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From: "aldot at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/110052] useless local variable not optimized away
Date: Wed, 31 May 2023 18:12:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110052

--- Comment #3 from Bernhard Reutner-Fischer <aldot at gcc dot gnu.org> ---
Note that in this particular case myrealloc() is static, maybe i should have
omitted the noipa attribute for it was only meant to simplify analysis and
there is no such attribute in the original code.
Furthermore, i would have hoped that given the placement of the definition =
of
myrealloc() before the typedef foo_t, it should be unlikely that the usage =
of
*foo would invalidate such an optimisation _in this particular case_. While=
 the
original code unfortunately does not mark *foo as restrict, adding restrict
would hopefully make it clear that myrealloc() has no business in comparing
foo's struct members, i'd hope?

When i drop the noipa attribute in the v1 testcase to make it similar to the
motivating case, i get:
    9: 0000000000000000    44 FUNC    GLOBAL DEFAULT    4 bloat
   12: 0000000000000000    38 FUNC    GLOBAL DEFAULT    6 ok


0000000000000000 <bloat>:
   0:   53                      push   %rbx
   1:   48 83 7f 08 00          cmpq   $0x0,0x8(%rdi)
   6:   48 89 fb                mov    %rdi,%rbx
   9:   8b 47 14                mov    0x14(%rdi),%eax
   c:   75 19                   jne    27 <bloat+0x27>
   e:   bf d0 01 00 00          mov    $0x1d0,%edi
  13:   e8 00 00 00 00          callq  18 <bloat+0x18>
  18:   48 85 c0                test   %rax,%rax
  1b:   75 05                   jne    22 <bloat+0x22>
  1d:   e8 00 00 00 00          callq  22 <bloat+0x22>
  22:   b8 d0 01 00 00          mov    $0x1d0,%eax
  27:   89 43 14                mov    %eax,0x14(%rbx)
  2a:   5b                      pop    %rbx
  2b:   c3                      retq=20=20=20

Disassembly of section .text.ok:

0000000000000000 <ok>:
   0:   48 83 7f 08 00          cmpq   $0x0,0x8(%rdi)
   5:   75 1e                   jne    25 <ok+0x25>
   7:   52                      push   %rdx
   8:   c7 47 14 d0 01 00 00    movl   $0x1d0,0x14(%rdi)
   f:   bf d0 01 00 00          mov    $0x1d0,%edi
  14:   e8 00 00 00 00          callq  19 <ok+0x19>
  19:   48 85 c0                test   %rax,%rax
  1c:   75 05                   jne    23 <ok+0x23>
  1e:   e8 00 00 00 00          callq  23 <ok+0x23>
  23:   58                      pop    %rax
  24:   c3                      retq=20=20=20
  25:   c3                      retq=