to think about @0 vs @@0. > > The patch you posted seems to accept the wrong > (int128t)LLONG_MAX-(int128_t)((int)LLONG_MAX-0) -> (int128_t)0 > > Using ANY_INTEGRAL_TYPE_P allows vectors, and TYPE_PRECISION mustn't be > used for vectors (maybe we should add more checking to catch such uses), > and they may not be very happy with convert either... > > >>> Once we'd "inline" nop_convert genmatch would complain about this. > > Maybe the new transform could be about scalars, and we could restrict the > old one to vectors, to simplify the code, Hmm, I guess that might indeed be a good idea. > but that wouldn't help genmatch > with the fact that the pattern x-(x-y) would appear twice. Is it really > that bad? It is suspicious, but can be justified. I think genmatch will either complain or might end up generating unreachable code. Note with the present patch there's probably order forcing patterns inbetween so it could simply work. Otherwise whether it works or not (as expected) depends on placing positions of the patterns ... So no, it's not inherently "bad" but it's not designed to work. > > Is someone working on inlining nop_convert? I'd like to avoid breaking > > someone else's work if that's being worked on right now. > > > > I've also added some extra tests to cover this new pattern. I've attached a > > patch with my latest changes. > > > > > > From: Richard Biener > > Sent: Wednesday, July 28, 2021 2:59 AM > > To: Victor Tong > > Cc: Marc Glisse ; gcc-patches@gcc.gnu.org > > > > Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division > > followed by multiply [PR95176] > > > > On Tue, Jun 29, 2021 at 1:10 AM Victor Tong wrote: > >> > >> Thanks Richard and Marc. > >> > >> I wrote the following test case to compare the outputs of fn1() and > >> fn1NoOpt() below with my extra pattern being applied. I tested the two &
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
On Fri, 6 Aug 2021, Victor Tong wrote:
Thanks for the feedback. Here's the updated pattern:
/* X - (X - Y) --> Y */
(simplify
(minus (convert1? @0) (convert2? (minus@2 (convert3? @@0) @1)))
(if (ANY_INTEGRAL_TYPE_P (type)
&& TYPE_OVERFLOW_UNDEFINED(type)
&& !TYPE_OVERFLOW_SANITIZED(type)
&& ANY_INTEGRAL_TYPE_P (TREE_TYPE(@2))
&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@2))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@2))
&& ANY_INTEGRAL_TYPE_P (TREE_TYPE(@0))
&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@0))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@0))
&& TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type)
&& TYPE_PRECISION (TREE_TYPE (@0)) <= TYPE_PRECISION (type))
(convert @1)))
I kept the TYPE_OVERFLOW_SANITIZED checks because I saw other patterns that
leverage undefined overflows check for it. I think this new pattern shouldn't
be applied if overflow sanitizer checks are enabled.
why is this testing TREE_TYPE (@0)?
I'm checking the type of @0 because I'm concerned that there could be a case
where @0's type isn't an integer type with undefined overflow. I tried creating
a test case and couldn't seem to create one where this is violated but I kept
the checks to avoid causing a regression. If I'm being overcautious and you
feel that the type checks on @0 aren't needed, I can remove them. I think the
precision check on TREE_TYPE(@0) is needed to avoid truncation cases though.
It doesn't matter if @0 has undefined overflow, but it can matter that it
be signed (yes, the 2 are correlated...) if it has the same precision as
@2. Otherwise (int64_t)(-1u)-(int64_t)((int)(-1u)-0) is definitely not 0
and it has type:int64_t, @2:int, @0:unsigned.
Ignoring the sanitizer, the confusing double matching of constants, and
restricting to scalars, I think the tightest condition (without vrp) that
works is
TYPE_PRECISION (type) <= TYPE_PRECISION (TREE_TYPE (@2)) ||
TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@2)) &&
(TYPE_PRECISION (TREE_TYPE (@0)) < TYPE_PRECISION (TREE_TYPE (@2)) ||
TYPE_PRECISION (TREE_TYPE (@0)) == TYPE_PRECISION (TREE_TYPE (@2)) &&
!TYPE_UNSIGNED (TREE_TYPE (@0))
)
(where implicitly undefined => signed) but of course it is ok to handle
only a subset. It is too late for me to think about @0 vs @@0.
The patch you posted seems to accept the wrong
(int128t)LLONG_MAX-(int128_t)((int)LLONG_MAX-0) -> (int128_t)0
Using ANY_INTEGRAL_TYPE_P allows vectors, and TYPE_PRECISION mustn't be
used for vectors (maybe we should add more checking to catch such uses),
and they may not be very happy with convert either...
Once we'd "inline" nop_convert genmatch would complain about this.
Maybe the new transform could be about scalars, and we could restrict the
old one to vectors, to simplify the code, but that wouldn't help genmatch
with the fact that the pattern x-(x-y) would appear twice. Is it really
that bad? It is suspicious, but can be justified.
Is someone working on inlining nop_convert? I'd like to avoid breaking someone
else's work if that's being worked on right now.
I've also added some extra tests to cover this new pattern. I've attached a
patch with my latest changes.
From: Richard Biener
Sent: Wednesday, July 28, 2021 2:59 AM
To: Victor Tong
Cc: Marc Glisse ; gcc-patches@gcc.gnu.org
Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
On Tue, Jun 29, 2021 at 1:10 AM Victor Tong wrote:
Thanks Richard and Marc.
