[Bug target/88402] inefficient code generation for mask from CC

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88402

--- Comment #3 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
However, this may be worthwhile when one of operands is an immediate, as in
that case there's no register pressure increase, and the xor just mutates the
immediate.

Essentially, we'd change e.g.

  (signed)a < 0xabcd ? -1ul : 0

to

  (a^0x80000000) < 0x8000abcd ? -1ul : 0

and emit a xorl/cmpl/sbbq sequence, plus a mov if a remains live.

Not for 64-bit operands though, where the 64-bits immediates would be costly.

But unfortunately today we don't manage to use the cmp-sbb trick for unsigned
comparison against an immediate, i.e. for

unsigned long baz (unsigned int a)
{
  return a < 123 ? -1ul : 0;
}

we emit

        xorl    %eax, %eax
        cmpl    $122, %edi
        setbe   %al
        negq    %rax