[SCCP] Resolve shifts beyond the bitwidth to undef

Shifts beyond the bitwidth are undef but SCCP resolved them to zero.
Instead, DTRT and resolve them to undef.

This reimplements the transform which caused PR27712.

llvm-svn: 269269

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
index 75414ef3864..c75c948329c 100644
--- a/llvm/lib/Transforms/Scalar/SCCP.cpp
+++ b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -1416,6 +1416,14 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
         // X >>a undef -> undef.
         if (Op1LV.isUndefined()) break;
 
+        // Shifting by the bitwidth or more is undefined.
+        if (Op1LV.isConstant()) {
+          auto *ShiftAmt = Op1LV.getConstantInt();
+          if (ShiftAmt->getLimitedValue() >=
+              ShiftAmt->getType()->getScalarSizeInBits())
+            break;
+        }
+
         // undef >>a X -> all ones
         markForcedConstant(&I, Constant::getAllOnesValue(ITy));
         return true;
@@ -1425,6 +1433,14 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
         // X >> undef -> undef.
         if (Op1LV.isUndefined()) break;
 
+        // Shifting by the bitwidth or more is undefined.
+        if (Op1LV.isConstant()) {
+          auto *ShiftAmt = Op1LV.getConstantInt();
+          if (ShiftAmt->getLimitedValue() >=
+              ShiftAmt->getType()->getScalarSizeInBits())
+            break;
+        }
+
         // undef << X -> 0
         // undef >> X -> 0
         markForcedConstant(&I, Constant::getNullValue(ITy));