[BasicAA] Fix linearization of shifts beyond the bitwidth.
Thanks to Simon Pilgrim for the reduced testcase.
Fixes PR35821.
llvm-svn: 321873
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 537813b..5a2401f 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -285,6 +285,19 @@
case Instruction::Shl:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
+
+ // We're trying to linearize an expression of the kind:
+ // shl i8 -128, 36
+ // where the shift count exceeds the bitwidth of the type.
+ // We can't decompose this further (the expression would return
+ // a poison value).
+ if (Offset.getBitWidth() < RHS.getLimitedValue() ||
+ Scale.getBitWidth() < RHS.getLimitedValue()) {
+ Scale = 1;
+ Offset = 0;
+ return V;
+ }
+
Offset <<= RHS.getLimitedValue();
Scale <<= RHS.getLimitedValue();
// the semantics of nsw and nuw for left shifts don't match those of