[ValueTracking] Improve ComputeNumSignBits to handle Trunc

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D140796

1 files changed, 11 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index b563154..d12e4c7 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3312,10 +3312,17 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
       return Tmp;
     }
 
-    case Instruction::Trunc:
-      // FIXME: it's tricky to do anything useful for this, but it is an
-      // important case for targets like X86.
-      break;
+    case Instruction::Trunc: {
+      // If the input contained enough sign bits that some remain after the
+      // truncation, then we can make use of that. Otherwise we don't know
+      // anything.
+      Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
+      unsigned OperandTyBits = U->getOperand(0)->getType()->getScalarSizeInBits();
+      if (Tmp > (OperandTyBits - TyBits))
+        return Tmp - (OperandTyBits - TyBits);
+
+      return 1;
+    }
 
     case Instruction::ExtractElement:
       // Look through extract element. At the moment we keep this simple and