[Bug middle-end/59521] New: __builtin_expect not effective in switch

drepper.fsp at gmail dot com gcc-bugzilla@gcc.gnu.org
Mon Dec 16 02:27:00 GMT 2013


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59521

            Bug ID: 59521
           Summary: __builtin_expect not effective in switch
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: drepper.fsp at gmail dot com

When used in switch, __builtin_expect should reorder the comparisons
appropriately.  Take this code:

#include <stdio.h>
void
f(int ch) {
  switch (__builtin_expect(ch, 333)) {
    case 3: puts("a"); break;                                                 
    case 42: puts("e"); break;                                                 
    case 333: puts("i"); break;                                                 
    } 
}

Current mainline (and also prior versions, I tested 4.8.2) produce with -O3
code like this:

0000000000000000 <f>:
   0:    83 ff 2a                 cmp    $0x2a,%edi
   3:    74 33                    je     38 <f+0x38>
   5:    81 ff 4d 01 00 00        cmp    $0x14d,%edi
   b:    74 1b                    je     28 <f+0x28>
   d:    83 ff 03                 cmp    $0x3,%edi
  10:    74 06                    je     18 <f+0x18>
  12:    c3                       retq   
  13:    0f 1f 44 00 00           nopl   0x0(%rax,%rax,1)
  18:    bf 00 00 00 00           mov    $0x0,%edi
  1d:    e9 00 00 00 00           jmpq   22 <f+0x22>
  22:    66 0f 1f 44 00 00        nopw   0x0(%rax,%rax,1)
  28:    bf 00 00 00 00           mov    $0x0,%edi
  2d:    e9 00 00 00 00           jmpq   32 <f+0x32>
  32:    66 0f 1f 44 00 00        nopw   0x0(%rax,%rax,1)
  38:    bf 00 00 00 00           mov    $0x0,%edi
  3d:    e9 00 00 00 00           jmpq   42 <f+0x42>

Instead the test for 333/$0x14d should have been moved to the front.



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