[llvm-dev] Implementing "-Wstack-usage=" warning flag

[Roman Lebedev via llvm-dev]

Thank you for looking into it!

I think while these diagnostics may be niche, they may be
quite important, especially given that clang static analyzer
does not implement a proper check for maximal stack usage
of a program.

How will this cooperate with parallel implementation in
https://reviews.llvm.org/D100509 ?

I think current clang's -Wframe-larger-than= does not match either one
of the GCC options: https://godbolt.org/z/ehTaEMjW5
I guess it only counts the allocas within the function?

There, it is visible that the stack usage is the cumulative size of all
the allocas within the function's body, while frame-size also includes
arguments into that computation.

Which, i think is consistent with
https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
* -Wstack-usage=byte-size
  Warn if the stack usage of a function might exceed byte-size.
  The computation done to determine the stack usage is conservative.
  Any space allocated via alloca, variable-length arrays, or related
  constructs is included by the compiler when determining whether or not
  to issue a warning.
* -Wframe-larger-than=byte-size
  Warn if the size of a function frame exceeds byte-size. The
  computation done to determine the stack frame size is approximate and
  not conservative. The actual requirements may be somewhat greater
  than byte-size even if you do not get a warning. In addition,
  any space allocated via alloca, variable-length arrays,
  or related constructs is not included by the compiler when
  determining whether or not to issue a warning
  -Wframe-larger-than=‘PTRDIFF_MAX’ is enabled by default.
  Warnings controlled by the option can be disabled either by
  specifying byte-size of ‘SIZE_MAX’ or more or by
  -Wno-frame-larger-than.

Roman

On Thu, May 6, 2021 at 4:55 AM David Blaikie <dblaikie at gmail.com> wrote:
>
> +Roman Lebedev
>
> Roman - what was your particular motivation for filing the bug? Is
> there a use case that -Wframe-larger-than doesn't cover? Would
> -Wstack-usage as an alias for -Wframe-larger-than be adequate for your
> use case (if directly using the existing flag is not sufficient for
> some reason)?
>
> On Wed, May 5, 2021 at 6:32 PM Ryan Santhirarajan via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hello everyone,
> >
> >
> >
> > I am trying to address the following bug:
> > https://bugs.llvm.org/show_bug.cgi?id=44418
> >
> > Which asks for the implementation of the missing “-Wstack-usage=” which will report, on a per-function basis, if the stack usage exceeds the user input.
> >
> >
> >
> > My first approach at implementing “-Wstack-usage” is the following:
> >
> > https://reviews.llvm.org/D101965
> >
> > This is virtually identical to the output of “-Wframe-larger-than” on clang. However when observing the behaviour of GCC I noticed that GCC outputs different values for “-Wstack-usage” and “-Wframe-larger-than”.
> >
> >
> >
> > This led me to my second approach which changes the way clang reports “-Wframe-larger-than”. This can be found here:
> > https://reviews.llvm.org/D101964
> >
> > With this second approach, “-Wframe-larger-than” now reports the stack size minus any space allocated on the stack for the parameters of called functions. This more closely resembles GCC’s behaviour.
> >
> >
> >
> > I am unsure which implementation is preferred/correct and I am wondering if anyone can comment or provide some information on what a better approach would be, and what exactly the two flags should be reporting.
> >
> >
> >
> > Thanks,
> >
> > Ryan Santhirarajan
> >
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