[Bug ipa/99862] [meta-issue] various missed optimizations for dead code elimination

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99862

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Zhendong Su from comment #0)
> [561] % 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 20210401 (experimental) [master revision
> e4bb1bd60a9:c23a685bf70:95d217ab52d31dc06fda42fc136dea165909e88b] (GCC) 
> [562] % 
> [562] % gcctk -O1 -S -o O1.s small.c
> [563] % gcctk -O3 -S -o O3.s small.c
> [564] % 
> [564] % wc O1.s O3.s
>   23   45  420 O1.s
>   39   74  669 O3.s
>   62  119 1089 total
> [565] % 
> [565] % grep foo O1.s
> [566] % grep foo O3.s
>         call    foo
> [567] % 
> [567] % cat small.c
> extern void foo(void);
> static int a, b;
> static int c() {
>   foo();
>   while (1)
>     while (b)
>       foo();
> }
> void d() {
>   if (a)
>     c();
> }
> int main() {
>   d();
>   return 0;
> }

I filed this as PR99991.