I wrote the following test case to compare the outputs of fn1() and fn1NoOpt()
below with my extra pattern being applied. I tested the two functions with all
of the integers from INT_MIN to INT_MAX.
long
fn1 (int x)
{
return 42L - (long)(42 - x);
}
#pragma GCC push_options
#pragma GCC optimize ("O0")
long
fn1NoOpt (int x)
{
volatile int y = (42 - x);
return 42L - (long)y;
}
#pragma GCC pop_options
int main ()
{
for (long i=INT_MIN; i<=INT_MAX;i++)
{
auto valNoOpt = fn1NoOpt(i);
auto valOpt = fn1(i);
if (valNoOpt != valOpt)
printf("valOpt=%ld, valNoOpt=%ld\n", valOpt, valNoOpt);
}
return 0;
}
I saw that the return values of fn1() and fn1NoOpt() differed when the input
was between INT_MIN and INT_MIN+42 inclusive. When passing values in this range
to fn1NoOpt(), a signed overflow is triggered which causes the value to differ
(undefined behavior). This seems to go in line with what Marc described and I
think the transformation is correct in the scenario above. I do think that type
casts that result in truncation (i.e. from a higher precision to a lower one)
or with unsigned types will result in an incorrect transformation so those
scenarios need t
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
Thanks for the feedback. Here's the updated pattern:
/* X - (X - Y) --> Y */
(simplify
(minus (convert1? @0) (convert2? (minus@2 (convert3? @@0) @1)))
(if (ANY_INTEGRAL_TYPE_P (type)
&& TYPE_OVERFLOW_UNDEFINED(type)
&& !TYPE_OVERFLOW_SANITIZED(type)
&& ANY_INTEGRAL_TYPE_P (TREE_TYPE(@2))
&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@2))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@2))
&& ANY_INTEGRAL_TYPE_P (TREE_TYPE(@0))
&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@0))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@0))
&& TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type)
&& TYPE_PRECISION (TREE_TYPE (@0)) <= TYPE_PRECISION (type))
(convert @1)))
I kept the TYPE_OVERFLOW_SANITIZED checks because I saw other patterns that
leverage undefined overflows check for it. I think this new pattern shouldn't
be applied if overflow sanitizer checks are enabled.
>> why is this testing TREE_TYPE (@0)?
I'm checking the type of @0 because I'm concerned that there could be a case
where @0's type isn't an integer type with undefined overflow. I tried creating
a test case and couldn't seem to create one where this is violated but I kept
the checks to avoid causing a regression. If I'm being overcautious and you
feel that the type checks on @0 aren't needed, I can remove them. I think the
precision check on TREE_TYPE(@0) is needed to avoid truncation cases though.
>> Once we'd "inline" nop_convert genmatch would complain
about this.
Is someone working on inlining nop_convert? I'd like to avoid breaking someone
else's work if that's being worked on right now.
I've also added some extra tests to cover this new pattern. I've attached a
patch with my latest changes.
From: Richard Biener
Sent: Wednesday, July 28, 2021 2:59 AM
To: Victor Tong
Cc: Marc Glisse ; gcc-patches@gcc.gnu.org
Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division
followed by multiply [PR95176]
On Tue, Jun 29, 2021 at 1:10 AM Victor Tong wrote:
>
> Thanks Richard and Marc.
>
> I wrote the following test case to compare the outputs of fn1() and
> fn1NoOpt() below with my extra pattern being applied. I tested the two
> functions with all of the integers from INT_MIN to INT_MAX.
>
> long
> fn1 (int x)
> {
> return 42L - (long)(42 - x);
> }
>
> #pragma GCC push_options
> #pragma GCC optimize ("O0")
> long
> fn1NoOpt (int x)
> {
> volatile int y = (42 - x);
> return 42L - (long)y;
> }
> #pragma GCC pop_options
>
> int main ()
> {
> for (long i=INT_MIN; i<=INT_MAX;i++)
> {
> auto valNoOpt = fn1NoOpt(i);
> auto valOpt = fn1(i);
> if (valNoOpt != valOpt)
> printf("valOpt=%ld, valNoOpt=%ld\n", valOpt,
>valNoOpt);
> }
> return 0;
> }
>
> I saw that the return values of fn1() and fn1NoOpt() differed when the input
> was between INT_MIN and INT_MIN+42 inclusive. When passing values in this
> range to fn1NoOpt(), a signed overflow is triggered which causes the value to
> differ (undefined behavior). This seems to go in line with what Marc
> described and I think the transformation is correct in the scenario above. I
> do think that type casts that result in truncation (i.e. from a higher
> precision to a lower one) or with unsigned types will result in an incorrect
> transformation so those scenarios need to be avoided.
>
> Given that the extra pattern I'm adding is taking advantage the undefined
> behavior of signed integer overflow, I'm considering keeping the existing
> nop_convert pattern in place and adding a new pattern to cover these new
> cases. I'd also like to avoid touching nop_convert given that it's used in a
> number of other patterns.
>
> This is the pattern I have currently:
>
> (simplify
> (minus (convert1? @0) (convert2? (minus (convert3? @2) @1)))
> (if (operand_equal_p(@0, @2, 0)
The operand_equal_p should be reflected by using @0 in place of @2.
> && INTEGRAL_TYPE_P (type)
> && TYPE_OVERFLOW_UNDEFINED(type)
> && !TYPE_OVERFLOW_SANITIZED(type)
> && INTEGRAL_TYPE_P (TREE_TYPE(@1))
> && TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@1))
> && !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@1))
> && !TYPE_UNSIGNED (TREE_TYPE (@1))
> && !TYPE_UNSIGNED (type)
please group checks on common argument. I think a single test
on INTEGRAL_TYPE_P (type) is enough, it could be ANY_INTEGRAL_TYPE_P
to include vector and complex types.
> && TYPE_PRECISION
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
On Tue, Jun 29, 2021 at 1:10 AM Victor Tong wrote:
>
> Thanks Richard and Marc.
>
> I wrote the following test case to compare the outputs of fn1() and
> fn1NoOpt() below with my extra pattern being applied. I tested the two
> functions with all of the integers from INT_MIN to INT_MAX.
