Reapply r165661, Patch by Shuxin Yang <shuxin.llvm@gmail.com>.
Original message:
The attached is the fix to radar://11663049. The optimization can be outlined by following rules:
(select (x != c), e, c) -> select (x != c), e, x),
(select (x == c), c, e) -> select (x == c), x, e)
where the <c> is an integer constant.
The reason for this change is that : on x86, conditional-move-from-constant needs two instructions;
however, conditional-move-from-register need only one instruction.
While the LowerSELECT() sounds to be the most convenient place for this optimization, it turns out to be a bad place. The reason is that by replacing the constant <c> with a symbolic value, it obscure some instruction-combining opportunities which would otherwise be very easy to spot. For that reason, I have to postpone the change to last instruction-combining phase.
The change passes the test of "make check-all -C <build-root/test" and "make -C project/test-suite/SingleSource".
Original message since r165661:
My previous change has a bug: I negated the condition code of a CMOV, and go ahead creating a new CMOV using the *ORIGINAL* condition code.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166017 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index b2537c6..2f09e9e 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -14632,6 +14632,7 @@
if (TrueC->getAPIntValue().ult(FalseC->getAPIntValue())) {
CC = X86::GetOppositeBranchCondition(CC);
std::swap(TrueC, FalseC);
+ std::swap(TrueOp, FalseOp);
}
// Optimize C ? 8 : 0 -> zext(setcc(C)) << 3. Likewise for any pow2/0.
@@ -14714,6 +14715,46 @@
}
}
}
+
+ // Handle these cases:
+ // (select (x != c), e, c) -> select (x != c), e, x),
+ // (select (x == c), c, e) -> select (x == c), x, e)
+ // where the c is an integer constant, and the "select" is the combination
+ // of CMOV and CMP.
+ //
+ // The rationale for this change is that the conditional-move from a constant
+ // needs two instructions, however, conditional-move from a register needs
+ // only one instruction.
+ //
+ // CAVEAT: By replacing a constant with a symbolic value, it may obscure
+ // some instruction-combining opportunities. This opt needs to be
+ // postponed as late as possible.
+ //
+ if (!DCI.isBeforeLegalize() && !DCI.isBeforeLegalizeOps()) {
+ // the DCI.xxxx conditions are provided to postpone the optimization as
+ // late as possible.
+
+ ConstantSDNode *CmpAgainst = 0;
+ if ((Cond.getOpcode() == X86ISD::CMP || Cond.getOpcode() == X86ISD::SUB) &&
+ (CmpAgainst = dyn_cast<ConstantSDNode>(Cond.getOperand(1))) &&
+ dyn_cast<ConstantSDNode>(Cond.getOperand(0)) == 0) {
+
+ if (CC == X86::COND_NE &&
+ CmpAgainst == dyn_cast<ConstantSDNode>(FalseOp)) {
+ CC = X86::GetOppositeBranchCondition(CC);
+ std::swap(TrueOp, FalseOp);
+ }
+
+ if (CC == X86::COND_E &&
+ CmpAgainst == dyn_cast<ConstantSDNode>(TrueOp)) {
+ SDValue Ops[] = { FalseOp, Cond.getOperand(0),
+ DAG.getConstant(CC, MVT::i8), Cond };
+ return DAG.getNode(X86ISD::CMOV, DL, N->getVTList (), Ops,
+ array_lengthof(Ops));
+ }
+ }
+ }
+
return SDValue();
}