[Bug ipa/99788] New: missed optimization for dead code elimination at -O3 (vs. -O1)
zhendong.su at inf dot ethz.ch
gcc-bugzilla@gcc.gnu.org
Fri Mar 26 10:44:50 GMT 2021
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99788
Bug ID: 99788
Summary: missed optimization for dead code elimination at -O3
(vs. -O1)
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: ipa
Assignee: unassigned at gcc dot gnu.org
Reporter: zhendong.su at inf dot ethz.ch
CC: marxin at gcc dot gnu.org
Target Milestone: ---
[606] % gcctk -v
Using built-in specs.
COLLECT_GCC=gcctk
COLLECT_LTO_WRAPPER=/local/suz-local/software/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/11.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--prefix=/local/suz-local/software/local/gcc-trunk --enable-languages=c,c++
--disable-werror --enable-multilib --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.1 20210326 (experimental) [master revision
9d45e848d02:ca344bbd24f:6081d8994ed1a0aef6b7f5fb34f091faa3580416] (GCC)
[607] %
[607] % gcctk -O1 -S -o O1.s small.c
[608] % gcctk -O3 -S -o O3.s small.c
[609] %
[609] % wc O1.s O3.s
79 162 986 O1.s
109 229 1433 O3.s
188 391 2419 total
[610] %
[610] % grep foo O1.s
[611] % grep foo O3.s
call foo
[612] %
[612] % cat small.c
extern void foo(void);
char a;
int b, *c, g, h = 1, i = 1, j, *k = &i;
static void d();
static int *e() {
for (a = 1; a; a = a+2)
;
foo();
h = (g % h) % i;
*k = -j;
return 0;
}
static void f() {
if (b)
d(e);
}
void d() {
for (;;)
c = e();
}
int main() {
f();
return 0;
}
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