[clang] [llvm] [ARM] musttail fixes (PR #102896)

[Eli Friedman via cfe-commits]

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@@ -3088,70 +3057,44 @@ bool ARMTargetLowering::IsEligibleForTailCallOptimization(
           getEffectiveCallingConv(CalleeCC, isVarArg),
           getEffectiveCallingConv(CallerCC, CallerF.isVarArg()), MF, C, Ins,
           CCAssignFnForReturn(CalleeCC, isVarArg),
-          CCAssignFnForReturn(CallerCC, CallerF.isVarArg())))
+          CCAssignFnForReturn(CallerCC, CallerF.isVarArg()))) {
+    LLVM_DEBUG(dbgs() << "false (incompatible results)\n");
     return false;
+  }
   // The callee has to preserve all registers the caller needs to preserve.
   const ARMBaseRegisterInfo *TRI = Subtarget->getRegisterInfo();
   const uint32_t *CallerPreserved = TRI->getCallPreservedMask(MF, CallerCC);
   if (CalleeCC != CallerCC) {
     const uint32_t *CalleePreserved = TRI->getCallPreservedMask(MF, CalleeCC);
-    if (!TRI->regmaskSubsetEqual(CallerPreserved, CalleePreserved))
+    if (!TRI->regmaskSubsetEqual(CallerPreserved, CalleePreserved)) {
+      LLVM_DEBUG(dbgs() << "false (not all registers preserved)\n");
       return false;
+    }
   }
 
-  // If Caller's vararg or byval argument has been split between registers and
+  // If Caller's vararg argument has been split between registers and
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efriedma-quic wrote:

What case is this trying to detect?  The varargs convention is exactly the same as the non-varargs convention, as far as I remember...

https://github.com/llvm/llvm-project/pull/102896