>
> long
> fn1 (int x)
> {
> return 42L - (long)(42 - x);
> }
>
> #pragma GCC push_options
> #pragma GCC optimize ("O0")
> long
> fn1NoOpt (int x)
> {
> volatile int y = (42 - x);
> return 42L - (long)y;
> }
> #pragma GCC pop_options
>
> int main ()
> {
> for (long i=INT_MIN; i<=INT_MAX;i++)
> {
> auto valNoOpt = fn1NoOpt(i);
> auto valOpt = fn1(i);
> if (valNoOpt != valOpt)
> printf("valOpt=%ld, valNoOpt=%ld\n", valOpt,
> valNoOpt);
> }
> return 0;
> }
>
> I saw that the return values of fn1() and fn1NoOpt() differed when the input
> was between INT_MIN and INT_MIN+42 inclusive. When passing values in this
> range to fn1NoOpt(), a signed overflow is triggered which causes the value to
> differ (undefined behavior). This seems to go in line with what Marc
> described and I think the transformation is correct in the scenario above. I
> do think that type casts that result in truncation (i.e. from a higher
> precision to a lower one) or with unsigned types will result in an incorrect
> transformation so those scenarios need to be avoided.
>
> Given that the extra pattern I'm adding is taking advantage the undefined
> behavior of signed integer overflow, I'm considering keeping the existing
> nop_convert pattern in place and adding a new pattern to cover these new
> cases. I'd also like to avoid touching nop_convert given that it's used in a
> number of other patterns.
>
> This is the pattern I have currently:
>
> (simplify
> (minus (convert1? @0) (convert2? (minus (convert3? @2) @1)))
> (if (operand_equal_p(@0, @2, 0)
The operand_equal_p should be reflected by using @0 in place of @2.
> && INTEGRAL_TYPE_P (type)
> && TYPE_OVERFLOW_UNDEFINED(type)
> && !TYPE_OVERFLOW_SANITIZED(type)
> && INTEGRAL_TYPE_P (TREE_TYPE(@1))
> && TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@1))
> && !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@1))
> && !TYPE_UNSIGNED (TREE_TYPE (@1))
> && !TYPE_UNSIGNED (type)
please group checks on common argument. I think a single test
on INTEGRAL_TYPE_P (type) is enough, it could be ANY_INTEGRAL_TYPE_P
to include vector and complex types.
> && TYPE_PRECISION (TREE_TYPE (@1)) <= TYPE_PRECISION (type)
> && INTEGRAL_TYPE_P (TREE_TYPE(@0))
> && TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@0))
why is this testing TREE_TYPE (@0)? The outer subtract is using 'type',
the inner subtract uses TREE_TYPE (@1) though you could place
a capture on the minus to make what you test more obvious, like
(minus (convert1? @0) (convert2? (minus@3 (convert3? @2) @1)))
TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@3))
there's one set of checks too much I think.
> && !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@0))
> && !TYPE_UNSIGNED (TREE_TYPE (@0))
we only ever have TYPE_OVERFLOW_UNDEFINED on singed types so
the !TYPE_UNSIGNED checks are superfluous.
> && TYPE_PRECISION (TREE_TYPE (@0)) <= TYPE_PRECISION (type)
> && TREE_TYPE(@1) == TREE_TYPE(@2))
This check means that convert3 is never present since a MINUS requires
compatible types.
Sorry for the late reply.
Note the pattern will necessarily be partly redundant with the
(simplify
(minus (nop_convert1? (minus (nop_convert2? @0) @1)) @0)
(if (!ANY_INTEGRAL_TYPE_P (type)
|| TYPE_OVERFLOW_WRAPS (type))
(negate (view_convert @1))
(view_convert (negate @1
pattern. Once we'd "inline" nop_convert genmatch would complain
about this.
> (convert @1)))
>
> Is there a more concise/better way of writing the pattern? I was looking for
> similar checks in match.pd and I couldn't find anything that I could leverage.
>
> I also kept my pattern to the specific scenario I'm seeing with the
> regression to lower the risk of something breaking. I've limited @1 and @2 to
> have the same type.
>
> I'm also in favor of adding/running computer verification to make sure the
> transformation is legal. I've written some tests to verify that the pattern
> is being applied in the right scenarios and not being applied in others, but
> I think there are too many possibilities to manually write them all. Is there
> anything in GCC that can be use
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
Gentle ping.
From: Gcc-patches on
behalf of Victor Tong via Gcc-patches
Sent: Monday, June 28, 2021 4:10 PM
To: Richard Biener ; Marc Glisse
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division
followed by multiply [PR95176]
Thanks Richard and Marc.
I wrote the following test case to compare the outputs of fn1() and fn1NoOpt()
below with my extra pattern being applied. I tested the two functions with all
of the integers from INT_MIN to INT_MAX.
long
fn1 (int x)
{
return 42L - (long)(42 - x);
}
#pragma GCC push_options
#pragma GCC optimize ("O0")
long
fn1NoOpt (int x)
{
volatile int y = (42 - x);
return 42L - (long)y;
}
#pragma GCC pop_options
int main ()
{
for (long i=INT_MIN; i<=INT_MAX;i++)
{
auto valNoOpt = fn1NoOpt(i);
auto valOpt = fn1(i);
if (valNoOpt != valOpt)
printf("valOpt=%ld, valNoOpt=%ld\n", valOpt, valNoOpt);
}
return 0;
}
I saw that the return values of fn1() and fn1NoOpt() differed when the input
was between INT_MIN and INT_MIN+42 inclusive. When passing values in this range
to fn1NoOpt(), a signed overflow is triggered which causes the value to differ
(undefined behavior). This seems to go in line with what Marc described and I
think the transformation is correct in the scenario above. I do think that type
casts that result in truncation (i.e. from a higher precision to a lower one)
or with unsigned types will result in an incorrect transformation so those
scenarios need to be avoided.
