http://llvm.org/bugs/show_bug.cgi?id=13564
Bug #: 13564
Summary: Extend BasicAliasAnalysis to recognize cyclic NoAlias
phis in loops
Product: tools
Version: trunk
Platform: PC
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P
Component: opt
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 9022
--> http://llvm.org/bugs/attachment.cgi?id=9022
Patch
BasicAliasAnalysis should recognize phis that have incoming
values from outside the loop that are known to not alias and whose other
incoming values do not change this fact because they perform congruent
operations.
Example: We want to prove that ptr_phi and ptr_phi2 do not alias each other.
bb:
ptr = ptr2 + 1
loop:
ptr_phi = phi [bb, ptr], [loop, ptr_plus_one]
ptr2_phi = phi [bb, ptr2], [loop, ptr2_plus_one]
...
ptr_plus_one = gep ptr_phi, 1
ptr2_plus_one = gep ptr2_phi, 1
This would enable the elimination of one load in code like the following:
extern int foo;
int test_noalias(int *ptr, int num, int* coeff) {
int *ptr2 = ptr;
int result = (*ptr++) * (*coeff--);
while (num--) {
*ptr2++ = *ptr;
result += (*coeff--) * (*ptr++);
}
*ptr = foo;
return result;
}
Currently, without the improvement to basic alias analysis we generate the
following code for the loop:
The loads from %r9 are the accesses to "ptr".
.LBB0_2: # %while.body
# =>This Inner Loop Header: Depth=1
movl (%r9), %ecx
movl %ecx, -4(%r9)
movl (%r9), %ecx
imull (%rdx), %ecx
addl %ecx, %eax
addq $-4, %rdx
addq $4, %r9
decl %esi
jne .LBB0_2
With the improvement we would generate:
.LBB0_2: # %while.body
# =>This Inner Loop Header: Depth=1
movl (%r9), %ecx
movl %ecx, -4(%r9)
imull (%rdx), %ecx
addl %ecx, %eax
addq $4, %r9
addq $-4, %rdx
decl %esi
jne .LBB0_2
The attached patch implements this enhancement.
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