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DAGCombiner: fold (xor (shl 1, x), -1) -> (rotl ~1, x)

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Targets which provide a rotate make it possible to replace a sequence of
(XOR (SHL 1, x), -1) with (ROTL ~1, x).  This saves an instruction on
architectures like X86 and POWER(64).

Differential Revision: http://reviews.llvm.org/D8350

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232572 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2015-03-18 00:03:36 +00:00
parent 7605cdd6e4
commit 8f01b96d93
2 changed files with 38 additions and 0 deletions
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26
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -3980,6 +3980,32 @@ SDValue DAGCombiner::visitXOR(SDNode *N) {
if (N0 == N1)
return tryFoldToZero(SDLoc(N), TLI, VT, DAG, LegalOperations, LegalTypes);
// fold (xor (shl 1, x), -1) -> (rotl ~1, x)
// Here is a concrete example of this equivalence:
// i16 x == 14
// i16 shl == 1 << 14 == 16384 == 0b0100000000000000
// i16 xor == ~(1 << 14) == 49151 == 0b1011111111111111
//
// =>
//
// i16 ~1 == 0b1111111111111110
// i16 rol(~1, 14) == 0b1011111111111111
//
// Some additional tips to help conceptualize this transform:
// - Try to see the operation as placing a single zero in a value of all ones.
// - There exists no value for x which would allow the result to contain zero.
// - Values of x larger than the bitwidth are undefined and do not require a
// consistent result.
// - Pushing the zero left requires shifting one bits in from the right.
// A rotate left of ~1 is a nice way of achieving the desired result.
if (TLI.isOperationLegalOrCustom(ISD::ROTL, VT))
if (auto *N1C = dyn_cast<ConstantSDNode>(N1.getNode()))
if (N0.getOpcode() == ISD::SHL)
if (auto *ShlLHS = dyn_cast<ConstantSDNode>(N0.getOperand(0)))
if (N1C->isAllOnesValue() && ShlLHS->isOne())
return DAG.getNode(ISD::ROTL, SDLoc(N), VT, DAG.getConstant(~1, VT),
N0.getOperand(1));
// Simplify: xor (op x...), (op y...) -> (op (xor x, y))
if (N0.getOpcode() == N1.getOpcode()) {
SDValue Tmp = SimplifyBinOpWithSameOpcodeHands(N);

12
test/CodeGen/X86/xor.ll
View File

@ -181,3 +181,15 @@ define i32 @PR17487(i1 %tobool) {
; X32-LABEL: PR17487:
; X32: andn
}
define i32 @test11(i32 %b) {
%shl = shl i32 1, %b
%neg = xor i32 %shl, -1
ret i32 %neg
; X64-LABEL: test11:
; X64: movl $-2, %[[REG:.*]]
; X64: roll %{{.*}}, %[[REG]]
; X32-LABEL: test11:
; X32: movl $-2, %[[REG:.*]]
; X32: roll %{{.*}}, %[[REG]]
}