[llvm] [X86] Fix CCMP miscompile for flag-source-folded SETCC leaves (PR #217682)

[Phoebe Wang via llvm-commits]

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@@ -25518,16 +25518,36 @@ static bool canEmitConjunctionForCCMP(SelectionDAG &DAG, SDValue Val,
     return false;
   unsigned Opcode = Val.getOpcode();
   if (Opcode == ISD::SETCC) {
-    EVT VT = Val.getOperand(0).getValueType();
+    SDValue LHS = Val.getOperand(0), RHS = Val.getOperand(1);
+    EVT VT = LHS.getValueType();
     if (!VT.isInteger())
       return false;
     CanNegate = true;
     MustBeFirst = false;
+    // A SETCC leaf whose flag source will be folded by EmitTest onto an
+    // OR/ADD/XOR node cannot appear in a non-root slot of a CCMP chain:
+    // CCMP is a subtract and CTEST is an AND, so neither can reproduce this
+    // leaf's SF/ZF. Force such a leaf to the chain root, where its flag
+    // source becomes a plain flag-setting instruction and its EFLAGS feed
+    // the next CCMP. This happens for icmp-vs-0 (and the icmp-vs-{-1,1}
+    // forms TranslateX86CC rewrites into icmp-vs-0) whose LHS is an
+    // OR/ADD/XOR the DAG will re-use for flags.
+    if (auto *C = dyn_cast<ConstantSDNode>(RHS)) {
+      ISD::CondCode CC = cast<CondCodeSDNode>(Val.getOperand(2))->get();
+      bool VsZero = C->isZero() || (CC == ISD::SETGT && C->isAllOnes()) ||
+                    (CC == ISD::SETLT && C->isOne());
+      if (VsZero && LHS.hasOneUse()) {
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phoebewang wrote:

Does hasOneUse matter here? What to do for multi use LOpc?

https://github.com/llvm/llvm-project/pull/217682