[PATCH] D98363: [Sema] Fold VLA types in compound literals to constant arrays.

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


================
Comment at: clang/test/Sema/vla.c:134
+  // expected-warning at +1{{variable length array folded to constant array as an extension}}
+  char (*a9)[] = (char[2][ksize]) {{1,2,3,4},{4,3,2,1}};
+
----------------
aaron.ballman wrote:
> efriedma wrote:
> > aaron.ballman wrote:
> > > efriedma wrote:
> > > > aaron.ballman wrote:
> > > > > Doesn't this violate the constraints in C17 6.5.2.5p1, "The type name shall specify a complete object type or an array of unknown size, but not a variable-length array type"?
> > > > Yes, this is a constraint violation.  This patch downgrades the error to a warning, for compatibility with older versions of clang.
> > > It was an error in older versions of Clang, so downgrading it to a warning can't make code *more* compatible with older versions. This also makes it harder to port C code to other compilers because Clang happily accepts code that the C standard says should be rejected.
> > > 
> > > I'm not strongly opposed to the change, but I also don't like ignoring constraint violations in the standard as that comes awfully close to what `-fpermissive` does (and at least with that awful flag you have to opt *into* the less strictly conforming behavior instead of getting it by default as with extensions).
> > > 
> > > I'm curious if @rsmith has thoughts here?
> > > It was an error in older versions of Clang
> > 
> > https://godbolt.org/z/rvbffY
> Oh, I hadn't realized this changed *that* recently! Is this breaking some significant amounts of code now that we err on it (or regressing performance by not folding the VLA)?
> 
> My point still stands about disliking when we ignore constraint violations. To expound a bit, in this particular case, I think the standard is being over-constraining because we reasonably *can* fold this to a more efficient form. Normally, I'd suggest filing a DR with WG14 over this and treating the constraint as UB we can extend. However, I think the "solution" that comes out of WG14 would (likely) be to make compound literal expressions of VLA types be undefined behavior rather than a constraint violation (because I don't see many other options in the standard wording for a more targeted fix to allow *just* this safe usage). Given the security concerns around misuse of VLAs already, I think that would be a worse world to live in even if it gets us what we want with this specific case in mind. This is where my caution is coming from. Knowing a bit more about the experience in the proprietary code bases would be helpful, if it's something you can share more details about.
> Is this breaking some significant amounts of code now that we err on it (or regressing performance by not folding the VLA)?

We only ever constant-fold VLAs to constant-length arrays in cases where a VLA would be invalid, so there's no regressing-performance concern. (We used to do such folding, but it resulted in our miscompiling certain testcases, which is why Clang's behavior changed recently.) As a result, there's no risk of someone seeing a surprise VLA here, that should have been a constraint violation.

Given the above, I don't think that changing this from a constraint violation to UB in the C standard would make a difference. Either way, I think a strictly conforming program cannot contain such a construct, and a conforming implementation can, as an extension, define the meaning of programs containing such a construct.

It's common for us to have conforming extensions that permit us to accept more real-world code. Accepting with an on-by-default warning is often the best tradeoff between accepting real-world code and diagnosing non-portable constructs, especially in cases where past versions of GCC (<= 4.4) and Clang accepted the code as an extension. I'm inclined to think that's the best we can do here.

If we want to more strongly discourage use of this extension (and I don't have any particular preferences on whether we should do so), we could promote the warning to `DefaultError` (but leave the extension in, so that people can still build legacy codebases), but I think we should do that consistently across all the places we fold VLAs to constant arrays, not only for compound literals.


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  https://reviews.llvm.org/D98363/new/

https://reviews.llvm.org/D98363