http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48316
Summary: missed CSE / reassoc with array offsets
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: rgue...@gcc.gnu.org
int
foo (int *p, int i)
{
int i1 = i + 1;
int i2 = i + 2;
return p[i1] + p[i2];
}
int
bar (int *p, unsigned long i)
{
unsigned long i1 = i + 1;
unsigned long i2 = i + 2;
return p[i1] + p[i2];
}
For both testcases (the latter being the more "optimal" input due to
pointer-plus-expr constraints) we miss to CSE the multiplication
of i by 4 which makes the memory references not trivially independent
(based on the same pointer, offsetted by different constants). Such
a case causes vectorization for alias checks being inserted for
gfortran.dg/reassoc_4.f with --param max-completely-peeled-insns=4000
IL on x86_64 is
<bb 2>:
i1_2 = i_1(D) + 1;
i2_3 = i_1(D) + 2;
D.2702_4 = (long unsigned int) i1_2;
D.2703_5 = D.2702_4 * 4;
D.2704_7 = p_6(D) + D.2703_5;
D.2705_8 = MEM[(int *)D.2704_7];
D.2706_9 = (long unsigned int) i2_3;
D.2707_10 = D.2706_9 * 4;
D.2708_11 = p_6(D) + D.2707_10;
D.2709_12 = MEM[(int *)D.2708_11];
D.2701_13 = D.2705_8 + D.2709_12;
return D.2701_13;
vs.
<bb 2>:
i1_2 = i_1(D) + 1;
i2_3 = i_1(D) + 2;
D.2694_4 = i1_2 * 4;
D.2695_6 = p_5(D) + D.2694_4;
D.2696_7 = MEM[(int *)D.2695_6];
D.2697_8 = i2_3 * 4;
D.2698_9 = p_5(D) + D.2697_8;
D.2699_10 = MEM[(int *)D.2698_9];
D.2693_11 = D.2696_7 + D.2699_10;
return D.2693_11;
For the reassoc_4.f testcase the question is whether either SCEV or
data-dependence can be enhanced to handle the cases (the multiplications
are in BB2, outside of any loop).
