[SCEV] Be careful with nuw/nsw/exact in InsertBinop

InsertBinop tries to find an appropriate instruction instead of
creating a new instruction. When it checks whether instruction is
the same as we need to create it ignores nuw/nsw/exact flags.

It leads to invalid behavior when poison instruction can be used
when it was not expected. Specifically, for example Expander
expands the SCEV built for instruction
%a = add i32 %v, 1
It is possible that InsertBinop can find an instruction
% b = add nuw nsw i32 %v, 1
and will use it instead of version w/o nuw nsw.
It is incorrect.

The patch conservatively ignores all instructions with any of
poison flags installed.

Reviewers: sanjoy, mkazantsev, sebpop, jbhateja
Reviewed By: sanjoy
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D41576

llvm-svn: 321475

1 files changed, 14 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
index 86f714b930d..3ceda677ba6 100644
--- a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
@@ -187,8 +187,21 @@ Value *SCEVExpander::InsertBinop(Instruction::BinaryOps Opcode,
       // generated code.
       if (isa<DbgInfoIntrinsic>(IP))
         ScanLimit++;
+
+      // Conservatively, do not use any instruction which has any of wrap/exact
+      // flags installed.
+      // TODO: Instead of simply disable poison instructions we can be clever
+      //       here and match SCEV to this instruction.
+      auto canGeneratePoison = [](Instruction *I) {
+        if (isa<OverflowingBinaryOperator>(I) &&
+            (I->hasNoSignedWrap() || I->hasNoUnsignedWrap()))
+          return true;
+        if (isa<PossiblyExactOperator>(I) && I->isExact())
+          return true;
+        return false;
+      };
       if (IP->getOpcode() == (unsigned)Opcode && IP->getOperand(0) == LHS &&
-          IP->getOperand(1) == RHS)
+          IP->getOperand(1) == RHS && !canGeneratePoison(&*IP))
         return &*IP;
       if (IP == BlockBegin) break;
     }