http://llvm.org/bugs/show_bug.cgi?id=12740
Bug #: 12740
Summary: BasicAliasAnalysis incorrectly concludes "NoAlias"
Product: libraries
Version: 3.0
Platform: All
OS/Version: All
Status: NEW
Severity: normal
Priority: P
Component: Global Analyses
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 8494
--> http://llvm.org/bugs/attachment.cgi?id=8494
Simple loop IR which basicaa incorrectly analyzes
After moving from 2.9 to 3.0 we noticed that -basicaa was giving us some false
"NoAlias" responses, even in very simple cases.
For example, in IR resulting from a C loop such as this:
for(i=0; i<100; i++){ A[i] = A[i] + A[i+1] }
-basicaa in 3.0 will say that A[i] and A[i+1] do not alias. While this is true
in the context of a single iteration, it is not true in general.
I have attached the IR for this loop. You can see the bad AA response with:
$ opt -disable-output -basicaa -count-aa -aa-eval t.ll
Relevant output:
No alias: [8B] double* %4, [8B] double* %2
After stepping through BasicAliasAnalysis, it looks like there is a test (+942)
for a partial alias which fails to "NoAlias" when this is not necessarily nor
usually correct.
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