[compiler-rt] r208603 - Move .subsections_via_symbols directives into DEFINE_COMPILERRT_PRIVATE_FUNCTION

Saleem Abdulrasool compnerd at compnerd.org
Mon May 12 17:45:37 PDT 2014

On Mon, May 12, 2014 at 5:30 PM, Nick Kledzik <kledzik at apple.com> wrote:

> On May 12, 2014, at 5:13 PM, Reid Kleckner <rnk at google.com> wrote:
> On Mon, May 12, 2014 at 4:44 PM, Nick Kledzik <kledzik at apple.com> wrote:
>> On May 12, 2014, at 4:18 PM, Jonathan Roelofs <jonathan at codesourcery.com>
>> wrote:
>> >
>> >
>> > On 5/12/14, 3:47 PM, Nick Kledzik wrote:
>> >> FYI, this is a semantic change.   The .subsection_via_symbols
>> directive tells the linker that the .o file has no functions that will fall
>> into the next one (the compiler never does that, but it is common enough in
>> hand written assembly).  That means the linker can dead strip functions not
>> referenced.  By moving .subsection_via_symbols into FILE_LEVEL_DIRECTIVE,
>> you’ve caused every file to opt-in to the directive, even if they have fall
>> through code.
>> >>
>> > I think the real gotcha here is that DEFINE_COMPILERRT_FUNCTION, which
>> defines something that looks like function level scope had/has a global
>> directive in it affecting file level scope…
>> Yes!
>> >> I just did a quick survey of the .S files in compiler-rt and do not
>> see any with fall through code.  Mostly because compiler-rt has the
>> convention of one function per file.
>> > Does it really though? I didn't move .subsection_via_symbols into
>> FILE_LEVEL_DIRECTIVE, it was already there. This only changes it for
>> private functions, and further it makes those behave the same way as
>> non-private ones, which already had it.
>> Oh, sorry I was looking at the final result and did not realize it was
>> half broke before your change.
>> >>
>> >> So, this change should be benign now, but may cause surprises on
>> darwin in the future if someone tries to write fall-through assembly code.
>> > So to eliminate this gotcha, it sounds like you are suggesting that I:
>> >  b) Maybe also remove .subsections_via_symbols from FILE_LEVEL_DIRECTIVE
>> >  c) Add .subsections_via_symbols to every *.S file (possibly inside
>> some other appropriately named macro)
>> >
>> > I'm happy to do this, but I want to make sure I'm clear on what you
>> want.
>> That would be a clean solution, but it is a bunch of work to support a
>> case we don’t have and probably don’t want.  A simpler solution would be:
>> b) Remove definition of FILE_LEVEL_DIRECTIVE
>> c) Having assembly.h out right use  .subsections_via_symbols , e.g.:
>> #if defined(__APPLE__) && !defined(COMPILERRT_NON_STD_DARWIN)
>>   /* Any assembly code that has multiple entry points needs to
>>     before including assembly.h
>>   */
>>   .subsections_via_symbols
>> #endif
>> So, by default it assumes all code follows the darwin conventions (as it
>> currently does), but if an issue arises, there is an easy way to disable
>> .subsections_via_symbols on a file-by-file basis.
> I'm not a compiler-rt person, but I don't think it's good design for
> headers to expect includers to define macros before inclusion.  I think the
> current situation is fine.
> Currently LLVM requires __STDC_LIMIT_MACROS and __STD_CONSTANT_MACROS to
> be defined before some inclusions.  So, the idea is not without precedent...
> .subsections_via_symbols is supposed to be opt-in.  The current state
> forces the opt-in with no way to opt-out.  My proposal is to opt-in by
> default (cause it is generally a good thing).  But it some problem arises,
> give the .S file a way to opt-out be defining COMPILERRT_NON_STD_DARWIN
> before including assembly.h.    I really hope no one ever uses

This would certainly be the solution with the least amount of work.

Unfortunately, there are exceptions to the general behavior of one function
per file (c.f. compares2f.S in lib/builtins/arm).

Having a separate macro for this and including it in explicitly is much
clearer and conscious, but it does leave the possibility of someone
accidentally leaving it off.  That said, CompilerRT is generally pretty
stable and doesn't see much churn in the builtins.

It sounded like your preference (it felt as though objection might be too
strong a term here) was mostly due to amount of effort involved.  I do
agree that it would be preferable to not actively encourage people to not
use .subsections_via_symbols, but, are reviews insufficient to prevent that?

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Saleem Abdulrasool
compnerd (at) compnerd (dot) org
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