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When adding the carry bit to another value on X86, exploit the fact that the carry-materialization

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(sbbl x, x) sets the registers to 0 or ~0.  Combined with two's complement arithmetic, we can fold
the intermediate AND and the ADD into a single SUB.

This fixes <rdar://problem/8449754>.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114460 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Owen Anderson 2010-09-21 18:41:19 +00:00
parent 89bfef003e
commit c004eec71b
2 changed files with 37 additions and 0 deletions
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23
lib/Target/X86/X86ISelLowering.cpp
View File

@ -1021,6 +1021,7 @@ X86TargetLowering::X86TargetLowering(X86TargetMachine &TM)
setTargetDAGCombine(ISD::OR);
setTargetDAGCombine(ISD::STORE);
setTargetDAGCombine(ISD::ZERO_EXTEND);
setTargetDAGCombine(ISD::ADD);
if (Subtarget->is64Bit())
setTargetDAGCombine(ISD::MUL);
@ -10452,6 +10453,27 @@ static SDValue PerformCMOVCombine(SDNode *N, SelectionDAG &DAG,
return SDValue();
}
/// PerformAddCombine - Optimize ADD when combined with X86 opcodes.
static SDValue PerformAddCombine(SDNode *N, SelectionDAG &DAG,
TargetLowering::DAGCombinerInfo &DCI) {
if (DCI.isBeforeLegalize() || DCI.isCalledByLegalizer())
return SDValue();
EVT VT = N->getValueType(0);
SDValue Op1 = N->getOperand(1);
if (Op1->getOpcode() == ISD::AND) {
SDValue AndOp0 = Op1->getOperand(0);
ConstantSDNode *AndOp1 = dyn_cast<ConstantSDNode>(Op1->getOperand(1));
// (add z, (and (sbbl x, x), 1)) -> (sub z, (sbbl x, x))
if (AndOp0->getOpcode() == X86ISD::SETCC_CARRY &&
AndOp1 && AndOp1->getZExtValue() == 1) {
DebugLoc DL = N->getDebugLoc();
return DAG.getNode(ISD::SUB, DL, VT, N->getOperand(0), AndOp0);
}
}
return SDValue();
}
/// PerformMulCombine - Optimize a single multiply with constant into two
/// in order to implement it with two cheaper instructions, e.g.
@ -10938,6 +10960,7 @@ SDValue X86TargetLowering::PerformDAGCombine(SDNode *N,
return PerformEXTRACT_VECTOR_ELTCombine(N, DAG, *this);
case ISD::SELECT: return PerformSELECTCombine(N, DAG, Subtarget);
case X86ISD::CMOV: return PerformCMOVCombine(N, DAG, DCI);
case ISD::ADD: return PerformAddCombine(N, DAG, DCI);
case ISD::MUL: return PerformMulCombine(N, DAG, DCI);
case ISD::SHL:
case ISD::SRA:

14
test/CodeGen/X86/add-of-carry.ll Normal file
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@ -0,0 +1,14 @@
; RUN: llc < %s -march=x86 | FileCheck %s
; <rdar://problem/8449754>
define i32 @add32carry(i32 %sum, i32 %x) nounwind readnone ssp {
entry:
; CHECK: sbbl %ecx, %ecx
; CHECK-NOT: addl
; CHECK: subl %ecx, %eax
%add4 = add i32 %x, %sum
%cmp = icmp ult i32 %add4, %x
%inc = zext i1 %cmp to i32
%z.0 = add i32 %add4, %inc
ret i32 %z.0
}