Jiufu Guo <guoji...@linux.ibm.com> writes:
> Jiufu Guo <guoji...@linux.ibm.com> writes:
>
>> Richard Biener <rguent...@suse.de> writes:
>>
>>> On Mon, 18 Oct 2021, Jiufu Guo wrote:
>>>
>>>> With reference the discussions in:
>>>> https://gcc.gnu.org/pipermail/gcc-patches/2021-July/574334.html
>>>> https://gcc.gnu.org/pipermail/gcc-patches/2021-June/572006.html
>>>> https://gcc.gnu.org/pipermail/gcc-patches/2021-September/578672.html
>>>>
>>>> Base on the patches in above discussion, we may draft a patch to fix the
>>>> issue.
>>>>
>>>> In this patch, to make sure it is ok to change '{b0,s0} op {b1,s1}' to
>>>> '{b0,s0-s1} op {b1,0}', we also compute the condition which could assume
>>>> both 2 ivs are not overflow/wrap: the niter "of '{b0,s0-s1} op {b1,0}'"
>>>> < the niter "of untill wrap for iv0 or iv1".
>>>>
>>>> Does this patch make sense?
>>>
>>> Hum, the patch is mightly complex :/ I'm not sure we can throw
>>> artficial IVs at number_of_iterations_cond and expect a meaningful
>>> result.
>>>
>>> ISTR the problem is with number_of_iterations_ne[_max], but I would
>>> have to go and dig in myself again for a full recap of the problem.
>>> I did plan to do that, but not before stage3 starts.
>>>
>>> Thanks,
>>> Richard.
>>
>> Hi Richard,
>>
>> Thanks for your comment! It is really complex, using artificial IVs and
>> recursively calling number_of_iterations_cond. We may use a simpler way.
>> Not sure if you had started to dig into the problem. I refined a patch.
>> Hope this patch is helpful. This patch enhances the conditions in some
>> aspects. Attached are two test cases that could be handled.
>
> Some questions, I want to consult here, it may help to make the patch
> works better.
>
> - 1. For signed type, I'm wondering if we could leverage the idea about
> "UB on signed overflow" in the phase to call number_of_iterations_cond
> where may be far from user source code.
> If we can, we may just ignore the assumption for signed type.
> But then, there would be inconsitent behavior between noopt(-O0) and
> opt (e.g. -O2/-O3). For example:
> "{INT_MAX-124, +5} < {INT_MAX-27, +1}".
> At -O0, the 'niter' would be 28; while, at -O3, it may result as 26.
>
> - 2. For NEQ, which you may also concern, the assumption
> "delta % step == 0" would make it safe. It seems current, we handle
> NEQ where no_overflow is true for both iv0 and iv1.
For overflow behavior on signed, here is a case. It runs a long time
when build with noopt. At opt level (e.g. -O3), it runs end quickly,
and gets a number of iteration(25).
------
#define TYPE int
#define FUNC(NAME, CODE, S0, S1) \
TYPE __attribute__ ((noinline)) NAME (TYPE b0, TYPE b1) \
{ \
__builtin_printf ("%s %d, %d\n", __FUNCTION__, b0, b1); \
TYPE n = 0; \
TYPE i0, i1; \
for (i0 = b0, i1 = b1; i0 CODE i1; i0 += S0, i1 += S1) \
n++; \
return n; \
}
FUNC (ne_4_0, !=, 4, 0);
int
main ()
{
TYPE r = ne_4_0 (1000, 1103); /* b0 < b1, niter % s != 0 */
__builtin_printf ("res: %ld\n", r);
return r;
}
----------
If using unsinged for TYPE, it runs a long time, even build with -O3.
For unsigned, the assumption checking "delta % step == 0" is added.
While for signed, there is no assumption checking. Here, signed
overflow is treated as UB. And then with option -fwrapv, it also runs
a long time, since this option defines the behavior on overflow.
So, in some aspects, it seems reasonable for current behavior including
it returns a niter(25).
Then we may keep current behavior for questions 1 and 2.
Thanks for comments!
BR,
Jiufu
>
> - 3. In the current patch, DIV_EXPR is used, the cost may be high in
> some cases. I'm wondering if the below idea is workable:
> Extent to longer type, and using MULT instead DIV, for example:
> a < b/c ===> a*c < b. a*c may be need to use longer type than 'a'.
>
> -- 3.1 For some special case, e.g. "{b0, 5} < {b1, -5}", the assumption
> may be able to simplied. For general case, still thinking to reduce
> the runtime cost from assumption.
