Re: [PATCH v2 3/5] C: Implement musttail attribute for returns

[Joseph Myers <josmyers@redhat.com>]

On Fri, 26 Jan 2024, Andi Kleen wrote:

> > > I don't have tests for that but since it's not new behavior I suppose
> > > that's sufficient.
> > 
> > Each attribute should have tests that invalid uses are appropriately 
> > diagnosed.  See gcc.dg/c23-attr-fallthrough-2.c for examples of such tests 
> > in the case of the [[fallthrough]] attribute.  Some invalid uses may be 
> > diagnosed by existing code that's generic across attributes, others 
> > require specific code for the individual attribute.
> 
> Okay I can add a test for the other statement and declaration cases like
> below.
> 
> Any other change you need for approval?

I use testcases as a key part of the review of a patch, to see if the 
behavior is as I'd expect, so will need to see the updated patch series.

As we're in regression-fixing mode for GCC 14, a new feature like this 
will need to wait for consideration until after GCC 14 branches.

> > The default parsing of an attribute without an entry in the table of 
> > attribute handlers is that arbitrary balanced token sequences are parsed 
> > and discarded as arguments.
> 
> And it triggers a warning too (see below)

For attribute arguments, the key test is [[gnu::musttail()]] on a return 
statement where the attribute would be valid were it not for the attribute 
arguments.

> BTW I noticed that [[musttail]] ; (empty statement with attribute) gives an error, which
> is probably a (unrelated) bug, afaik that should be legal for C23.

That's defined in the standard as an attribute declaration, not an 
attribute on a statement (empty statements can't have attributes).  The 
only currently supported attribute valid in an attribute declaration is 
[[fallthrough]].

When you give an attribute in C23 syntax without a namespace (so 
[[musttail]] as opposed to [[gnu::musttail]]), if it's not a known 
standard attribute then it fails the constraint in 6.7.12.1p2, "The 
identifier in a standard attribute shall be one of: [list]".

-- 
Joseph S. Myers
josmyers@redhat.com