[llvm] [X86] LowerSELECTWithCmpZero - extend branchless OR/XOR select codegen to handle ADD/SUB as well (PR #107612)

[Simon Pilgrim via llvm-commits]

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@@ -24084,18 +24084,32 @@ static SDValue LowerSELECTWithCmpZero(SDValue CmpVal, SDValue LHS, SDValue RHS,
   if (!CmpVT.isScalarInteger() || !VT.isScalarInteger())
     return SDValue();
 
-  // Convert OR/XOR 'identity' patterns (iff X is 0 or 1):
-  // select (X != 0), Y, (OR Y, Z) -> (OR Y, (AND (0 - X), Z))
-  // select (X != 0), Y, (XOR Y, Z) -> (XOR Y, (AND (0 - X), Z))
+  // Convert 'identity' patterns (iff X is 0 or 1):
+  // SELECT (X != 0), Y, (OR Y, Z) -> (OR Y, (AND (0 - X), Z))
+  // SELECT (X != 0), Y, (XOR Y, Z) -> (XOR Y, (AND (0 - X), Z))
+  // SELECT (X != 0), Y, (ADD Y, Z) -> (ADD Y, (AND (0 - X), Z))
+  // SELECT (X != 0), Y, (SUB Y, Z) -> (SUB Y, (AND (0 - X), Z))
   if (!Subtarget.canUseCMOV() && X86CC == X86::COND_E &&
       CmpVal.getOpcode() == ISD::AND && isOneConstant(CmpVal.getOperand(1))) {
     SDValue Src1, Src2;
     auto isIdentityPattern = [&]() {
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RKSimon wrote:

Yes, DAG has `llvm::isNeutralConstant` which is similar, but no getNeutralConstant equivalent - we could add something that LowerSELECTWithCmpZero could use.

We should probably rename isNeutralConstant to isIdentityConstant as well to be consistent with other implementations.

https://github.com/llvm/llvm-project/pull/107612