[dragonegg PATCH] Turn off libcall simplification in C and C++ when -fno-builtin is specified

[Peter Collingbourne]

On Mon, Apr 15, 2013 at 10:28:23AM +0200, Duncan Sands wrote:
> Hi Peter,
>
>> This patch turns off libcall simplification in C and C++ when
>> -fno-builtin is specified.
>>
>> For now I decided to leave the other languages alone, as this is
>> probably insufficient for those languages [1].
>>
>> OK to commit?
>
> I think this patch has several problems.  First off, I'm pretty sure that only
> the C-like front-ends define flag_no_builtin, so your patch means that you would
> no longer be able to use the plugin for non-C languages as the reference to
> flag_no_builtin would fail to resolve when loading the plugin.  Secondly, it
> doesn't do the right thing if someone uses -fno-builtin-FUNCTION.
>
> That said, I think something like this is needed, for the following reason.
> The -fno-builtin-FUNCTION flags can be supported easily enough because it is
> possible to query GCC to find out if the FUNCTION builtin is available (this
> doesn't work well for non-C languages because they tend to only set things up
> for a sprinkling of builtins they are interested in, not all builtins, but that
> can be worked around).  With this implemented, -fno-builtin will automatically
> turn off all libcall simplifications for functions that GCC knows about.  But
> LLVM has standard library optimizations for functions that GCC doesn't know
> about, and this mechanism won't ever be able to turn those off.  The only
> solution I see to that is turning off all LLVM libcall simplifications if the
> user passes -fno-builtin.  Conclusion: your patch is needed, but you are going
> to have to define flag_no_builtin as a weak symbol so that non-C languages
> continue to work.

Hi Duncan,

I agree with your reasoning.  I looked into this some more and it
looks like flag_no_builtin is defined as a macro representing a field
reference starting from gcc 4.6 onwards, while in previous versions
it is an ordinary variable as you describe.  So I'll modify the patch
to use the weak reference for gcc <= 4.5.

Thanks,
-- 
Peter