[Bug tree-optimization/39075] alignment for unsigned short a[10000] vs extern unsigned short a[10000]

2009-02-02 Thread dann at godzilla dot ics dot uci dot edu 


--- Comment #1 from dann at godzilla dot ics dot uci dot edu  2009-02-02 
14:50 ---
This code:
unsigned short a[1];
void test()
{
  int i;
  for (i = 0; i  1; ++i)  a[i] = 5;
}

will be vectorized with -O3 -march=core2 to this:

.L2:
movdqa  %xmm0, a(%eax)
addl$16, %eax
cmpl$2, %eax
jne .L2


but this one:

extern unsigned short a[1];

void test()
{
  int i;
  for (i = 0; i  1; ++i) a[i] = 5;
}

will get a lot of extra code before the loop because the vectorizer thinks it
needs to do peeling for alignment:
test.c:7: note: Alignment of access forced using peeling.

Intel's compiler does not generate the extra peeling code.


-- 

dann at godzilla dot ics dot uci dot edu changed:

   What|Removed |Added

Summary|alignment for unsigned |alignment for unsigned
   |short a[1   |short a[1] vs extern
   ||unsigned short a[1]


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39075

[Bug tree-optimization/39075] alignment for unsigned short a[10000] vs extern unsigned short a[10000]

2009-02-02 Thread rguenth at gcc dot gnu dot org 


--- Comment #2 from rguenth at gcc dot gnu dot org  2009-02-02 14:53 ---
The ABI does not guarantee alignment bigger than 2 for the external array.  The
vectorizer adjusts the alignment for the internal one.


-- 

rguenth at gcc dot gnu dot org changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution||INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39075

[Bug tree-optimization/39075] alignment for unsigned short a[10000] vs extern unsigned short a[10000]

2009-02-02 Thread rguenth at gcc dot gnu dot org 


--- Comment #3 from rguenth at gcc dot gnu dot org  2009-02-02 14:55 ---
Err, it seems at least the x86_64 ABI guarantees alignment of 16 bytes for
arrays bigger than 16 bytes (including variable length arrays).


-- 

rguenth at gcc dot gnu dot org changed:

   What|Removed |Added

   Severity|normal  |enhancement
 Status|RESOLVED|UNCONFIRMED
 Resolution|INVALID |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39075