Teach computeKnownBits to look through returned-argument functions

If a function is known to return one of its arguments, we can use that in order
to compute known bits of the return value.

Differential Revision: http://reviews.llvm.org/D9397

llvm-svn: 275036

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 5c5d213..3277c3c 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1279,11 +1279,16 @@ static void computeKnownBitsFromOperator(Operator *I, APInt &KnownZero,
   }
   case Instruction::Call:
   case Instruction::Invoke:
+    // If range metadata is attached to this call, set known bits from that,
+    // and then intersect with known bits based on other properties of the
+    // function.
     if (MDNode *MD = cast<Instruction>(I)->getMetadata(LLVMContext::MD_range))
       computeKnownBitsFromRangeMetadata(*MD, KnownZero, KnownOne);
-    // If a range metadata is attached to this IntrinsicInst, intersect the
-    // explicit range specified by the metadata and the implicit range of
-    // the intrinsic.
+    if (Value *RV = CallSite(I).getReturnedArgOperand()) {
+      computeKnownBits(RV, KnownZero2, KnownOne2, Depth + 1, Q);
+      KnownZero |= KnownZero2;
+      KnownOne |= KnownOne2;
+    }
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       default: break;