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

Tue May 18 11:13:32 PDT 2021

On Tue, May 18, 2021 at 7:51 AM Ryan Santhirarajan <rsanthir at quicinc.com>
wrote:

> Hi Roman,
>
> It seems that there isn't really a consensus on what the most "correct"
> approach is as there already exists a discrepancy between LLVM and GCC
> behaviour. I suggest that we proceeed with the less invasive of the two
> approaches, https://reviews.llvm.org/D101965, and if we decide to update
> "-Wframe-larger-than=" and "-Wstack-usage=" that can be addressed later. As
> it stands now the two flags will report similar information.
>
> My change cooperates well with the change you pointed out,
> https://reviews.llvm.org/D100509 , it actually reports very similar
> results albeit in different forms.

Is there any particular benefit to them being in different forms, rather
than having the stack-usage be an alias for frame-larger-than for now?

>
>
> Regards,
> Ryan
>
>
> -----Original Message-----
> From: Roman Lebedev <lebedev.ri at gmail.com>
> Sent: Thursday, May 6, 2021 12:35 AM
> To: David Blaikie <dblaikie at gmail.com>
> Cc: Ryan Santhirarajan <rsanthir at quicinc.com>; llvm-dev at lists.llvm.org;
> Chris Jenneisch <chrisj at quicinc.com>; Pengxuan Zheng <pzheng at quicinc.com>
> Subject: [EXT] Re: [llvm-dev] Implementing "-Wstack-usage=" warning flag
>
> 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
> > >
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > llvm-dev at lists.llvm.org
> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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