[Bug other/66112] New: __builtin_mul_overflow for int16_t emits poor code

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66112

            Bug ID: 66112
           Summary: __builtin_mul_overflow for int16_t emits poor code
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc.hall at gmail dot com
  Target Milestone: ---

I am using all the three signed integer overflow builtin's for the four 
types: int8_t, int16_t, int32_t and int64_t on Intel x64.  In all cases except
one this produces optimal code, the operation followed by "jo".  See the test
case below.
For the int32_t type, from  the last atoi down:

    call    atoi    #
    imull   %eax, %ebx  # b, tmp100
    jo  .L3 #,

However, for int16_t this produces much larger code that does a 32 bit multiply
and checks for overflow by hand:

    call    atoi    #
    movswl  %bx, %esi   # D.5222, D.5222
    cwtl
    imull   %esi, %eax  # D.5222, tmp109
    movl    %eax, %edx  # tmp109, tmp110
    movl    %eax, %ecx  # tmp109, tmp111
    sarl    $16, %edx   #, tmp110
    sarw    $15, %cx    #, tmp111
    cmpw    %dx, %cx    # tmp110, tmp111
    jne .L7 #,

Even though a simple "imulw" followed by "jo" would work.

-------------------------------------------------------------------
#include <stdio.h>
#include <inttypes.h>
int
main( int argc, const char *argv[] )
{
  int16_t result, a = (int16_t)atoi(argv[1]), b = (int16_t)atoi( argv[2] );
  if( __builtin_mul_overflow( a, b, &result ) )
    printf( "Overflow\n");
 else
    printf( "Product is %d\n", result );
}