[InstCombine] try to fold (select C, (sext A), B) into logical ops
Summary:
Turn (select C, (sext A), B) into (sext (select C, A, B')) when A is i1 and
B is a compatible constant, also for zext instead of sext. This will then be
further folded into logical operations.
The transformation would be valid for non-i1 types as well, but other parts of
InstCombine prefer to have sext from non-i1 as an operand of select.
Motivated by the shader compiler frontend in Mesa for AMDGPU, which emits i32
for boolean operations. With this change, the boolean logic is fully
recovered.
Reviewers: majnemer, spatel, tstellarAMD
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D22747
llvm-svn: 277801
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 018cdcc..b76241a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -912,6 +912,37 @@
return nullptr;
}
+/// If one of the operands is a sext/zext from i1 and the other is a constant,
+/// we may be able to create an i1 select which can be further folded to
+/// logical ops.
+static Instruction *foldSelectExtConst(InstCombiner::BuilderTy &Builder,
+ SelectInst &SI, Instruction *EI,
+ const APInt &C, bool isExtTrueVal,
+ bool isSigned) {
+ Value *SmallVal = EI->getOperand(0);
+ Type *SmallType = SmallVal->getType();
+
+ // TODO Handle larger types as well? Note this requires adjusting
+ // FoldOpIntoSelect as well.
+ if (!SmallType->getScalarType()->isIntegerTy(1))
+ return nullptr;
+
+ if (C != 0 && (isSigned || C != 1) &&
+ (!isSigned || !C.isAllOnesValue()))
+ return nullptr;
+
+ Value *SmallConst = ConstantInt::get(SmallType, C.trunc(1));
+ Value *TrueVal = isExtTrueVal ? SmallVal : SmallConst;
+ Value *FalseVal = isExtTrueVal ? SmallConst : SmallVal;
+ Value *Select = Builder.CreateSelect(SI.getOperand(0), TrueVal, FalseVal,
+ "fold." + SI.getName());
+
+ if (isSigned)
+ return new SExtInst(Select, SI.getType());
+
+ return new ZExtInst(Select, SI.getType());
+}
+
Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
Value *CondVal = SI.getCondition();
Value *TrueVal = SI.getTrueValue();
@@ -1098,6 +1129,31 @@
if (Instruction *IV = FoldSelectOpOp(SI, TI, FI))
return IV;
+ // (select C, (sext X), const) -> (sext (select C, X, const')) and
+ // variations thereof when extending from i1, as that allows further folding
+ // into logic ops. When the sext is from a larger type, we prefer to have it
+ // as an operand.
+ if (TI &&
+ (TI->getOpcode() == Instruction::ZExt || TI->getOpcode() == Instruction::SExt)) {
+ bool IsSExt = TI->getOpcode() == Instruction::SExt;
+ const APInt *C;
+ if (match(FalseVal, m_APInt(C))) {
+ if (Instruction *IV =
+ foldSelectExtConst(*Builder, SI, TI, *C, true, IsSExt))
+ return IV;
+ }
+ }
+ if (FI &&
+ (FI->getOpcode() == Instruction::ZExt || FI->getOpcode() == Instruction::SExt)) {
+ bool IsSExt = FI->getOpcode() == Instruction::SExt;
+ const APInt *C;
+ if (match(TrueVal, m_APInt(C))) {
+ if (Instruction *IV =
+ foldSelectExtConst(*Builder, SI, FI, *C, false, IsSExt))
+ return IV;
+ }
+ }
+
// See if we can fold the select into one of our operands.
if (SelType->isIntOrIntVectorTy() || SelType->isFPOrFPVectorTy()) {
if (Instruction *FoldI = FoldSelectIntoOp(SI, TrueVal, FalseVal))
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index daa22da..7ffe34a 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -790,7 +790,7 @@
if (isa<Constant>(TV) || isa<Constant>(FV)) {
// Bool selects with constant operands can be folded to logical ops.
- if (SI->getType()->isIntegerTy(1)) return nullptr;
+ if (SI->getType()->getScalarType()->isIntegerTy(1)) return nullptr;
// If it's a bitcast involving vectors, make sure it has the same number of
// elements on both sides.