[InstCombine] canonicalize rotate right by constant to rotate left
This was noted as a backend problem:
https://bugs.llvm.org/show_bug.cgi?id=41057
...and subsequently fixed for x86:
rL356121
But we should canonicalize these in IR for the benefit of all targets
and improve IR analysis such as CSE.
llvm-svn: 356338
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 1c292ea..bdc235f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1993,22 +1993,36 @@
case Intrinsic::fshl:
case Intrinsic::fshr: {
- // Canonicalize a shift amount constant operand to be modulo the bit-width.
- unsigned BitWidth = II->getType()->getScalarSizeInBits();
+ Value *Op0 = II->getArgOperand(0), *Op1 = II->getArgOperand(1);
+ Type *Ty = II->getType();
+ unsigned BitWidth = Ty->getScalarSizeInBits();
Constant *ShAmtC;
if (match(II->getArgOperand(2), m_Constant(ShAmtC)) &&
!isa<ConstantExpr>(ShAmtC) && !ShAmtC->containsConstantExpression()) {
- Constant *WidthC = ConstantInt::get(II->getType(), BitWidth);
+ // Canonicalize a shift amount constant operand to modulo the bit-width.
+ Constant *WidthC = ConstantInt::get(Ty, BitWidth);
Constant *ModuloC = ConstantExpr::getURem(ShAmtC, WidthC);
if (ModuloC != ShAmtC) {
II->setArgOperand(2, ModuloC);
return II;
}
+ // Canonicalize rotate right by constant to rotate left. This is not
+ // entirely arbitrary. For historical reasons, the backend may recognize
+ // rotate left patterns but miss rotate right patterns.
+ if (II->getIntrinsicID() == Intrinsic::fshr && Op0 == Op1) {
+ // fshr X, X, C --> fshl X, X, (BitWidth - C)
+ assert(ConstantExpr::getICmp(ICmpInst::ICMP_UGT, WidthC, ShAmtC) ==
+ ConstantInt::getTrue(CmpInst::makeCmpResultType(Ty)) &&
+ "Shift amount expected to be modulo bitwidth");
+ Constant *LeftShiftC = ConstantExpr::getSub(WidthC, ShAmtC);
+ Module *Mod = II->getModule();
+ Function *Fshl = Intrinsic::getDeclaration(Mod, Intrinsic::fshl, Ty);
+ return CallInst::Create(Fshl, { Op0, Op0, LeftShiftC });
+ }
}
const APInt *SA;
if (match(II->getArgOperand(2), m_APInt(SA))) {
- Value *Op0 = II->getArgOperand(0), *Op1 = II->getArgOperand(1);
uint64_t ShiftAmt = SA->urem(BitWidth);
assert(ShiftAmt != 0 && "SimplifyCall should have handled zero shift");
// Normalize to funnel shift left.
@@ -2018,14 +2032,13 @@
// fshl(X, 0, C) -> shl X, C
// fshl(X, undef, C) -> shl X, C
if (match(Op1, m_Zero()) || match(Op1, m_Undef()))
- return BinaryOperator::CreateShl(
- Op0, ConstantInt::get(II->getType(), ShiftAmt));
+ return BinaryOperator::CreateShl(Op0, ConstantInt::get(Ty, ShiftAmt));
// fshl(0, X, C) -> lshr X, (BW-C)
// fshl(undef, X, C) -> lshr X, (BW-C)
if (match(Op0, m_Zero()) || match(Op0, m_Undef()))
return BinaryOperator::CreateLShr(
- Op1, ConstantInt::get(II->getType(), BitWidth - ShiftAmt));
+ Op1, ConstantInt::get(Ty, BitWidth - ShiftAmt));
}
// The shift amount (operand 2) of a funnel shift is modulo the bitwidth,