https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95622
Thomas Schwinge <tschwinge at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|force_output flag on a |[11 Regression]
|variable prevents |force_output flag on a
|optimization / regresses |variable prevents
|c-c++-common/goacc/kernels- |optimization / regresses
|alias-ipa-pta-2.c |c-c++-common/goacc/kernels-
| |alias-ipa-pta{-2,-4,}.c
CC| |tschwinge at gcc dot gnu.org
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
Last reconfirmed| |2020-06-12
--- Comment #2 from Thomas Schwinge <tschwinge at gcc dot gnu.org> ---
(In reply to Tobias Burnus from comment #0)
> The following
Specifically:
PASS: c-c++-common/goacc/kernels-alias-ipa-pta-2.c (test for excess errors)
[-PASS:-]{+FAIL:+} c-c++-common/goacc/kernels-alias-ipa-pta-2.c
scan-tree-dump-times optimized "(?n)= 0;$" 2
PASS: c-c++-common/goacc/kernels-alias-ipa-pta-2.c scan-tree-dump-times
optimized "(?n)= 1;$" 1
PASS: c-c++-common/goacc/kernels-alias-ipa-pta-2.c scan-tree-dump-times
optimized "(?n)= \\*a" 0
PASS: c-c++-common/goacc/kernels-alias-ipa-pta-4.c (test for excess errors)
[-PASS:-]{+FAIL:+} c-c++-common/goacc/kernels-alias-ipa-pta-4.c
scan-tree-dump-times optimized "(?n)= 0;$" 2
PASS: c-c++-common/goacc/kernels-alias-ipa-pta-4.c scan-tree-dump-times
optimized "(?n)= 1;$" 1
[-PASS:-]{+FAIL:+} c-c++-common/goacc/kernels-alias-ipa-pta-4.c
scan-tree-dump-times optimized "(?n)= \\*_[0-9];$" 0
PASS: c-c++-common/goacc/kernels-alias-ipa-pta.c (test for excess errors)
[-PASS:-]{+FAIL:+} c-c++-common/goacc/kernels-alias-ipa-pta.c
scan-tree-dump-times optimized "(?n)= 0;$" 2
PASS: c-c++-common/goacc/kernels-alias-ipa-pta.c scan-tree-dump-times
optimized "(?n)= 1;$" 1
PASS: c-c++-common/goacc/kernels-alias-ipa-pta.c scan-tree-dump-times
optimized "(?n)= \\*_[0-9]\\[0\\];$" 0
Same for C++.
Thus: "[11 Regression] force_output flag on a variable prevents optimization /
regresses c-c++-common/goacc/kernels-alias-ipa-pta{-2,-4,}.c".
We'll need to analyze if we need to restore that functionality now, or if it's
OK to lose that optimization.
> only shows up as FAIL in the testsuite if ENABLE_OFFLOAD is
> true, even though it is only a compile-time test. [Hence, it can make
> sense to not only "make check-target-libgomp" with an offloading compiler
> but also gcc/.]
Somewhat unfortunate, but yes.
The test cases need to be XFAILed conditionally "if ENABLE_OFFLOAD is true".
> Somewhat similar to PR 68773.
>
> The testcase: c-c++-common/goacc/kernels-alias-ipa-pta-2.c
> …
> {
> a[0] = 0;
> b[0] = 1;
> c[0] = a[0];
> }
> …
>
> Without ENABLE_OFFLOAD or before
> r11-1075-g1c0fdaf79e3618fd7512608a2e5c62b6b306e9e8 (for PR94848 + PR95551):
>
> MEM[(unsigned int *)_5] = 0; // a[0] = 0
> MEM[(unsigned int *)_4] = 1; // b[0] = 1
> MEM[(unsigned int *)_3] = 0; // c[0] = a[0]
>
> With r11-1075 + offloading compiler, force_output is on offloading variables
> (to ensure they are not optimized away), the last line becomes
>
> _8 = MEM[(unsigned int *)_5]; // _8 = a[0]
> MEM[(unsigned int *)_3] = _8; // c[0] = _8
(... as is visible from the 'kernels-alias-ipa-pta-2.c.104t.fre3' dump
onwards.)
> Expected: force_output does not affect the optimization. (Especially as "a"
> cannot be optimized away as it is also used for "copyout".)
>
> [A very well optimizing compiler could see that nothing uses a/b/c after the
> target section and could replace the whole function body by "{ }"…]
That's PR90591 etc.?
