[X86] combineXor - limit fold to non-opaque constants (PR50254)

Ensure we don't try to fold when one might be an opaque constant - the constant fold will fail and then the reverse fold will happen in DAGCombine.....

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -46998,15 +46998,17 @@ static SDValue combineXor(SDNode *N, SelectionDAG &DAG,
   // Fold xor(truncate(xor(x,c1)),c2) -> xor(truncate(x),xor(truncate(c1),c2))
   // TODO: Under what circumstances could this be performed in DAGCombine?
   if ((N0.getOpcode() == ISD::TRUNCATE || N0.getOpcode() == ISD::ZERO_EXTEND) &&
-      N0.getOperand(0).getOpcode() == N->getOpcode() &&
-      isa<ConstantSDNode>(N1) &&
-      isa<ConstantSDNode>(N0.getOperand(0).getOperand(1))) {
-    SDLoc DL(N);
+      N0.getOperand(0).getOpcode() == N->getOpcode()) {
     SDValue TruncExtSrc = N0.getOperand(0);
-    SDValue LHS = DAG.getZExtOrTrunc(TruncExtSrc.getOperand(0), DL, VT);
-    SDValue RHS = DAG.getZExtOrTrunc(TruncExtSrc.getOperand(1), DL, VT);
-    return DAG.getNode(ISD::XOR, DL, VT, LHS,
-                       DAG.getNode(ISD::XOR, DL, VT, RHS, N1));
+    auto *N1C = dyn_cast<ConstantSDNode>(N1);
+    auto *N001C = dyn_cast<ConstantSDNode>(TruncExtSrc.getOperand(1));
+    if (N1C && !N1C->isOpaque() && N001C && !N001C->isOpaque()) {
+      SDLoc DL(N);
+      SDValue LHS = DAG.getZExtOrTrunc(TruncExtSrc.getOperand(0), DL, VT);
+      SDValue RHS = DAG.getZExtOrTrunc(TruncExtSrc.getOperand(1), DL, VT);
+      return DAG.getNode(ISD::XOR, DL, VT, LHS,
+                         DAG.getNode(ISD::XOR, DL, VT, RHS, N1));
+    }
   }
 
   if (SDValue FPLogic = convertIntLogicToFPLogic(N, DAG, Subtarget))

llvm/test/CodeGen/X86/pr50254.ll

@@ -0,0 +1,49 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=i686-unknown | FileCheck %s --check-prefix=X86
+; RUN: llc < %s -mtriple=x86_64-unknown | FileCheck %s --check-prefix=X64
+
+@d.e = external dso_local unnamed_addr global i32, align 4
+
+define void @PR50254() {
+; X86-LABEL: PR50254:
+; X86:       # %bb.0: # %entry
+; X86-NEXT:    movswl d.e, %eax
+; X86-NEXT:    xorl %ecx, %ecx
+; X86-NEXT:    testb %cl, %cl
+; X86-NEXT:    jne .LBB0_2
+; X86-NEXT:  # %bb.1: # %for.end
+; X86-NEXT:    movw %ax, d.e
+; X86-NEXT:  .LBB0_2: # %for.body.1
+; X86-NEXT:    retl
+;
+; X64-LABEL: PR50254:
+; X64:       # %bb.0: # %entry
+; X64-NEXT:    movswq {{.*}}(%rip), %rax
+; X64-NEXT:    xorl %ecx, %ecx
+; X64-NEXT:    testb %cl, %cl
+; X64-NEXT:    jne .LBB0_2
+; X64-NEXT:  # %bb.1: # %for.end
+; X64-NEXT:    movw %ax, {{.*}}(%rip)
+; X64-NEXT:  .LBB0_2: # %for.body.1
+; X64-NEXT:    retq
+entry:
+  %load = load i16, i16* bitcast (i32* @d.e to i16*), align 4
+  %xor1 = xor i16 %load, 0
+  %xor2 = xor i64 undef, 3821908120
+  %xor3 = xor i16 %load, -1
+  %xor4 = sext i16 %xor3 to i64
+  %xor5 = and i64 %xor4, 4294967295
+  %xor6 = xor i64 %xor5, 3821908120
+  br label %for.body
+
+for.body:                                         ; preds = %entry
+  br i1 undef, label %for.end, label %for.body.1
+
+for.end:                                          ; preds = %for.body
+  store i16 %xor1, i16* bitcast (i32* @d.e to i16*), align 4
+  ret void
+
+for.body.1:                                       ; preds = %for.body
+  %add.1 = add i64 %xor6, undef
+  ret void
+}