[Bug target/42497] New: GCC can do less work in the frequently executed path

["carrot at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Compile following code with options -march=armv5te -O2

extern void *memcpy(void *dst, const void *src, int n);

void *memmove(void *dst, const void *src, int n)
{
   const char *p = src;
   char *q = dst;
   if (__builtin_expect(q < p, 1)) {
          return memcpy(dst, src, n);
   } else {
          int i=0;
          for (; i<n; i++)
               q[i] = p[i];
   }
   return dst;
}


gcc generates:

memmove:
        cmp     r1, r0              
        str     r4, [sp, #-4]!
        mov     r3, r0
        mov     ip, r1
        mov     r4, r2
        bls     .L8
        ldmfd   sp!, {r4}
        b       memcpy
.L8:
        cmp     r2, #0
        movgt   r2, #0
        ble     .L4
.L5:
        ldrb    r1, [ip, r2]    @ zero_extendqisi2
        strb    r1, [r3, r2]
        add     r2, r2, #1
        cmp     r2, r4
        bne     .L5
.L4:
        mov     r0, r3
        ldmfd   sp!, {r4}
        bx      lr

The if block is expected to be more frequent than the else block, but the
generated code is not very efficient. Better code could be:

        cmp     r1, r0              
        bhi     memcpy
        str     r4, [sp, #-4]!
        mov     r3, r0
        mov     ip, r1
        mov     r4, r2
L8:
        ...


-- 
           Summary: GCC can do less work in the frequently executed path
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42497