[llvm] [RISCV] Support tail calls for functions with sret parameters (PR #223999)

[Craig Topper via llvm-commits]

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@@ -27620,12 +27587,23 @@ bool RISCVTargetLowering::isEligibleForTailCallOptimization(
     if (VA.getLocInfo() == CCValAssign::Indirect)
       return false;
 
-  // Do not tail call opt if either caller or callee uses struct return
-  // semantics.
-  auto IsCallerStructRet = Caller.hasStructRetAttr();
+  // If the callee has an sret parameter, conservatively require it to receive
+  // the caller's sret pointer. If only the caller has an sret parameter, treat
+  // that pointer like an ordinary pointer when passing call arguments.
+  // TODO: Support other sret buffers that outlive the caller, such as globals.
+  auto IsCallerStructRet =
+      !Caller.arg_empty() && Caller.getArg(0)->hasStructRetAttr();
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topperc wrote:

Does `!Caller.arg_empty() && Caller.getArg(0)->hasStructRetAttr()` return something different than the original `Caller.hasStructRetAttr()`?

https://github.com/llvm/llvm-project/pull/223999