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Change this DAGCombine to build AND of SHR instead of SHR of AND; this matches the ordering we prefer in instcombine. Part of rdar://9562809. 

The potential DAGCombine which enforces this more generally messes up some other very fragile patterns, so I'm leaving that alone, at least for now.

llvm-svn: 132809
This commit is contained in:
Eli Friedman 2011-06-09 22:14:44 +00:00
parent 4b6fed4bab
commit 1877ac9937
2 changed files with 27 additions and 15 deletions
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31
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -3092,26 +3092,27 @@ SDValue DAGCombiner::visitSHL(SDNode *N) {
}
}
// fold (shl (srl x, c1), c2) -> (shl (and x, (shl -1, c1)), (sub c2, c1)) or
// (srl (and x, (shl -1, c1)), (sub c1, c2))
// fold (shl (srl x, c1), c2) -> (and (shl x, (sub c2, c1), MASK) or
// (and (srl x, (sub c1, c2), MASK)
if (N1C && N0.getOpcode() == ISD::SRL &&
N0.getOperand(1).getOpcode() == ISD::Constant) {
uint64_t c1 = cast<ConstantSDNode>(N0.getOperand(1))->getZExtValue();
if (c1 < VT.getSizeInBits()) {
uint64_t c2 = N1C->getZExtValue();
SDValue HiBitsMask =
DAG.getConstant(APInt::getHighBitsSet(VT.getSizeInBits(),
VT.getSizeInBits() - c1),
VT);
SDValue Mask = DAG.getNode(ISD::AND, N0.getDebugLoc(), VT,
N0.getOperand(0),
HiBitsMask);
if (c2 > c1)
return DAG.getNode(ISD::SHL, N->getDebugLoc(), VT, Mask,
DAG.getConstant(c2-c1, N1.getValueType()));
else
return DAG.getNode(ISD::SRL, N->getDebugLoc(), VT, Mask,
DAG.getConstant(c1-c2, N1.getValueType()));
APInt Mask = APInt::getHighBitsSet(VT.getSizeInBits(),
VT.getSizeInBits() - c1);
SDValue Shift;
if (c2 > c1) {
Mask = Mask.shl(c2-c1);
Shift = DAG.getNode(ISD::SHL, N->getDebugLoc(), VT, N0.getOperand(0),
DAG.getConstant(c2-c1, N1.getValueType()));
} else {
Mask = Mask.lshr(c1-c2);
Shift = DAG.getNode(ISD::SRL, N->getDebugLoc(), VT, N0.getOperand(0),
DAG.getConstant(c1-c2, N1.getValueType()));
}
return DAG.getNode(ISD::AND, N0.getDebugLoc(), VT, Shift,
DAG.getConstant(Mask, VT));
}
}
// fold (shl (sra x, c1), c1) -> (and x, (shl -1, c1))

11
llvm/test/CodeGen/X86/shift-pair.ll Normal file
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@ -0,0 +1,11 @@
; RUN: llc < %s -march=x86-64 | FileCheck %s
define i64 @test(i64 %A) {
; CHECK: @test
; CHECK: shrq $54
; CHECK: andq $1020
; CHECK: ret
%B = lshr i64 %A, 56
%C = shl i64 %B, 2
ret i64 %C
}