[PATCH] D24542: Make vector operation with variable index unsafe to speculate

[Eli Friedman via llvm-commits]

efriedma added a subscriber: efriedma.
efriedma added a comment.

LangRef says "If idx exceeds the length of val, the results are undefined."  That implies that the operation itself isn't undefined behavior, and therefore it's safe to speculate.

That said, the current implementation of DAGTypeLegalizer::GetVectorElementPointer can lead to undefined behavior... so we need to either fix that to mask the index, or change LangRef.  I would lean towards fixing GetVectorElementPointer.


================
Comment at: lib/Analysis/ValueTracking.cpp:3239
@@ +3238,3 @@
+  case Instruction::ExtractElement:
+    return isa<Constant>(Inst->getOperand(1));
+  case Instruction::InsertElement:
----------------
`isa<Constant>` doesn't prove anything useful.


https://reviews.llvm.org/D24542