[clang] [llvm] [Clang][inlineasm] Add special support for "rm" output constraints (PR #92040)

[Nikita Popov via llvm-commits]

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@@ -2892,13 +2892,19 @@ void CodeGenFunction::EmitAsmStmt(const AsmStmt &S) {
     if (!Constraints.empty())
       Constraints += ',';
 
-    // If this is a register output, then make the inline asm return it
-    // by-value.  If this is a memory result, return the value by-reference.
+    // - If this is a register output, then make the inline asm return it
+    //   by-value.
+    // - If this is an "rm" constraint, then treat it like a register output.
+    //   (We'll correct this before ISel if using the fast register allocator.)
+    // - If this is a memory result, return the value by-reference.
     QualType QTy = OutExpr->getType();
     const bool IsScalarOrAggregate = hasScalarEvaluationKind(QTy) ||
                                      hasAggregateEvaluationKind(QTy);
-    if (!Info.allowsMemory() && IsScalarOrAggregate) {
+    const bool RegisterMemoryConstraints =
+        OutputConstraint == "rm" || OutputConstraint == "mr";
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nikic wrote:

Should avoid matching the constraint string directly. I think the right logic here may be to replace !allowsMemory with allowsRegister?

https://github.com/llvm/llvm-project/pull/92040