[PATCH] D64883: Add new warning -Walloca for use of builtin alloca function

[George Burgess IV via Phabricator via cfe-commits]

george.burgess.iv added inline comments.


================
Comment at: clang/include/clang/Basic/DiagnosticSemaKinds.td:2776
+def warn_alloca : Warning<
+  "use of builtin function %0">,
+  InGroup<DiagGroup<"alloca">>, DefaultIgnore;
----------------
aaron.ballman wrote:
> george.burgess.iv wrote:
> > nit: I'd just say "use of function '%0'" here; "builtin" doesn't really add much.
> > 
> > I also wonder if we should be saying anything more than "we found a use of this function." Looks like GCC doesn't (https://godbolt.org/z/sYs_8G), but since this warning is sort of opinionated in itself, might it be better to expand this to "use of '%0' is discouraged"?
> > 
> > WDYT, Aaron?
> What is the purpose to this diagnostic, aside from GCC compatibility? What does it protect against?
> 
> If there's a reason users should not use alloc(), it would be better for the diagnostic to spell it out.
> 
> Btw, I'm okay with this being default-off because the GCC warning is as well. I'm mostly hoping we can do better with our diagnostic wording.
> I'm mostly hoping we can do better with our diagnostic wording

+1

> What is the purpose to this diagnostic?

I think the intent boils down to that `alloca` is easily misused, and leads to e.g., https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt . Since its use often boils down to nothing but a tiny micro-optimization, some parties would like to discourage its use.

Both glibc and bionic recommend against the use of `alloca` in their documentation, though glibc's docs are less assertive than bionic's, and explicitly call out "[alloca's] use can improve efficiency compared to the use of malloc plus free."

Greping a codebase and investigating the first 15 results:
- all of them look like microoptimizations; many of them also sit close to other `malloc`/`new` ops, so allocating on these paths presumably isn't prohibitively expensive
- all but two of the uses of `alloca` have no logic to fall back to the heap `malloc` if the array they want to allocate passes a certain threshold. Some of the uses make it look *really* easy for the array to grow very large.
- one of the uses compares the result of `alloca` to `NULL`. Since `alloca`'s behavior is undefined if it fails, ...

I'm having trouble putting this into a concise and actionable diagnostic message, though. The best I can come up with at the moment is something along the lines of "use of function %0 is subtle; consider using heap allocation instead."


Repository:
  rC Clang

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D64883/new/

https://reviews.llvm.org/D64883