https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95622
Bug ID: 95622
Summary: force_output flag on a variable prevents optimization
/ regresses
c-c++-common/goacc/kernels-alias-ipa-pta-2.c
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization, openacc, openmp
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: burnus at gcc dot gnu.org
CC: jakub at gcc dot gnu.org
Target Milestone: ---
The following 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 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
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 "{ }"…]
