[InstCombine] fold funnel shift amount based on demanded bits

The shift amount of a funnel shift is modulo the scalar bitwidth:
http://llvm.org/docs/LangRef.html#llvm-fshl-intrinsic
...so we can use demanded bits analysis on that operand to simplify it
when we have a power-of-2 bitwidth.

This is another step towards canonicalizing {shift/shift/or} to the 
intrinsics in IR.

Differential Revision: https://reviews.llvm.org/D54478

llvm-svn: 346814

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index fae47ec93b9..a99eaf013e6 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1990,6 +1990,20 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
       return I;
     break;
 
+  case Intrinsic::fshl:
+  case Intrinsic::fshr: {
+    // The shift amount (operand 2) of a funnel shift is modulo the bitwidth,
+    // so only the low bits of the shift amount are demanded if the bitwidth is
+    // a power-of-2.
+    unsigned BitWidth = II->getType()->getScalarSizeInBits();
+    if (!isPowerOf2_32(BitWidth))
+      break;
+    APInt Op2Demanded = APInt::getLowBitsSet(BitWidth, Log2_32_Ceil(BitWidth));
+    KnownBits Op2Known(BitWidth);
+    if (SimplifyDemandedBits(II, 2, Op2Demanded, Op2Known))
+      return &CI;
+    break;
+  }
   case Intrinsic::uadd_with_overflow:
   case Intrinsic::sadd_with_overflow:
   case Intrinsic::umul_with_overflow: