[PATCH] D79121: Add nomerge function attribute to clang

[Reid Kleckner via Phabricator via cfe-commits]

rnk added inline comments.


================
Comment at: clang/include/clang/Basic/Attr.td:1799
+  let Spellings = [Clang<"nomerge">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [NoMergeDocs];
----------------
aaron.ballman wrote:
> zequanwu wrote:
> > rnk wrote:
> > > Clang supports many function-like things that are not functions, for example, Objective C method declarations are not considered FunctionDecls in the AST. Blocks are another function-like thing. I see below there is a `FunctionLike` subject type. Lets use that here.
> > > 
> > > I would like to make it possible to apply this attribute directly to call expressions as well, but I don't see a good example of how to do that in the existing attribute. I'd like to be able to do this, for example:
> > > 
> > > ```
> > > void __attribute__((noreturn)) bar();
> > > void foo(bool cond) {
> > >   if (cond)
> > >     [[clang::nomerge]] bar();
> > >   else
> > >     [[clang::nomerge]] bar();
> > > }
> > > ```
> > > 
> > > This gives the user fine-grained control. It allows them to call some existing API, like perhaps RaiseException, of which they do not control the declaration, and mark the calls as unmergeable.
> > It seems like there is no exiting example of call site attributes. https://github.com/llvm/llvm-project/blob/master/clang/lib/CodeGen/CGCall.cpp#L4767. And the function `EmitCall` may not know existence of call site attributes.
> > I would like to make it possible to apply this attribute directly to call expressions as well, but I don't see a good example of how to do that in the existing attribute.
> 
> You would have to use a statement attribute rather than a declaration attribute for this, and we have a few of those but none that work on call expressions specifically.
> 
> One question I have about that idea is whether attributes give you fine enough levels of control (I don't know enough about `nomerge` to answer this myself). e.g., `foo(bar(), bar()); // Do you want to nomerge the bar() calls, the foo() call, or all three?` (You can only add the attribute to the statement expression, not to individual expressions.)
I guess what I really wanted was an expression attribute, so you can put it on the calls to `bar`. :)

But given that that doesn't exist, I think it would be OK to do either one of the following:
- Apply nomerge attribute to all call sites in the whole Stmt
- Apply nomerge only to the first CallExpr found inside the Stmt

I think the simplest implementation is to, in CodeGenFunction, set a flag (NoMerge) on CodeGenFunction when emitting such an attributed statement with this attribute. Then set it back on exit. Something like that.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D79121/new/

https://reviews.llvm.org/D79121