[Bug middle-end/81376] New: unnecessary cast before comparison

["drepper.fsp+rhbz at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81376

            Bug ID: 81376
           Summary: unnecessary cast before comparison
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: drepper.fsp+rhbz at gmail dot com
  Target Milestone: ---

Take the following code:

typedef double c_t;
typedef int a_t;
int f(a_t a1, a_t a2) {
  return (c_t) a1 < (c_t) a2;
}

With IEEE 754 double we have a 52 bits mantissa which is wide enough to
represent all of the 'int' values exactly.  No possibility for imprecision and
especially not for Inf or NaN.

Still gcc (trunk as of today but likely older versions as well) generate the
code for the conversion.  This is for x86-64 with -O2:

f:
        vxorpd  %xmm0, %xmm0, %xmm0
        vxorpd  %xmm1, %xmm1, %xmm1
        xorl    %eax, %eax
        vcvtsi2sd       %edi, %xmm0, %xmm0
        vcvtsi2sd       %esi, %xmm1, %xmm1
        vucomisd        %xmm0, %xmm1
        seta    %al
        ret

A simple

        xorl    %eax, %eax
        cmpl    %esi, %edi
        setl    %al
        ret

is sufficient, just as if c_t above would be defined as 'int'.