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From: "zhendong.su at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug ipa/99419] New: possible missed optimization for dead code
 elimination
Date: Fri, 05 Mar 2021 19:21:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99419

            Bug ID: 99419
           Summary: possible missed optimization for dead code elimination
           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: ---

[552] % gcctk -v
Using built-in specs.
COLLECT_GCC=3Dgcctk
COLLECT_LTO_WRAPPER=3D/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=3D/local/suz-local/software/local/gcc-trunk --enable-languages=3Dc=
,c++
--disable-werror --enable-multilib --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.1 20210305 (experimental) [master revision
9ee91079fd5:70357716fd6:3d9577c254003f2d18185015b75ce6e3e4af9ca2] (GCC)=20
[553] %=20
[553] % gcctk -O3 -S small.c
[554] %=20
[554] % cat small.c
extern void foo(void);

int a;
static int b[2] =3D {0,0};

int main() {
  if (b[a])
    foo();
  return 0;
}
[555] %=20
[555] % cat small.s
        .file   "small.c"
        .text
        .section        .text.startup,"ax",@progbits
        .p2align 4
        .globl  main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        movslq  a(%rip), %rax
        movl    b(,%rax,4), %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
        .section        .rodata
        .align 8
        .type   b, @object
        .size   b, 8
b:
        .zero   8
        .globl  a
        .bss
        .align 4
        .type   a, @object
        .size   a, 4
a:
        .zero   4
        .ident  "GCC: (GNU) 11.0.1 20210305 (experimental) [master revision
9ee91079fd5:70357716fd6:3d9577c254003f2d18185015b75ce6e3e4af9ca2]"
        .section        .note.GNU-stack,"",@progbits=