Given that the extra pattern I'm adding is taking advantage the undefined
behavior of signed integer overflow, I'm considering keeping the existing
nop_convert pattern in place and adding a new pattern to cover these new cases.
I'd also like to avoid touching nop_convert given that it's used in a number of
other patterns.
This is the pattern I have currently:
(simplify
(minus (convert1? @0) (convert2? (minus (convert3? @2) @1)))
(if (operand_equal_p(@0, @2, 0)
&& INTEGRAL_TYPE_P (type)
&& TYPE_OVERFLOW_UNDEFINED(type)
&& !TYPE_OVERFLOW_SANITIZED(type)
&& INTEGRAL_TYPE_P (TREE_TYPE(@1))
&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@1))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@1))
&& !TYPE_UNSIGNED (TREE_TYPE (@1))
&& !TYPE_UNSIGNED (type)
&& TYPE_PRECISION (TREE_TYPE (@1)) <= TYPE_PRECISION (type)
&& INTEGRAL_TYPE_P (TREE_TYPE(@0))
&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@0))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@0))
&& !TYPE_UNSIGNED (TREE_TYPE (@0))
&& TYPE_PRECISION (TREE_TYPE (@0)) <= TYPE_PRECISION (type)
&& TREE_TYPE(@1) == TREE_TYPE(@2))
(convert @1)))
Is there a more concise/better way of writing the pattern? I was looking for
similar checks in match.pd and I couldn't find anything that I could leverage.
I also kept my pattern to the specific scenario I'm seeing with the regression
to lower the risk of something breaking. I've limited @1 and @2 to have the
same type.
I'm also in favor of adding/running computer verification to make sure the
transformation is legal. I've written some tests to verify that the pattern is
being applied in the right scenarios and not being applied in others, but I
think there are too many possibilities to manually write them all. Is there
anything in GCC that can be used to verify that match.pd transformations are
correct? I'm thinking of something like Alive
https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FAliveToolkit%2Falive2data=04%7C01%7Cvitong%40microsoft.com%7Cba7d8f9f9b774462148608d93a8a0471%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637605186726283785%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000sdata=ugx%2FTw58OPjLzamE9yqQThV5u4EfQ8JLrurnIy00AzQ%3Dreserved=0.
Thanks,
Victor
From: Richard Biener
Sent: Monday, June 21, 2021 12:08 AM
To: Marc Glisse
Cc: Victor Tong ; gcc-patches@gcc.gnu.org
Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division
followed by multiply [PR95176]
On Sat, Jun 19, 2021 at 7:05 PM Marc Glisse wrote:
>
> On Fri, 18 Jun 2021, Richard Biener wrote:
>
> >> Option 2: Add a new pattern to support scenarios that the existing
> >> nop_convert pattern bails out on.
> >>
> >> Existing pattern:
> >>
> >> (simplify
> >>(minus (nop_convert1? @0) (nop_convert2? (minus (nop_convert3? @@0)
> >> @1)))
> >>(view_convert @1))
>
> I tried to check with a program when
>
> T3 x;
> T1 y;
> (T2)x-(T2)((T1)x-y)
>
> can be safely repla
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
Thanks Richard and Marc.
I wrote the following test case to compare the outputs of fn1() and fn1NoOpt()
below with my extra pattern being applied. I tested the two functions with all
of the integers from INT_MIN to INT_MAX.
long
fn1 (int x)
{
return 42L - (long)(42 - x);
}
#pragma GCC push_options
#pragma GCC optimize ("O0")
long
fn1NoOpt (int x)
{
volatile int y = (42 - x);
return 42L - (long)y;
}
#pragma GCC pop_options
int main ()
{
for (long i=INT_MIN; i<=INT_MAX;i++)
{
auto valNoOpt = fn1NoOpt(i);
auto valOpt = fn1(i);
if (valNoOpt != valOpt)
printf("valOpt=%ld, valNoOpt=%ld\n", valOpt, valNoOpt);
}
return 0;
}
I saw that the return values of fn1() and fn1NoOpt() differed when the input
was between INT_MIN and INT_MIN+42 inclusive. When passing values in this range
to fn1NoOpt(), a signed overflow is triggered which causes the value to differ
(undefined behavior). This seems to go in line with what Marc described and I
think the transformation is correct in the scenario above. I do think that type
casts that result in truncation (i.e. from a higher precision to a lower one)
or with unsigned types will result in an incorrect transformation so those
scenarios need to be avoided.
Given that the extra pattern I'm adding is taking advantage the undefined
behavior of signed integer overflow, I'm considering keeping the existing
nop_convert pattern in place and adding a new pattern to cover these new cases.
I'd also like to avoid touching nop_convert given that it's used in a number of
other patterns.
This is the pattern I have currently:
(simplify
(minus (convert1? @0) (convert2? (minus (convert3? @2) @1)))
(if (operand_equal_p(@0, @2, 0)
&& INTEGRAL_TYPE_P (type)
&& TYPE_OVERFLOW_UNDEFINED(type)
&& !TYPE_OVERFLOW_SANITIZED(type)
&& INTEGRAL_TYPE_P (TREE_TYPE(@1))
&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@1))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@1))
&& !TYPE_UNSIGNED (TREE_TYPE (@1))
&& !TYPE_UNSIGNED (type)
&& TYPE_PRECISION (TREE_TYPE (@1)) <= TYPE_PRECISION (type)
&& INTEGRAL_TYPE_P (TREE_TYPE(@0))
&& TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@0))
&& !TYPE_OVERFLOW_SANITIZED(TREE_TYPE(@0))
&& !TYPE_UNSIGNED (TREE_TYPE (@0))
&& TYPE_PRECISION (TREE_TYPE (@0)) <= TYPE_PRECISION (type)
&& TREE_TYPE(@1) == TREE_TYPE(@2))
(convert @1)))
Is there a more concise/better way of writing the pattern? I was looking for
similar checks in match.pd and I couldn't find anything that I could leverage.
