[Bug tree-optimization/16913] New: [3.5] Regression in alias analysis

["martin at mpa-garching dot mpg dot de" <gcc-bugzilla at gcc dot gnu dot org>]

for the following C code the current mainline produces worse code than
the current 3.4 branch. Mainline doesn't seem to recognize that the pointers
a and b may not alias.

#define N 200

void foo ( float * restrict a, float * restrict b, int n, int j)
  {
  int i;
  for( i=0; i<n; ++i )
    a[i] = (b[j+N/4] + b[j-N/4]) * 0.5f;
  }

Compiling with mainline using -O3 -fstrict-alilasing -std=c99 -S produces
the following assembler for the loop:

.L4:
	flds	(%eax)
	fadds	(%ecx)
	fmul	%st(1), %st
	fstps	(%esi,%edx,4)
	incl	%edx
	cmpl	%edx, %ebx
	jg	.L4


The current 3.4 version produces: 
.L5:
	fsts	(%ecx,%eax,4)
	incl	%eax
	cmpl	%edx, %eax
	jl	.L5

In this case, all floating point computations are hoisted out of the loop.

-- 
           Summary: [3.5] Regression in alias analysis
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: martin at mpa-garching dot mpg dot de
                CC: gcc-bugs at gcc dot gnu dot org,martin at mpa-garching
                    dot mpg dot de
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16913