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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/103674] New: Poor codegen for C++ casts
Date: Sun, 12 Dec 2021 20:10:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103674

            Bug ID: 103674
           Summary: Poor codegen for C++ casts
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hubicka at gcc dot gnu.org
  Target Milestone: ---

Compiling:

struct a {int a;};
struct b {int b;};
struct c : a, b {};
int
test(struct c *c)
{
        struct b *b=3Dc;
        return b->b;
}

with -O2 leads to:

        .text
.LHOTB0:
        .p2align 4
        .globl  _Z4testP1c
        .type   _Z4testP1c, @function
_Z4testP1c:
.LFB0:
        .cfi_startproc
        testq   %rdi, %rdi
        je      .L2
        movl    4(%rdi), %eax
        ret
        .cfi_endproc
        .section        .text.unlikely
        .cfi_startproc
        .type   _Z4testP1c.cold, @function
_Z4testP1c.cold:
.LFSB0:
.L2:
        movl    0, %eax
        ud2
        .cfi_endproc
.LFE0:


while clang does:
_Z4testP1c:                             # @_Z4testP1c
        .cfi_startproc
# %bb.0:
        leaq    4(%rdi), %rax
        testq   %rdi, %rdi
        cmoveq  %rdi, %rax
        movl    (%rax), %eax
        retq

And so does gcc 4.8.5:

test(c*):
        leaq    4(%rdi), %rdx
        xorl    %eax, %eax
        testq   %rdi, %rdi
        cmovne  %rdx, %rax
        movl    (%rax), %eax
        ret

So this is technically a regression.=