[llvm] [X86] Fold some (truncate (srl (add X, C1), C2)) patterns to (add (truncate (srl X, C2), C1')) (PR #126448)

[Simon Pilgrim via llvm-commits]

=?utf-8?q?João?= Gouveia <jtalonegouveia at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/126448 at github.com>


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@@ -48472,6 +48472,64 @@ static SDValue combineSetCCMOVMSK(SDValue EFLAGS, X86::CondCode &CC,
   return SDValue();
 }
 
+// Attempt to fold some (truncate (srl (add X, C1), C2)) patterns to
+// (add (truncate (srl X, C2), C1')). C1' will be smaller than C1 so we are able
+// to avoid generating code with MOVABS and large constants in certain cases.
+static SDValue combineSetCCTruncAdd(SDValue EFLAGS, X86::CondCode &CC,
+                                    SelectionDAG &DAG) {
+  if (!(CC == X86::COND_E || CC == X86::COND_NE || CC == X86::COND_AE ||
+        CC == X86::COND_B))
+    return SDValue();
+
+  EVT VT = EFLAGS.getValueType();
+  if (EFLAGS.getOpcode() == X86ISD::SUB && VT == MVT::i32) {
+    SDValue CmpLHS = EFLAGS.getOperand(0);
+    auto *CmpConstant = dyn_cast<ConstantSDNode>(EFLAGS.getOperand(1));
+
+    if (CmpLHS.getOpcode() != ISD::TRUNCATE || !CmpConstant)
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RKSimon wrote:

Can you rework this to use sd_match? It should simplify a lot of the pattern matching below.

https://github.com/llvm/llvm-project/pull/126448