[Bug tree-optimization/99428] New: possible missed optimization for dead code elimination

[Bug tree-optimization/99428] New: possible missed optimization for dead code elimination

[zhendong.su at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99428

            Bug ID: 99428
           Summary: possible missed optimization for dead code elimination
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhendong.su at inf dot ethz.ch
  Target Milestone: ---

[604] % 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 20210306 (experimental) [master revision
02a35548790:4487c1830cf:ceae9533826aabaf4c78d173c60e3bedeffc6955] (GCC)
[605] %
[605] % gcctk -O3 -S small.c
[606] %
[606] % cat small.c
extern void foo(void);

static int a;
static int *b = &a;

int main() {
  if (*b)
    foo();
  return 0;
}
[607] %
[607] % cat small.s
        .file   "small.c"
        .text
        .section        .text.startup,"ax",@progbits
        .p2align 4
        .globl  main
        .type   main, @function
main:
.LFB0:  
        .cfi_startproc
        movl    a(%rip), %ecx
        testl   %ecx, %ecx
        jne     .L8
        xorl    %eax, %eax
        ret
.L8:
        pushq   %rax
        .cfi_def_cfa_offset 16
        call    foo
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc
.LFE0:  
        .size   main, .-main
        .local  a
        .comm   a,4,4
        .ident  "GCC: (GNU) 11.0.1 20210306 (experimental) [master revision
02a35548790:4487c1830cf:ceae9533826aabaf4c78d173c60e3bedeffc6955]"
        .section        .note.GNU-stack,"",@progbits

[Bug ipa/99428] possible missed optimization for dead code elimination

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99428

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |ipa
           Keywords|                            |missed-optimization
   Last reconfirmed|                            |2021-03-08
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
            Version|unknown                     |11.0

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
IPA - gimple "phase ordering" issue.  Alternatively when 'b' is discovered
read-only its analysis would need to consider the initializer propagated and
thus eventually not address-taken to make 'a' readonly as well ... (or apply
modref to tell 'a' is not written to?)