[WebAssembly] Add no-prototype attribute to prototype-less C functions

[sbc100]

Summary

The WebAssembly backend in particular benefits from being
able to distinguish between varargs functions (...) and prototype-less
C functions.

See https://reviews.llvm.org/D48471

[sbc100]

wrong test

[jgravelle-google]

Looks very reasonable and straightforward. LGTM in spirit, but I'll wait for someone who knows Clang better.

[dschuff]

lib/CodeGen/CGCall.cpp

1850 ↗

(On Diff #152335)

Would it make sense to put this in AddAttributesFormFunctionProtoType Does this have the same effect as that would? For that matter, what's the practical difference between this call and the one on line 1826?

[aheejin]

lib/CodeGen/CGCall.cpp

1849 ↗

(On Diff #152335)

Is there a reason why this is not something like llvm::Attribute::NoPrototype like other attributes?

[dschuff]

lib/CodeGen/CGCall.cpp

1849 ↗

(On Diff #152335)

Target-independent attributes get enums, and target-specific attributes are just strings: https://llvm.org/docs/LangRef.html#attribute-groups
We could potentially make this attribute target-independent if there is wider interest in removing the reliance on bare (...) signatures to signify prototypeless C functions. I would like to see more details of how we plan to handle this in the backend before I have any idea about what anyone else might thing.

[dschuff]

lib/CodeGen/CGCall.cpp

1849 ↗

(On Diff #152335)

Actually, I just noticed that we don't appear to have made this actually target-dependent (the test uses X86). So we should either make this attribute actually wasm-only, or make it an enum.

[sbc100]

• feedback

[sbc100]

OK, I made this target specific. I think this is cleaner now.

[sunfish]

I haven't thought through all the possibilities related to !FD->doesThisDeclarationHaveABody(), but overall this looks good.

[sbc100]

Without the check for doesThisDeclarationHaveABody() the attribute will also be added to baz which is IIRC is not what we want since the C notions of "prototypeless" only applies to forward declarations I think.

lib/CodeGen/TargetInfo.cpp

private:
   Address EmitVAArg(CodeGenFunction &CGF, Address VAListAddr,
                     QualType Ty) const override;
 };
 
 class WebAssemblyTargetCodeGenInfo final : public TargetCodeGenInfo {
 public:
   explicit WebAssemblyTargetCodeGenInfo(CodeGen::CodeGenTypes &CGT)
       : TargetCodeGenInfo(new WebAssemblyABIInfo(CGT)) {}
+
+  void setTargetAttributes(const Decl *D, llvm::GlobalValue *GV,
+                           CodeGen::CodeGenModule &CGM) const override {
+    if (auto *FD = dyn_cast_or_null<FunctionDecl>(D)) {
+      llvm::Function *Fn = cast<llvm::Function>(GV);
+      if (!FD->doesThisDeclarationHaveABody() && !FD->hasPrototype())
+        Fn->addFnAttr("no-prototype");
+    }
+  }
 };
 
 /// Classify argument of given type \p Ty.
 ABIArgInfo WebAssemblyABIInfo::classifyArgumentType(QualType Ty) const {
   Ty = useFirstFieldIfTransparentUnion(Ty);
 
   if (isAggregateTypeForABI(Ty)) {
     // Records with non-trivial destructors/copy-constructors should not be

test/CodeGen/no-prototype.c

+// REQUIRES: webassembly-registered-target
+// RUN: %clang_cc1 -triple wasm32 -emit-llvm %s -o - | FileCheck %s
+
+int foo();
+
+int bar(int a) {
+  return foo();
+}
+
+int baz() {
+  return foo();
+}
+
+// CHECK: define i32 @bar(i32 %a) [[BAR_ATTR:#[0-9]+]] {
+// CHECK: declare i32 @foo(...) [[FOO_ATTR:#[0-9]+]]
+// CHECK: define i32 @baz() [[BAZ_ATTR:#[0-9]+]] {
+
+// CHECK: attributes [[FOO_ATTR]] = {  {{.*}}"no-prototype"{{.*}} }
+// CHECK-NOT: attributes [[BAR_ATTR]] = {  {{.*}}"no-prototype"{{.*}} }
+// CHECK-NOT: attributes [[BAZ_ATTR]] = {  {{.*}}"no-prototype"{{.*}} }