[llvm-bugs] [Bug 32314] New: Miscompile in GVN due to bug in BasisAA
llvm-bugs at lists.llvm.org
Thu Mar 16 09:31:03 PDT 2017
Bug ID: 32314
Summary: Miscompile in GVN due to bug in BasisAA
OS: Windows NT
Component: Global Analyses
Assignee: unassignedbugs at nondot.org
Reporter: craig.topper at gmail.com
CC: llvm-bugs at lists.llvm.org
Created attachment 18113
IR just before GVN runs.
In the attached IR, GVN removes the following load
%6 = load i32, i32* %p.017, align 4, !tbaa !1
as it believes it can get the value from the previous iteration of the loop.
What it fails to realize is that this store
store i32 50, i32* %arrayidx, align 4, !tbaa !1
aliases the load.
This is because during BasicAA we go through this PHI feeding the load
%p.017 = phi i32* [ %r, %entry ], [ %arrayidx3, %for.body ]
to find this GEP
%arrayidx3 = getelementptr inbounds [3 x i32], [3 x i32]* %a, i64 0, i64
then compare it to this GEP which feeds the store
%arrayidx = getelementptr inbounds [3 x i32], [3 x i32]* %a, i64 0, i64 %4
Then we ask ValueTracking if %4 is known non-equal to %indvars.iv.
ValueTracking sees that %4 is equal to %indvars.iv - 1, so returns that they
are known non-equal and BasicAA returns NoAlias. But this is wrong because we
crossed a PHI to get to the GEP using %indvars.i so we're really looking at
%indvars.iv from two different loop iterations.
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