https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63986
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |NEW
CC| |ppalka at gcc dot gnu.org,
| |rguenth at gcc dot gnu.org
Assignee|rguenth at gcc dot gnu.org |unassigned at gcc dot
gnu.org
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ok, now already existing forwprop code gets fed with
<bb 2>:
_3 = a_2(D) == 0;
x_4 = (char) _3;
_7 = ~_3;
_8 = (int) _7;
MEM[(int *)d_5(D) + 8B] = _8;
if (x_4 != 0)
where we now in the first forwprop pass identified the opportunity
to use ~_3 instead of x_4 == 0 thus x_4 is now no longer multi-use.
This makes us optimize if (x_4 != 0) to if (_3 != 0) which we
re-optimize in fold_gimple_cond now to '_3' and then of course
if (_3 != 0) (err, and we return "changed"....) which means we
now propagate _again_ via forward_propagate_into_gimple_cond
which now specifically allows aggressive forwarding of compares,
bypassing single-use restrictions. See
2014-11-16 Patrick Palka <ppa...@gcc.gnu.org>
PR middle-end/63790
* tree-ssa-forwprop.c (forward_propagate_into_comparison_1):
Always combine comparisons or conversions from booleans.
thus me fixing my "mistake" does not help anymore.
I suppose RTL CSE cannot CSE flag register sets...?
Btw, my previous comment was incorrect - the code is what is now produced
on trunk while on the 4.9 branch we create
test_0:
.LFB0:
.cfi_startproc
testl %edi, %edi
movl %edx, %eax
sete %r8b
movl %r8d, %edi
xorl $1, %edi
testb %r8b, %r8b
movzbl %dil, %edi
cmovne %esi, %eax
movl %edi, 8(%rcx)
ret
which means code generation improved for x86...
