[llvm] [BOLT][AArch64] Transform cmpbr ~> cmp + br when inversion not possible (PR #185731)

[Paschalis Mpeis via llvm-commits]

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@@ -2155,38 +2175,76 @@ class AArch64MCPlusBuilder : public MCPlusBuilder {
     }
   }
 
-  bool isReversibleBranch(const MCInst &Inst) const override {
+  bool is32BitVariant(unsigned Opcode) const {
+    switch (Opcode) {
+    default:
+      return false;
+    case AArch64::CBGTWri:
+    case AArch64::CBLTWri:
+    case AArch64::CBHIWri:
+    case AArch64::CBLOWri:
+      return true;
+    }
+  }
+
+  bool isReversibleBranch(const MCInst &Inst,
+                          DataflowInfoManager *DIM = nullptr) const override {
     if (isCompAndBranch(Inst)) {
+      bool IsReversible =
+          DIM ? !DIM->getLivenessAnalysis().getLiveIn(Inst).test(getFlagsReg())
+              : false;
       unsigned InvertedOpcode = getInvertedBranchOpcode(Inst.getOpcode());
-      if (needsImmDec(InvertedOpcode) && Inst.getOperand(1).getImm() == 0)
+      if (needsImmDec(InvertedOpcode) && Inst.getOperand(1).getImm() == 0 &&
+          !IsReversible)
         return false;
-      if (needsImmInc(InvertedOpcode) && Inst.getOperand(1).getImm() == 63)
+      if (needsImmInc(InvertedOpcode) && Inst.getOperand(1).getImm() == 63 &&
+          !IsReversible)
         return false;
     }
     return MCPlusBuilder::isReversibleBranch(Inst);
   }
 
-  void reverseBranchCondition(MCInst &Inst, const MCSymbol *TBB,
+  void reverseBranchCondition(BinaryBasicBlock *Parent, MCInst &Inst,
+                              const MCSymbol *TBB,
                               MCContext *Ctx) const override {
-    if (!isReversibleBranch(Inst)) {
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paschalis-mpeis wrote:

Can you add a comment about this in the header file?

Also, should we add that check in SimplifyConditionalTailCalls too? We don't have to pass DIM in that call.

https://github.com/llvm/llvm-project/pull/185731