[Bug c/43406] New: __builtin_popcountll fails with -O0 and -mpopcnt

[rbarreira at gmail dot com]

Sample code:

#include 

int main (void)
{
long long a = 0xLL; // 48 bits set
int popcount;

#if 1
popcount = __builtin_popcountll (a);
#else
popcount = __popcountdi2 (a);
#endif

printf ("%llx popcount = %d\n", a, popcount);
return 0;
}

If -mpopcnt is enabled, this code only outputs the correct value (48) when -O3
is on (apparently it's calculating it at compile time). Without optimizations,
it is apparently only counting the bits in the lower dword of the long long
variable.

OTOH, If __popcountdi2 is used, it works correctly (but according to the
assembly code it's not really using the popcnt instruction which means it's
much slower).


Test runs and output follow (note you need a CPU which supports the popcnt
instruction):

=> gcc popcnt.c -o popcnt -Wall -O0 -mpopcnt && ./popcnt
 popcount = 32
=> gcc popcnt.c -o popcnt -Wall -O3 -mpopcnt && ./popcnt
 popcount = 48
=> gcc popcnt.c -o popcnt -Wall -O0 && ./popcnt
 popcount = 48
=> gcc popcnt.c -o popcnt -Wall -O3 && ./popcnt
 popcount = 48


-- 
   Summary: __builtin_popcountll fails with -O0 and -mpopcnt
   Product: gcc
   Version: 4.1.2
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
    AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: rbarreira at gmail dot com
 GCC build triplet: i586-suse-linux
  GCC host triplet: i586-suse-linux
GCC target triplet: i586-suse-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43406

[Bug c/43406] __builtin_popcountll fails with -O0 and -mpopcnt

[rbarreira at gmail dot com]

--- Comment #1 from rbarreira at gmail dot com  2010-03-17 16:35 ---
Note that if you use scanf ("%llx", &a) and input "" instead of
having a hardcoded value for a, the bug happens both with -O3 and -O0.

To sum up it seems that when the popcnt instruction is actually issued, it's
using a 32-bit instruction instead of 64-bit.


-- 

rbarreira at gmail dot com changed:

   What|Removed |Added

Summary|__builtin_popcountll fails  |__builtin_popcountll fails
   |with -O0 and -mpopcnt   |with -O0 and -mpopcnt


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43406