I also kept my pattern to the specific scenario I'm seeing with the regression
to lower the risk of something breaking. I've limited @1 and @2 to have the
same type.
I'm also in favor of adding/running computer verification to make sure the
transformation is legal. I've written some tests to verify that the pattern is
being applied in the right scenarios and not being applied in others, but I
think there are too many possibilities to manually write them all. Is there
anything in GCC that can be used to verify that match.pd transformations are
correct? I'm thinking of something like Alive
https://github.com/AliveToolkit/alive2.
Thanks,
Victor
From: Richard Biener
Sent: Monday, June 21, 2021 12:08 AM
To: Marc Glisse
Cc: Victor Tong ; gcc-patches@gcc.gnu.org
Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division
followed by multiply [PR95176]
On Sat, Jun 19, 2021 at 7:05 PM Marc Glisse wrote:
>
> On Fri, 18 Jun 2021, Richard Biener wrote:
>
> >> Option 2: Add a new pattern to support scenarios that the existing
> >> nop_convert pattern bails out on.
> >>
> >> Existing pattern:
> >>
> >> (simplify
> >> (minus (nop_convert1? @0) (nop_convert2? (minus (nop_convert3? @@0)
> >>@1)))
> >> (view_convert @1))
>
> I tried to check with a program when
>
> T3 x;
> T1 y;
> (T2)x-(T2)((T1)x-y)
>
> can be safely replaced with
>
> (T2)y
>
> From the output, it looks like this is safe when T1 is at least as large
> as T2. It is wrong when T1 is unsigned and smaller than T2. And when T1 is
> signed and smaller than T2, it is ok if T3 is the same type as T1 (signed
> then) or has strictly less precision (any sign), and not in other cases.
>
> Note that this is when signed implies undefined overflow and unsigned
> implies wrapping, and I wouldn't put too much faith in this recently
> dusted program. And it doesn't say how to write the match.pd pattern with
> '?', "@@", disabling it if TYPE_OVERFLOW_SANITIZED, etc.
>
> Mostly, I wa
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
On Sat, Jun 19, 2021 at 7:05 PM Marc Glisse wrote: > > On Fri, 18 Jun 2021, Richard Biener wrote: > > >> Option 2: Add a new pattern to support scenarios that the existing > >> nop_convert pattern bails out on. > >> > >> Existing pattern: > >> > >> (simplify > >>(minus (nop_convert1? @0) (nop_convert2? (minus (nop_convert3? @@0) > >> @1))) > >>(view_convert @1)) > > I tried to check with a program when > > T3 x; > T1 y; > (T2)x-(T2)((T1)x-y) > > can be safely replaced with > > (T2)y > > From the output, it looks like this is safe when T1 is at least as large > as T2. It is wrong when T1 is unsigned and smaller than T2. And when T1 is > signed and smaller than T2, it is ok if T3 is the same type as T1 (signed > then) or has strictly less precision (any sign), and not in other cases. > > Note that this is when signed implies undefined overflow and unsigned > implies wrapping, and I wouldn't put too much faith in this recently > dusted program. And it doesn't say how to write the match.pd pattern with > '?', "@@", disabling it if TYPE_OVERFLOW_SANITIZED, etc. > > Mostly, I wanted to say that if we are going to go handle more than > nop_convert for more than just 1 or 2 easy transformations, I think some > kind of computer verification would be useful, it would save a lot of time > and headaches. True. I wonder if auto-generating such tests from match.pd rules would be a good project to work on (GSoC?). When there's define_match involved things get a little tricky, but then one item on the TODO list is "inlining" those at the use site (exploding the decision tree even more). Richard. > (I just check by brute force all possible precisions (from 1 to 6) and > signedness for T1, T2 and T3, all possible values for x and y, compute > the before and after formulas, accepting if there is UB before, rejecting > if there is UB after (and not before), and manually try to see a pattern > in the list of types that work) > > -- > Marc Glisse
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
On Fri, 18 Jun 2021, Richard Biener wrote: Option 2: Add a new pattern to support scenarios that the existing nop_convert pattern bails out on. Existing pattern: (simplify (minus (nop_convert1? @0) (nop_convert2? (minus (nop_convert3? @@0) @1))) (view_convert @1)) I tried to check with a program when T3 x; T1 y; (T2)x-(T2)((T1)x-y) can be safely replaced with (T2)y From the output, it looks like this is safe when T1 is at least as large as T2. It is wrong when T1 is unsigned and smaller than T2. And when T1 is signed and smaller than T2, it is ok if T3 is the same type as T1 (signed then) or has strictly less precision (any sign), and not in other cases. Note that this is when signed implies undefined overflow and unsigned implies wrapping, and I wouldn't put too much faith in this recently dusted program. And it doesn't say how to write the match.pd pattern with '?', "@@", disabling it if TYPE_OVERFLOW_SANITIZED, etc. Mostly, I wanted to say that if we are going to go handle more than nop_convert for more than just 1 or 2 easy transformations, I think some kind of computer verification would be useful, it would save a lot of time and headaches. (I just check by brute force all possible precisions (from 1 to 6) and signedness for T1, T2 and T3, all possible values for x and y, compute the before and after formulas, accepting if there is UB before, rejecting if there is UB after (and not before), and manually try to see a pattern in the list of types that work) -- Marc Glisse
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
On Wed, Jun 16, 2021 at 8:49 PM Victor Tong wrote:
>
> Hi Richard,
>
> Thanks for the feedback. From what you said, I can think of two possible
> solutions (though I'm not sure if either is feasible/fully correct):
>
> Option 1: Have the new X * (Y / X) --> Y - (Y % X) optimization only run in
> scenarios that don't interfere with the existing X - (X / Y) * Y --> X % Y
> optimization.
