[Bug target/30315] optimize unsigned-add overflow test on x86 to use cpu flags from addl

["scovich at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #1 from scovich at gmail dot com  2007-06-06 03:39 -------
Happens on x86_64-unknown-linux-gnu as well, for both 4.2.0 and 4.3 (20070605)

The problem is even worse for 128-bit arithmetic because it has to check two
registers (with associated branches) before making a decision. This in spite of
the fact that sbb sets the flags properly AFAIK:

bool sub128(__uint128_t &dest, __uint128_t a, __uint128_t b) {
  dest = a - b;
  if(dest > a) abort();
}

_Z6sub128Rooo:
.LFB557:
        movq    %rsi, %rax
        movq    %rdx, %r10
        pushq   %rbx
.LCFI0:
        subq    %rcx, %rax
        sbbq    %r8, %rdx
        movq    %rax, (%rdi)
        cmpq    %rdx, %r10
        movq    %rdx, 8(%rdi)
        ja      .L23
        jae     .L24
.L21:
        call    abort
        .p2align 4,,7
.L24:
        cmpq    %rax, %rsi
        .p2align 4,,6
        jb      .L21
        .p2align 4,,7
.L23:
        popq    %rbx
        .p2align 4,,5
        ret

There's not really a way to work around it with inline asm, either, because of
the branch on overflow that will most likely come right afterward...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30315