[llvm] cc4f462 - [X86][ISel] Improve VPTERNLOG matching for negated logic trees (#164863)

[via llvm-commits]

Author: Yi-Chi Lee
Date: 2025-10-25T09:48:35+08:00
New Revision: cc4f462ba46c288dc26aa155775444cd9a7c4d0e

URL: https://github.com/llvm/llvm-project/commit/cc4f462ba46c288dc26aa155775444cd9a7c4d0e
DIFF: https://github.com/llvm/llvm-project/commit/cc4f462ba46c288dc26aa155775444cd9a7c4d0e.diff

LOG: [X86][ISel] Improve VPTERNLOG matching for negated logic trees (#164863)

This patch extends VPTERNLOG pattern matching to handle cases where an
outer NOT wraps a pure logical tree, such as `~(A | B | C)`. By
recognizing these negated logic trees, the instruction selector can now
emit a single vpternlog instruction.

The change preserves the match for patterns like `(x != C1) & (x !=
C2)`, which also have the xor-with-all-ones pattern outside. The patch
conservatively peels the outer XOR-with-all-ones only when it directly
wraps a foldable logical operator (AND, OR, XOR, or ANDNP).

Resolves #163738

Added: 
    llvm/test/CodeGen/X86/issue163738.ll

Modified: 
    llvm/lib/Target/X86/X86ISelDAGToDAG.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index 62073ec125e8f..4393f6ecaa033 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -4721,9 +4721,6 @@ bool X86DAGToDAGISel::tryVPTERNLOG(SDNode *N) {
   if (!(Subtarget->hasVLX() || NVT.is512BitVector()))
     return false;
 
-  SDValue N0 = N->getOperand(0);
-  SDValue N1 = N->getOperand(1);
-
   auto getFoldableLogicOp = [](SDValue Op) {
     // Peek through single use bitcast.
     if (Op.getOpcode() == ISD::BITCAST && Op.hasOneUse())
@@ -4740,13 +4737,47 @@ bool X86DAGToDAGISel::tryVPTERNLOG(SDNode *N) {
     return SDValue();
   };
 
-  SDValue A, FoldableOp;
-  if ((FoldableOp = getFoldableLogicOp(N1))) {
-    A = N0;
-  } else if ((FoldableOp = getFoldableLogicOp(N0))) {
-    A = N1;
-  } else
-    return false;
+  SDValue N0, N1, A, FoldableOp;
+
+  // Identify and (optionally) peel an outer NOT that wraps a pure logic tree
+  auto tryPeelOuterNotWrappingLogic = [&](SDNode *Op) {
+    if (Op->getOpcode() == ISD::XOR && Op->hasOneUse() &&
+        ISD::isBuildVectorAllOnes(Op->getOperand(1).getNode())) {
+      SDValue InnerOp = Op->getOperand(0);
+
+      if (!getFoldableLogicOp(InnerOp))
+        return SDValue();
+
+      N0 = InnerOp.getOperand(0);
+      N1 = InnerOp.getOperand(1);
+      if ((FoldableOp = getFoldableLogicOp(N1))) {
+        A = N0;
+        return InnerOp;
+      }
+      if ((FoldableOp = getFoldableLogicOp(N0))) {
+        A = N1;
+        return InnerOp;
+      }
+    }
+    return SDValue();
+  };
+
+  bool PeeledOuterNot = false;
+  SDNode *OriN = N;
+  if (SDValue InnerOp = tryPeelOuterNotWrappingLogic(N)) {
+    PeeledOuterNot = true;
+    N = InnerOp.getNode();
+  } else {
+    N0 = N->getOperand(0);
+    N1 = N->getOperand(1);
+
+    if ((FoldableOp = getFoldableLogicOp(N1)))
+      A = N0;
+    else if ((FoldableOp = getFoldableLogicOp(N0)))
+      A = N1;
+    else
+      return false;
+  }
 
   SDValue B = FoldableOp.getOperand(0);
   SDValue C = FoldableOp.getOperand(1);
@@ -4798,7 +4829,10 @@ bool X86DAGToDAGISel::tryVPTERNLOG(SDNode *N) {
   case ISD::XOR: Imm ^= TernlogMagicA; break;
   }
 
-  return matchVPTERNLOG(N, ParentA, ParentB, ParentC, A, B, C, Imm);
+  if (PeeledOuterNot)
+    Imm = ~Imm;
+
+  return matchVPTERNLOG(OriN, ParentA, ParentB, ParentC, A, B, C, Imm);
 }
 
 /// If the high bits of an 'and' operand are known zero, try setting the

diff  --git a/llvm/test/CodeGen/X86/issue163738.ll b/llvm/test/CodeGen/X86/issue163738.ll
new file mode 100644
index 0000000000000..61fe043a970dd
--- /dev/null
+++ b/llvm/test/CodeGen/X86/issue163738.ll
@@ -0,0 +1,13 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc < %s -mtriple=x86_64-- -mattr=+avx512f,+avx512vl | FileCheck %s --check-prefixes=CHECK
+
+define <8 x i64> @foo(<8 x i64> %a, <8 x i64> %b, <8 x i64> %c) {
+; CHECK-LABEL: foo:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vpternlogq {{.*#+}} zmm0 = ~(zmm0 | zmm2 | zmm1)
+; CHECK-NEXT:    retq
+  %and.demorgan = or <8 x i64> %b, %a
+  %and3.demorgan = or <8 x i64> %and.demorgan, %c
+  %and3 = xor <8 x i64> %and3.demorgan, splat (i64 -1)
+  ret <8 x i64> %and3
+}