> -----Original Message-----
> From: Jeff Law [mailto:l...@redhat.com]
> Sent: Thursday, October 10, 2013 11:05 AM
> To: Zhenqiang Chen; gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH] Reassociate X == CST1 || X == CST2 if popcount (CST2
-
> CST1) == 1 into ((X - CST1) & ~(CST2 - CST1)) == 0
>
> On 08/05/13 02:08, Zhenqiang Chen wrote:
> > Hi
> >
> > The patch reassociates X == CST1 || X == CST2 if popcount (CST2 -
> > CST1) == 1 into ((X - CST1) & ~(CST2 - CST1)) == 0.
> >
> > Bootstrap on x86-64 and ARM chromebook.
> > No make check regression on x86-64 and panda board.
> >
> > For some targets/options, the "(X == CST1 || X == CST2)" might be
> > converted to "if (x == CST1) else if (x == CST2)" at the beginning. In
> > such case, the patch will not be executed. It is hard to write a
> > reliable testcase to detect the transformation. So the testcase does
> > not check the dump logs from
> > reassoc1 pass. It just checks the runtime result.
> >
> > Is it OK for trunk?
> >
> > Thanks!
> > -Zhenqiang
> >
> > ChangeLog
> > 2013-08-05 Zhenqiang Chen <zhenqiang.c...@arm.com>
> >
> > * tree-ssa-reassoc.c (optimize_range_tests): Reasociate
> > X == CST1 || X == CST2 if popcount (CST2 - CST1) == 1 into
> > ((X - CST1) & ~(CST2 - CST1)) == 0.
> >
> > testsuite/ChangeLog
> > 2013-08-05 Zhenqiang Chen <zhenqiang.c...@arm.com>
> >
> > * gcc.dg/reassoc1.c: New testcase.
> This is an interesting transformation. I suspect we'd want to gate it on
> something like BRANCH_COST. For a related example, see how we handle
> (a != 0) || (b != 0) -> (a | b) != 0 in fold-const.c. I suspect the
conditions
> under which we want to do your transformation are going to be similar if
not
> the same as those transformations in fold-const.c.
Thank you for the comments.
I will update the patch to check BRANCH_COST.
> Note I've been suggesting the bits I'm referring to in fold-const.c move
out
> into the tree-ssa optimizers. If they fit well into tree-ssa-reassoc.c
I'd look
> favorably upon a patch which moved them.
The code is similar with the code (in tree-ssa-reassoc.c) for
Optimize X == CST1 || X == CST2
if popcount (CST1 ^ CST2) == 1 into
(X & ~(CST1 ^ CST2)) == (CST1 & ~(CST1 ^ CST2))
> As far as testing, the way to go will be to look at the reassoc1 dumps,
but skip
> the test on targets with the "wrong" branch cost.
> tree-ssa/vrp87.c has a suitable condition to filter out the test on
targets were
> the branch cost is too small.
I will update the test case.
> Out of curiosity, what prompted you to make this transformation? Was it
> mostly to deal with a codesize problem or is it a significant win on some
hot
> code in a benchmark, or something else? Closely related, have you done
> anything like instrument the transformation to see how often it applies
> during a GCC bootstrap?
It comes from Coremark. The code is:
if (NEXT_SYMBOL == '+' || NEXT_SYMBOL == '-')
For ARM, there are three instructions rather than 4 (in thumb state).
For some older gcc, I got performance improvement on ARM chromebook. But I
can not reproduce the performance gain now.
I will collect logs during GCC bootstrap.
Thanks!
-Zhenqiang
