[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

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

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

[dschuff]

lib/CodeGen/CGCall.cpp

1849

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

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/CGCall.cpp

if (TargetDecl) {
     if (TargetDecl->hasAttr<ColdAttr>())
       FuncAttrs.addAttribute(llvm::Attribute::Cold);
     if (TargetDecl->hasAttr<NoDuplicateAttr>())
       FuncAttrs.addAttribute(llvm::Attribute::NoDuplicate);
     if (TargetDecl->hasAttr<ConvergentAttr>())
       FuncAttrs.addAttribute(llvm::Attribute::Convergent);
 
     if (const FunctionDecl *Fn = dyn_cast<FunctionDecl>(TargetDecl)) {
+      if (!AttrOnCallSite && !Fn->hasPrototype())
+        FuncAttrs.addAttribute("no-prototype");
       AddAttributesFromFunctionProtoType(
           getContext(), FuncAttrs, Fn->getType()->getAs<FunctionProtoType>());
       // Don't use [[noreturn]] or _Noreturn for a call to a virtual function.
       // These attributes are not inherited by overloads.
       const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(Fn);
       if (Fn->isNoReturn() && !(AttrOnCallSite && MD && MD->isVirtual()))
         FuncAttrs.addAttribute(llvm::Attribute::NoReturn);
     }
 

test/CodeGen/no-prototype.c

+// RUN: %clang_cc1 -triple x86_64-apple-darwin -emit-llvm %s -o - | FileCheck %s
+
+extern void foo();
+
+void bar(int a) {
+  foo(1);
+}
+
+// CHECK: declare void @foo(...) [[NOPROTO:#[0-9]+]]
+// CHECK: attributes [[NOPROTO]] = {  {{.*}}"no-prototype"{{.*}} }