Missed optimization

missed-optimization.txt

richard@deodand:~$ cat t.c
unsigned long f();

unsigned long h() {
  unsigned long x = f();
  return x == (unsigned long)-1 ? 0 : x + 1;
}
richard@deodand:~$ gcc -O3 -S t.c && cat t.s
        .file   "t.c"
        .text
        .p2align 4,,15
        .globl  h
        .type   h, @function
h:
.LFB0:
        .cfi_startproc
        subq    $8, %rsp
        .cfi_def_cfa_offset 16
        xorl    %eax, %eax
        call    f@PLT
        leaq    1(%rax), %rdx
        cmpq    $-1, %rax
        movl    $0, %eax
        cmovne  %rdx, %rax
        addq    $8, %rsp
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc
.LFE0:
        .size   h, .-h
        .ident  "GCC: (Debian 6.3.0-14) 6.3.0 20170415"
        .section        .note.GNU-stack,"",@progbits
richard@deodand:~$ clang-4.0 -O3 -S t.c && cat t.s
        .text
        .file   "t.c"
        .globl  h
        .p2align        4, 0x90
        .type   h,@function
h:                                      # @h
        .cfi_startproc
# BB#0:
        pushq   %rax
.Lcfi0:
        .cfi_def_cfa_offset 16
        xorl    %eax, %eax
        callq   f
        incq    %rax
        popq    %rcx
        retq
.Lfunc_end0:
        .size   h, .Lfunc_end0-h
        .cfi_endproc


        .ident  "clang version 4.0.0-3 (tags/RELEASE_400/rc1)"
        .section        ".note.GNU-stack","",@progbits

Why would you write that code? Because the natural way of doing it triggers a false positive from a sanitizer in the x=(unsigned long)-1 case.