>
> This would involve checking the expression one level up to see if there's a
> subtraction that would trigger the existing optimization. I looked through
> the match.pd file and couldn't find a bail condition like this. It doesn't
> seem like there's a link from an expression to its parent expression one
> level up. This also feels a bit counter-intuitive since it would be doing the
> opposite of the bottom-up expression matching where the compiler would like
> to match a larger expression rather than a smaller one.
Yes, that option is not really possible from match.pd.
> Option 2: Add a new pattern to support scenarios that the existing
> nop_convert pattern bails out on.
>
> Existing pattern:
>
> (simplify
>(minus (nop_convert1? @0) (nop_convert2? (minus (nop_convert3? @@0) @1)))
>(view_convert @1))
>
> New pattern to add:
>
> /* X - (X - Y) --> Y */
> (simplify
> (minus @0 (convert? (minus @@0 @1)))
> (if (INTEGRAL_TYPE_P (type)
> && TYPE_OVERFLOW_UNDEFINED(type)
> && INTEGRAL_TYPE_P (TREE_TYPE(@1))
> && TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@1))
> && !TYPE_UNSIGNED (TREE_TYPE (@1))
> && !TYPE_UNSIGNED (type)
> && TYPE_PRECISION (TREE_TYPE (@1)) <= TYPE_PRECISION (type))
> (convert @1)))
>
> I think the truncation concerns that you brought up should be covered if the
> external expression type precision is greater than or equal to the internal
> expression type. There may be a sign extension operation (which is why the
> nop_convert check fails) but that shouldn't affect the value of the
> expression. And if the types involved are signed integers where
> overflow/underflow results in undefined behavior, the X - (X - Y) --> Y
> optimization should be legal.
>
> Please correct me if I'm wrong with either one of these options, or if you
> can think of a better option to fix the regression.
So to recap, we're looking to simplify 42 - (long int) (42 - 42 % x)
(simplified from gcc.dg/fold-minus-6.c), or
simply (new testcase):
long
fn1 (int x)
{
return 42L - (long)(42 - x);
}
where the existing pattern does not apply because the conversion is
not a NOP one:
(simplify
(minus (nop_convert1? (minus (nop_convert2? @0) @1)) @0)
(if (!ANY_INTEGRAL_TYPE_P (type)
|| TYPE_OVERFLOW_WRAPS (type))
(negate (view_convert @1))
(view_convert (negate @1
so let's consider replacing nop_convert1? with convert1? and thus obtain
(simplify
(minus (convert1? (minus (nop_convert2? @0) @1)) @0)
(if (!ANY_INTEGRAL_TYPE_P (type)
|| TYPE_OVERFLOW_WRAPS (type))
(negate (view_convert @1))
(view_convert (negate @1
given we still require a matching @0 (as in operand_requal_p) it looks like
a convert1 that is not the inverse of the nop_convert2, and thus also
a nop_convert
is only possible for constants (because operand_equal_p does not verify type
equality). Now - can we construct any testcase for which this conversion would
be wrong?
Richard.
> Thanks,
> Victor
>
>
>
>
> From: Richard Biener
> Sent: Monday, June 7, 2021 1:25 AM
> To: Victor Tong
> Cc: gcc-patches@gcc.gnu.org
> Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division
> followed by multiply [PR95176]
>
> On Wed, Jun 2, 2021 at 10:55 PM Victor Tong wrote:
> >
> > Hi Richard,
> >
> > Thanks for reviewing my patch. I did a search online and you're right --
> > there isn't a vector modulo instruction. I'll remove the X * (Y / X) --> Y
> > - (Y % X) pattern and the existing X - (X / Y) * Y --> X % Y from
> > triggering on vector types.
> >
> > I looked into why the following pattern isn't triggering:
> >
> > (simplify
> >(minus @0 (nop_convert1? (minus (nop_convert2? @0) @1)))
> >(view_convert @1))
> >
> > The nop_converts expand into tree_nop_conversion_p checks. In fn2() of the
> > testsuite/gcc.dg/fold-minus-6.c, the expression during generic matching
> > looks like:
> >
> > 42 - (long int) (42 - 42 % x)
> >
> > When looking at the right-hand side of the expression (the (long int) (42 -
> > 42 % x)), the tree_nop_conversion_p check fails because of the type
> > precision difference. The expression
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
Hi Richard, Thanks for the feedback. From what you said, I can think of two possible solutions (though I'm not sure if either is feasible/fully correct): Option 1: Have the new X * (Y / X) --> Y - (Y % X) optimization only run in scenarios that don't interfere with the existing X - (X / Y) * Y --> X % Y optimization. This would involve checking the expression one level up to see if there's a subtraction that would trigger the existing optimization. I looked through the match.pd file and couldn't find a bail condition like this. It doesn't seem like there's a link from an expression to its parent expression one level up. This also feels a bit counter-intuitive since it would be doing the opposite of the bottom-up expression matching where the compiler would like to match a larger expression rather than a smaller one. Option 2: Add a new pattern to support scenarios that the existing nop_convert pattern bails out on. Existing pattern: (simplify (minus (nop_convert1? @0) (nop_convert2? (minus (nop_convert3? @@0) @1))) (view_convert @1)) New pattern to add: /* X - (X - Y) --> Y */ (simplify (minus @0 (convert? (minus @@0 @1))) (if (INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_UNDEFINED(type) && INTEGRAL_TYPE_P (TREE_TYPE(@1)) && TYPE_OVERFLOW_UNDEFINED(TREE_TYPE(@1)) && !TYPE_UNSIGNED (TREE_TYPE (@1)) && !TYPE_UNSIGNED (type) && TYPE_PRECISION (TREE_TYPE (@1)) <= TYPE_PRECISION (type)) (convert @1))) I think the truncation concerns that you brought up should be covered if the external expression type precision is greater than or equal to the internal expression type. There may be a sign extension operation (which is why the nop_convert check fails) but that shouldn't affect the value of the expression. And if the types involved are signed integers where overflow/underflow results in undefined behavior, the X - (X - Y) --> Y optimization should be