>
>
> Thanks again!
>
> BR,
> Jiufu
>
>>
>> ---
>> gcc/tree-ssa-loop-niter.c | 92 +++++++++++++++----
>> .../gcc.c-torture/execute/pr100740.c | 11 +++
>> gcc/testsuite/gcc.dg/vect/pr102131.c | 47 ++++++++++
>> 3 files changed, 134 insertions(+), 16 deletions(-)
>> create mode 100644 gcc/testsuite/gcc.c-torture/execute/pr100740.c
>> create mode 100644 gcc/testsuite/gcc.dg/vect/pr102131.c
>>
>> diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
>> index 06954e437f5..ee1d7293c5c 100644
>> --- a/gcc/tree-ssa-loop-niter.c
>> +++ b/gcc/tree-ssa-loop-niter.c
>> @@ -1788,6 +1788,70 @@ dump_affine_iv (FILE *file, affine_iv *iv)
>> }
>> }
>>
>> +/* Generate expr: (HIGH - LOW) / STEP, under UTYPE. */
>> +
>> +static tree
>> +get_step_count (tree high, tree low, tree step, tree utype,
>> + bool end_inclusive = false)
>> +{
>> + tree delta = fold_build2 (MINUS_EXPR, TREE_TYPE (low), high, low);
>> + delta = fold_convert (utype,delta);
>> + if (end_inclusive)
>> + delta = fold_build2 (PLUS_EXPR, utype, delta, build_one_cst (utype));
>> +
>> + if (tree_int_cst_sign_bit (step))
>> + step = fold_build1 (NEGATE_EXPR, TREE_TYPE (step), step);
>> + step = fold_convert (utype, step);
>> +
>> + return fold_build2 (FLOOR_DIV_EXPR, utype, delta, step);
>> +}
>> +
>> +/* Get the additional assumption if both two steps are not zero.
>> + Assumptions satisfy that there is no overflow or wrap during
>> + v0 and v1 chasing. */
>> +
>> +static tree
>> +extra_iv_chase_assumption (affine_iv *iv0, affine_iv *iv1, tree step,
>> + enum tree_code code)
>> +{
>> + /* No need additional assumptions. */
>> + if (code == NE_EXPR)
>> + return boolean_true_node;
>> +
>> + /* it not safe to transform {b0, 1} < {b1, 2}. */
>> + if (tree_int_cst_sign_bit (step))
>> + return boolean_false_node;
>> +
>> + /* No need addition assumption for pointer. */
>> + tree type = TREE_TYPE (iv0->base);
>> + if (POINTER_TYPE_P (type))
>> + return boolean_true_node;
>> +
>> + bool positive0 = !tree_int_cst_sign_bit (iv0->step);
>> + bool positive1 = !tree_int_cst_sign_bit (iv1->step);
>> + bool positive = !tree_int_cst_sign_bit (step);
>> + tree utype = unsigned_type_for (type);
>> + bool add1 = code == LE_EXPR;
>> + tree niter = positive
>> + ? get_step_count (iv1->base, iv0->base, step, utype, add1)
>> + : get_step_count (iv0->base, iv1->base, step, utype, add1);
>> +
>> + int prec = TYPE_PRECISION (type);
>> + signop sgn = TYPE_SIGN (type);
>> + tree max = wide_int_to_tree (type, wi::max_value (prec, sgn));
>> + tree min = wide_int_to_tree (type, wi::min_value (prec, sgn));
>> + tree valid_niter0, valid_niter1;
>> +
>> + valid_niter0 = positive0 ? get_step_count (max, iv0->base, iv0->step,
>> utype)
>> + : get_step_count (iv0->base, min, iv0->step, utype);
>> + valid_niter1 = positive1 ? get_step_count (max, iv1->base, iv1->step,
>> utype)
>> + : get_step_count (iv1->base, min, iv1->step, utype);
>> +
>> + tree e0 = fold_build2 (LT_EXPR, boolean_type_node, niter, valid_niter0);
>> + tree e1 = fold_build2 (LT_EXPR, boolean_type_node, niter, valid_niter1);
>> + return fold_build2 (TRUTH_AND_EXPR, boolean_type_node, e0, e1);
>> +}
>> +
>> /* Determine the number of iterations according to condition (for staying
>> inside loop) which compares two induction variables using comparison
>> operator CODE. The induction variable on left side of the comparison
>> @@ -1879,30 +1943,26 @@ number_of_iterations_cond (class loop *loop,
>> {iv0.base, iv0.step - iv1.step} cmp_code {iv1.base, 0}
>>
>> provided that either below condition is satisfied:
>> + a. iv0.step and iv1.step are integer.
