Description of problem:

I used __builtin_clz() to implement a roundup_pwr2() function. I do get three different results when changing optimization level from zero to 2. Neither of those are correct.

Also tried this on fc22, again, three different results, but none correct.

Of the six combinations O0..O2, and 6.6 -> fc22, I get 5 different results.

Running on SPARC64 with gcc 4.5.2, all three optimization levels return the correct result.

Version-Release number of selected component (if applicable):

On 6.6, gcc 4.4.7

On fc22, gcc 5.1.1

On SPARC64 (which works correctly), gcc 4.5.2. 

How reproducible:

Program:
-----------------------
#include <stdio.h>

unsigned roundup_pwr2(unsigned x) {
  int leading_zero = __builtin_clz((x ? x - 1 : 0));

  return 1UL << (32UL - leading_zero);
}

int main () {
  unsigned n;

  for (n = 0; n < 10; ++n) {
    printf("0x%08x %2d %2d 0x%08x\n", n, __builtin_clz(n), (n ? n - 1 : 0), roundup_pwr2(n));
  }

  return 0;
}
--------------------------

Steps to Reproduce:
1.

(change gcc version and architecture as appropriate in the output file).

for O in 0 1 2; do gcc -Wall -Wextra -O$O  roundup_pwr2.c; ./a.out > gcc_4.4.7_x86_64_bug_O${O}.txt; done

2.

md5sum gcc*txt


3.

Actual results:

gcc 4.4.7_x86_64:

0b973118faeb09f7a3af99b7df9b19e1  gcc_4.4.7_x86_64_bug_O0.txt
e8c2515621198862b49be891f95d1c26  gcc_4.4.7_x86_64_bug_O1.txt
4baff5266b4593189591133962913c72  gcc_4.4.7_x86_64_bug_O2.txt

gcc 5.1.1_x86_64 (fc22):

0b973118faeb09f7a3af99b7df9b19e1  gcc_5.1.1_x86_64_bug_O0.txt
158b9254067ef5ca35f870fb485ced66  gcc_5.1.1_x86_64_bug_O1.txt
111d0183e49181a3556c3907eb795851  gcc_5.1.1_x86_64_bug_O2.txt


Expected results:

gcc 4.5.2_SPARC64 (SunOS 5.11):

0ff9225a380dc978cd6aa30a02ef922f  gcc_4.5.2_SPARC64_bug_O0.txt
0ff9225a380dc978cd6aa30a02ef922f  gcc_4.5.2_SPARC64_bug_O1.txt
0ff9225a380dc978cd6aa30a02ef922f  gcc_4.5.2_SPARC64_bug_O2.txt

Additional info: