[clang] [CIR] Add implicit return zero handling (PR #194490)

[Erich Keane via cfe-commits]

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@@ -514,6 +514,21 @@ void CIRGenFunction::startFunction(GlobalDecl gd, QualType returnType,
     }
     emitAndUpdateRetAlloca(returnType, getLoc(bodyEndLoc),
                            getContext().getTypeAlignInChars(returnType));
+
+    // If this is an implicit-return-zero function, initialize the return
+    // value. This mirrors the implicit-return-zero handling in classic
+    // codegen's EmitFunctionProlog (CGCall.cpp). It is done here, after
+    // emitAndUpdateRetAlloca, because in CIR the return slot is created
+    // after the prolog (the opposite of classic codegen, where ReturnValue
+    // is set up before EmitFunctionProlog runs).
+    // TODO(cir): Align prolog handling with classic codegen.
+    if (fd && fd->hasImplicitReturnZero()) {
+      mlir::Type cirRetTy = convertType(returnType.getUnqualifiedType());
+      mlir::Location bodyBeginMLIRLoc = getLoc(bodyBeginLoc);
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erichkeane wrote:

Silly question: should this be the END of the body?  In case it has actual stuff in it? 

https://github.com/llvm/llvm-project/pull/194490