>> + b. Additional condition: before iv0 chase up v1, iv0 and iv1 should not
>> + step over min or max of the type. */
>>
>> - a) the test is NE_EXPR;
>> - b) iv0.step - iv1.step is integer and iv0/iv1 don't overflow.
>> -
>> - This rarely occurs in practice, but it is simple enough to manage. */
>> if (!integer_zerop (iv0->step) && !integer_zerop (iv1->step))
>> {
>> + if (TREE_CODE (iv0->step) != INTEGER_CST
>> + || TREE_CODE (iv1->step) != INTEGER_CST)
>> + return false;
>> +
>> tree step_type = POINTER_TYPE_P (type) ? sizetype : type;
>> - tree step = fold_binary_to_constant (MINUS_EXPR, step_type,
>> - iv0->step, iv1->step);
>> -
>> - /* No need to check sign of the new step since below code takes care
>> - of this well. */
>> - if (code != NE_EXPR
>> - && (TREE_CODE (step) != INTEGER_CST
>> - || !iv0->no_overflow || !iv1->no_overflow))
>> + tree step
>> + = fold_binary_to_constant (MINUS_EXPR, step_type, iv0->step, iv1->step);
>> +
>> + niter->assumptions = extra_iv_chase_assumption (iv0, iv1, step, code);
>> + if (integer_zerop (niter->assumptions))
>> return false;
>>
>> iv0->step = step;
>> - if (!POINTER_TYPE_P (type))
>> - iv0->no_overflow = false;
>> -
>> iv1->step = build_int_cst (step_type, 0);
>> - iv1->no_overflow = true;
>> }
>>
>> /* If the result of the comparison is a constant, the loop is weird.
>> More
>> diff --git a/gcc/testsuite/gcc.c-torture/execute/pr100740.c
>> b/gcc/testsuite/gcc.c-torture/execute/pr100740.c
>> new file mode 100644
>> index 00000000000..8fcdaffef3b
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.c-torture/execute/pr100740.c
>> @@ -0,0 +1,11 @@
>> +/* PR tree-optimization/100740 */
>> +
>> +unsigned a, b;
>> +int main() {
>> + unsigned c = 0;
>> + for (a = 0; a < 2; a++)
>> + for (b = 0; b < 2; b++)
>> + if (++c < a)
>> + __builtin_abort ();
>> + return 0;
>> +}
>> diff --git a/gcc/testsuite/gcc.dg/vect/pr102131.c
>> b/gcc/testsuite/gcc.dg/vect/pr102131.c
>> new file mode 100644
>> index 00000000000..23975cfeadb
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.dg/vect/pr102131.c
>> @@ -0,0 +1,47 @@
>> +/* { dg-require-effective-target vect_int } */
>> +/* { dg-additional-options "-O3" } */
>> +#define MAX ((unsigned int) 0xffffffff)
>> +#define MIN ((unsigned int) (0))
>> +
>> +int arr[512];
>> +
>> +#define FUNC(NAME, CODE, S0, S1)
>> \
>> + unsigned __attribute__ ((noinline)) NAME (unsigned int b0, unsigned int
>> b1) \
>> + {
>> \
>> + unsigned int n = 0;
>> \
>> + unsigned int i0, i1;
>> \
>> + int *p = arr;
>> \
>> + for (i0 = b0, i1 = b1; i0 CODE i1; i0 += S0, i1 += S1)
>> \
>> + {
>> \
>> + n++; \
>> + *p++ = i0 + i1; \
>> + }
>> \
>> + return n;
>> \
>> + }
>> +
>> +FUNC (lt_5_1, <, 5, 1);
>> +FUNC (le_1_m5, <=, 1, -5);
>> +FUNC (lt_1_10, <, 1, 10);
>> +
>> +int
>> +main ()
>> +{
>> + int fail = 0;
>> + if (lt_5_1 (MAX - 124, MAX - 27) != 28)
>> + fail++;
>> +
>> + /* to save time, do not run this. */
>> + /*
>> + if (le_1_m5 (MIN + 1, MIN + 9) != 715827885)
>> + fail++; */
>> +
>> + if (lt_1_10 (MAX - 1000, MAX - 500) != 51)
>> + fail++;
>> +
>> + if (fail)
>> + __builtin_abort ();
>> +
>> + return 0;
>> +}
>> +
>> +/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 2 "vect" } } */
