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

[Eli Friedman via cfe-commits]

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@@ -3056,18 +3014,26 @@ bool ARMTargetLowering::IsEligibleForTailCallOptimization(
   // Exception-handling functions need a special set of instructions to indicate
   // a return to the hardware. Tail-calling another function would probably
   // break this.
-  if (CallerF.hasFnAttribute("interrupt"))
+  if (CallerF.hasFnAttribute("interrupt")) {
+    LLVM_DEBUG(dbgs() << "false (interrupt attribute)\n");
     return false;
+  }
 
-  if (canGuaranteeTCO(CalleeCC, getTargetMachine().Options.GuaranteedTailCallOpt))
+  if (canGuaranteeTCO(CalleeCC,
+                      getTargetMachine().Options.GuaranteedTailCallOpt)) {
+    LLVM_DEBUG(dbgs() << (CalleeCC == CallerCC ? "true" : "false")
+                      << " (guaranteed tail-call CC)\n");
     return CalleeCC == CallerCC;
+  }
 
   // Also avoid sibcall optimization if either caller or callee uses struct
   // return semantics.
   bool isCalleeStructRet = Outs.empty() ? false : Outs[0].Flags.isSRet();
   bool isCallerStructRet = MF.getFunction().hasStructRetAttr();
-  if (isCalleeStructRet || isCallerStructRet)
+  if (isCalleeStructRet != isCallerStructRet) {
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efriedma-quic wrote:

Nevermind, got it confused with a different ABI.  I don't think we need to impose a restriction on sret for 32-bit Arm.

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