legal. Please correct me if I'm wrong with either one of these options, or if you can think of a better option to fix the regression. Thanks, Victor From: Richard Biener Sent: Monday, June 7, 2021 1:25 AM To: Victor Tong Cc: gcc-patches@gcc.gnu.org Subject: Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176] On Wed, Jun 2, 2021 at 10:55 PM Victor Tong wrote: > > Hi Richard, > > Thanks for reviewing my patch. I did a search online and you're right -- > there isn't a vector modulo instruction. I'll remove the X * (Y / X) --> Y - > (Y % X) pattern and the existing X - (X / Y) * Y --> X % Y from triggering on > vector types. > > I looked into why the following pattern isn't triggering: > > (simplify > (minus @0 (nop_convert1? (minus (nop_convert2? @0) @1))) > (view_convert @1)) > > The nop_converts expand into tree_nop_conversion_p checks. In fn2() of the > testsuite/gcc.dg/fold-minus-6.c, the expression during generic matching looks > like: > > 42 - (long int) (42 - 42 % x) > > When looking at the right-hand side of the expression (the (long int) (42 - > 42 % x)), the tree_nop_conversion_p check fails because of the type precision > difference. The expression inside of the cast has a 32-bit precision and the > outer expression has a 64-bit precision. > > I looked around at other patterns and it seems like nop_convert and > view_convert are used because of underflow/overflow concerns. I'm not > familiar with the two constructs. What's the difference between using them > and checking TYPE_OVERFLOW_UNDEFINED? In the scenario above, since > TYPE_OVERFLOW_UNDEFINED is true, the second pattern that I added (X - (X - Y) > --> Y) gets triggered. But TYPE_OVERFLOW_UNDEFINED is not a good condition here since the conversion is the problematic one and conversions have implementation defined behavior. Now, the above does not match because it wasn't designed to, and for non-constant '42' it would have needed a (convert ...) around the first @0 as well (matching of constants is by value, not by value + type). That said, your +/* X - (X - Y) --> Y */ +(simplify + (minus (convert1? @0) (convert2? (minus @@0 @1))) + (if ((INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type)) && TYPE_OVERFLOW_UNDEFINED(type)) + (convert @1))) would match (int)x - (int)(x - y) where you assert the outer subtract has undefined behavior on overflow but the inner subtract could wrap and the (int) conversion can be truncating or widening. Is that really always a valid transform then? Richard. > Thanks, > Victor > > > From: Richard Biener > Sent: Tuesday, April 27, 2021 1:29 AM > To: Victor Tong > Cc: gcc-patches@gcc.gnu.org > Subject: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed > by multiply [PR951
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
On Wed, Jun 2, 2021 at 10:55 PM Victor Tong wrote: > > Hi Richard, > > Thanks for reviewing my patch. I did a search online and you're right -- > there isn't a vector modulo instruction. I'll remove the X * (Y / X) --> Y - > (Y % X) pattern and the existing X - (X / Y) * Y --> X % Y from triggering on > vector types. > > I looked into why the following pattern isn't triggering: > > (simplify >(minus @0 (nop_convert1? (minus (nop_convert2? @0) @1))) >(view_convert @1)) > > The nop_converts expand into tree_nop_conversion_p checks. In fn2() of the > testsuite/gcc.dg/fold-minus-6.c, the expression during generic matching looks > like: > > 42 - (long int) (42 - 42 % x) > > When looking at the right-hand side of the expression (the (long int) (42 - > 42 % x)), the tree_nop_conversion_p check fails because of the type precision > difference. The expression inside of the cast has a 32-bit precision and the > outer expression has a 64-bit precision. > > I looked around at other patterns and it seems like nop_convert and > view_convert are used because of underflow/overflow concerns. I'm not > familiar with the two constructs. What's the difference between using them > and checking TYPE_OVERFLOW_UNDEFINED? In the scenario above, since > TYPE_OVERFLOW_UNDEFINED is true, the second pattern that I added (X - (X - Y) > --> Y) gets triggered. But TYPE_OVERFLOW_UNDEFINED is not a good condition here since the conversion is the problematic one and conversions have implementation defined behavior. Now, the above does not match because it wasn't designed to, and for non-constant '42' it would have needed a (convert ...) around the first @0 as well (matching of constants is by value, not by value + type). That said, your +/* X - (X - Y) --> Y */ +(simplify + (minus (convert1? @0) (convert2? (minus @@0 @1))) + (if ((INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type)) && TYPE_OVERFLOW_UNDEFINED(type)) + (convert @1))) would match (int)x - (int)(x - y) where you assert the outer subtract has undefined behavior on overflow but the inner subtract could wrap and the (int) conversion can be truncating or widening. Is that really always a valid transform then? Richard. > Thanks, > Victor > > > From: Richard Biener > Sent: Tuesday, April 27, 2021 1:29 AM > To: Victor Tong > Cc: gcc-patches@gcc.gnu.org > Subject: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed > by multiply [PR95176] > > On Thu, Apr 1, 2021 at 1:03 AM Victor Tong via Gcc-patches > wrote: > > > > Hello, > > > > This patch fixes PR tree-optimization/95176. A new pattern in match.pd was > > added to transform "a * (b / a)" --> "b - (b % a)". A new test case was > > also added to cover this scenario. > > > > The new pattern interfered with the existing pattern of "X - (X / Y) * Y". > > In some cases (such as in fn4() in gcc/testsuite/gcc.dg/fold-minus-6.c), > > the new pattern is applied causing the existing pattern to no longer apply. > > This results in worse code generation because the expression is left as "X > > - (X - Y)". An additional subtraction pattern of "X - (X - Y) --> Y" was > > added to this patch to avoid this regression. > > > > I also didn't remove the existing