[llvm-dev] RFC: Adding argument allocas

[Reid Kleckner via llvm-dev]

On Thu, Dec 8, 2016 at 5:37 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:

> So IIUC basically the *only* reason for this IR change is that we don’t
> want to pattern match in debug build?
> I don't understand right now why we wouldn’t want to do this?
>

If we need to pattern match such a basic construct, it suggests to me that
we have the wrong representation, and we should instead make our
representation more accurately model reality. To me, it feels like this
representation allows several good things to just "fall out" without any
additional work, and that suggests it's a good representation.

On Thu, Dec 8, 2016 at 5:47 PM, Friedman, Eli <efriedma at codeaurora.org>
wrote:
>
> One other questionable side benefit of doing this is that it would make it
>> possible to implement va_start by taking the address of a parameter and
>> doing pointer arithmetic. While that code is fairly invalid, it’s exactly
>> what the MSVC STL headers do for 32-bit x86. If we make this work in Clang,
>> we can remove our stdarg.h and vadefs.h wrapper headers. Users often pass
>> flags that cause clang to skip these wrapper headers, and then they file
>> bugs complaining that va lists don't work.
>>
>
> This ignores the other reason why this doesn't work at the moment: reading
> off the end of an alloca returns undef, whether or not the actual pointer
> value points at the data you want.  I mean, it's not impossible to make it
> work, but it involves a bunch of unrelated changes which I don't think we
> want to pursue.


I guess I'm more willing to pursue those changes at some vague unspecified
point in the future. It doesn't seem that crazy to add a special case to
allow OOB GEPs of off the last argument alloca of a variadic function.
Other OOB GEPs can continue to be UB as they are today. We have other
loopholes for these kinds of OOB GEP optimizations around zero-sized
globals so that people can use __start_section/__stop_section symbols to
implement .init_array style registration systems. This doesn't seem that
different.
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