[PATCH] D152279: [Driver] Default -msmall-data-limit= to 0

[Aditya Kumar via Phabricator via cfe-commits]

hiraditya added a comment.

In D152279#4406896 <https://reviews.llvm.org/D152279#4406896>, @MaskRay wrote:

> In D152279#4405901 <https://reviews.llvm.org/D152279#4405901>, @asb wrote:
>
>> One of the key things we've been discussing on this at the LLVM call is that we probably want to keep the small data limit for embedded targets.
>
> It'd be useful to hear from some concrete embedded target users, whether they customize `-msmall-data-limit=`, whether they are happy with `-msmall-data-limit=8`, or whether they are just unaware of this threshold.
>
> If an embedded system typically customizes compiler driver options a lot, I think they can consider adding `-msmall-data-limit=` (with a value suitable for their use cases) in a configuration file <https://clang.llvm.org/docs/UsersManual.html#configuration-files>, not letting their use cases dictate Linux systems.
>
> I putting up the patch partly came from my stance finding UX of this option is really confusing and misleading. I wish that the embedded target users can give me compelling arguments to keep `-msmall-data-limit=8` (and not another value).

The default of 8 is probably to make it consistent with gcc. Here is text from man gcc <https://man7.org/linux/man-pages/man1/gcc.1.html>

  -msmall-data
  -mlarge-data
      When -mexplicit-relocs is in effect, static data is accessed
      via gp-relative relocations.  When -msmall-data is used,
      objects 8 bytes long or smaller are placed in a small data
      area (the ".sdata" and ".sbss" sections) and are accessed via
      16-bit relocations off of the $gp register.  This limits the
      size of the small data area to 64KB, but allows the variables
      to be directly accessed via a single instruction.
  
      The default is -mlarge-data.  With this option the data area
      is limited to just below 2GB.  Programs that require more
      than 2GB of data must use "malloc" or "mmap" to allocate the
      data in the heap instead of in the program's data segment.
  
      When generating code for shared libraries, -fpic implies
      -msmall-data and -fPIC implies -mlarge-data.


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