pattern because it triggered in more > > cases than the new pattern because of a tree_invariant_p check that's > > inserted by genmatch for the new pattern. > > Yes, we do not handle using Y multiple times when it might contain > side-effects in GENERIC folding > (comments in genmatch suggest we can use save_expr but we don't > implement this [anymore]). > > On GIMPLE there's also the issue that your new pattern creates a > complex expression which > makes it failed to be used by value-numbering for example where the > old pattern was OK > (eventually, if no conversion was required). > > So indeed it looks OK to preserve both. > > I wonder why you needed the > > +/* X - (X - Y) --> Y */ > +(simplify > + (minus (convert1? @0) (convert2? (minus @@0 @1))) > + (if ((INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type)) && > TYPE_OVERFLOW_UNDEFINED(type)) > + (convert @1))) > > pattern since it should be handled by > > /* Match patterns that allow contracting a plus-minus pair > irrespective of overflow issues. */ > /* (A +- B) - A -> +- B */ > /* (A +- B) -+ B -> A */ > /* A - (A +- B) -> -+ B */ > /* A +- (B -+ A) -> +- B */ > > in particular > > (simplify >(minus @0 (nop_convert1? (minus (nop_convert2? @0) @1))) >(view_convert @1)) >
Re: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176]
Hi Richard, Thanks for reviewing my patch. I did a search online and you're right -- there isn't a vector modulo instruction. I'll remove the X * (Y / X) --> Y - (Y % X) pattern and the existing X - (X / Y) * Y --> X % Y from triggering on vector types. I looked into why the following pattern isn't triggering: (simplify (minus @0 (nop_convert1? (minus (nop_convert2? @0) @1))) (view_convert @1)) The nop_converts expand into tree_nop_conversion_p checks. In fn2() of the testsuite/gcc.dg/fold-minus-6.c, the expression during generic matching looks like: 42 - (long int) (42 - 42 % x) When looking at the right-hand side of the expression (the (long int) (42 - 42 % x)), the tree_nop_conversion_p check fails because of the type precision difference. The expression inside of the cast has a 32-bit precision and the outer expression has a 64-bit precision. I looked around at other patterns and it seems like nop_convert and view_convert are used because of underflow/overflow concerns. I'm not familiar with the two constructs. What's the difference between using them and checking TYPE_OVERFLOW_UNDEFINED? In the scenario above, since TYPE_OVERFLOW_UNDEFINED is true, the second pattern that I added (X - (X - Y) --> Y) gets triggered. Thanks, Victor From: Richard Biener Sent: Tuesday, April 27, 2021 1:29 AM To: Victor Tong Cc: gcc-patches@gcc.gnu.org Subject: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed by multiply [PR95176] On Thu, Apr 1, 2021 at 1:03 AM Victor Tong via Gcc-patches wrote: > > Hello, > > This patch fixes PR tree-optimization/95176. A new pattern in match.pd was > added to transform "a * (b / a)" --> "b - (b % a)". A new test case was also > added to cover this scenario. > > The new pattern interfered with the existing pattern of "X - (X / Y) * Y". In > some cases (such as in fn4() in gcc/testsuite/gcc.dg/fold-minus-6.c), the new > pattern is applied causing the existing pattern to no longer apply. This > results in worse code generation because the expression is left as "X - (X - > Y)". An additional subtraction pattern of "X - (X - Y) --> Y" was added to > this patch to avoid this regression. > > I also didn't remove the existing pattern because it triggered in more cases > than the new pattern because of a tree_invariant_p check that's inserted by > genmatch for the new pattern. Yes, we do not handle using Y multiple times when it might contain side-effects in GENERIC folding (comments in genmatch suggest we can use save_expr but we don't implement this [anymore]). On GIMPLE there's also the issue that your new pattern creates a complex expression which makes it failed to be used by value-numbering for example where the old pattern was OK (eventually, if no conversion was required). So indeed it looks OK to preserve both. I wonder why you needed the +/* X - (X - Y) --> Y */ +(simplify + (minus (convert1? @0) (convert2? (minus @@0 @1))) + (if ((INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type)) && TYPE_OVERFLOW_UNDEFINED(type)) + (convert @1))) pattern since it should be handled by /* Match patterns that allow contracting a plus-minus pair irrespective of overflow issues. */ /* (A +- B) - A -> +- B */ /* (A +- B) -+ B -> A */ /* A - (A +- B) -> -+ B */ /* A +- (B -+ A) -> +- B */ in particular (simplify (minus @0 (nop_convert1? (minus (nop_convert2? @0) @1))) (view_convert @1)) if there's supported cases missing I'd rather extend this pattern than replicating it. +/* X * (Y / X) is the same as Y - (Y % X). */ +(simplify + (mult:c (convert1? @0) (convert2? (trunc_div @1 @@0))) + (if (INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type)) + (minus (convert @1) (convert (trunc_mod @1 @0) note that if you're allowing vector types you have to use (view_convert ...) in the transform and you also need to make sure that the target can expand the modulo - I suspect that's an issue with the existing pattern as well. I don't know of any vector ISA that supports modulo (or integer division, that is). Restricting the patterns to integer types is probably the most sensible solution. Thanks, Richard. > I verified that all "make -k check" tests pass when targeting > x86_64-pc-linux-gnu. > > 2021-03-31 Victor Tong > > gcc/ChangeLog: > > * match.pd: Two new patterns: One to optimize division followed by >multiply and the other to avoid a regression as explained above > > gcc/testsuite/ChangeLog: > > * gcc.dg/tree-ssa/20030807-10.c: Update existing test to look for a >subtraction because a shift is no longer emitted > * gcc.dg/pr95176.c: New test to cover optimizing division followed by >multiply > > I don't have write access to the GCC repo b
