[InstCombine] recognize popcount.
This patch recognizes popcount intrinsic according to algorithm from website
http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
Differential Revision: https://reviews.llvm.org/D68189
llvm-svn: 374512
diff --git a/llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp b/llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp
index 6e2867f..a24de3c 100644
--- a/llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp
+++ b/llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp
@@ -250,6 +250,72 @@
return true;
}
+// Try to recognize below function as popcount intrinsic.
+// This is the "best" algorithm from
+// http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
+// Also used in TargetLowering::expandCTPOP().
+//
+// int popcount(unsigned int i) {
+// i = i - ((i >> 1) & 0x55555555);
+// i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
+// i = ((i + (i >> 4)) & 0x0F0F0F0F);
+// return (i * 0x01010101) >> 24;
+// }
+static bool tryToRecognizePopCount(Instruction &I) {
+ if (I.getOpcode() != Instruction::LShr)
+ return false;
+
+ Type *Ty = I.getType();
+ if (!Ty->isIntOrIntVectorTy())
+ return false;
+
+ unsigned Len = Ty->getScalarSizeInBits();
+ // FIXME: fix Len == 8 and other irregular type lengths.
+ if (!(Len <= 128 && Len > 8 && Len % 8 == 0))
+ return false;
+
+ APInt Mask55 = APInt::getSplat(Len, APInt(8, 0x55));
+ APInt Mask33 = APInt::getSplat(Len, APInt(8, 0x33));
+ APInt Mask0F = APInt::getSplat(Len, APInt(8, 0x0F));
+ APInt Mask01 = APInt::getSplat(Len, APInt(8, 0x01));
+ APInt MaskShift = APInt(Len, Len - 8);
+
+ Value *Op0 = I.getOperand(0);
+ Value *Op1 = I.getOperand(1);
+ Value *MulOp0;
+ // Matching "(i * 0x01010101...) >> 24".
+ if ((match(Op0, m_Mul(m_Value(MulOp0), m_SpecificInt(Mask01)))) &&
+ match(Op1, m_SpecificInt(MaskShift))) {
+ Value *ShiftOp0;
+ // Matching "((i + (i >> 4)) & 0x0F0F0F0F...)".
+ if (match(MulOp0, m_And(m_c_Add(m_LShr(m_Value(ShiftOp0), m_SpecificInt(4)),
+ m_Deferred(ShiftOp0)),
+ m_SpecificInt(Mask0F)))) {
+ Value *AndOp0;
+ // Matching "(i & 0x33333333...) + ((i >> 2) & 0x33333333...)".
+ if (match(ShiftOp0,
+ m_c_Add(m_And(m_Value(AndOp0), m_SpecificInt(Mask33)),
+ m_And(m_LShr(m_Deferred(AndOp0), m_SpecificInt(2)),
+ m_SpecificInt(Mask33))))) {
+ Value *Root, *SubOp1;
+ // Matching "i - ((i >> 1) & 0x55555555...)".
+ if (match(AndOp0, m_Sub(m_Value(Root), m_Value(SubOp1))) &&
+ match(SubOp1, m_And(m_LShr(m_Specific(Root), m_SpecificInt(1)),
+ m_SpecificInt(Mask55)))) {
+ LLVM_DEBUG(dbgs() << "Recognized popcount intrinsic\n");
+ IRBuilder<> Builder(&I);
+ Function *Func = Intrinsic::getDeclaration(
+ I.getModule(), Intrinsic::ctpop, I.getType());
+ I.replaceAllUsesWith(Builder.CreateCall(Func, {Root}));
+ return true;
+ }
+ }
+ }
+ }
+
+ return false;
+}
+
/// This is the entry point for folds that could be implemented in regular
/// InstCombine, but they are separated because they are not expected to
/// occur frequently and/or have more than a constant-length pattern match.
@@ -268,6 +334,7 @@
for (Instruction &I : make_range(BB.rbegin(), BB.rend())) {
MadeChange |= foldAnyOrAllBitsSet(I);
MadeChange |= foldGuardedRotateToFunnelShift(I);
+ MadeChange |= tryToRecognizePopCount(I);
}
}