[cfe-dev] Applying custom filters after preprocessing (and compile-time sprintf optimization)

[Richard Smith]

On 2 Oct 2014 18:56, "Kazuho Oku" <kazuhooku at gmail.com> wrote:
>
>
>
> 2014-10-03 9:31 GMT+09:00 Richard Smith <richard at metafoo.co.uk>:
>>
>> On Thu, Oct 2, 2014 at 5:18 PM, Kazuho Oku <kazuhooku at gmail.com> wrote:
>>>
>>> Hi,
>>>
>>> Recently I have started writing a preprocessor that wraps the C
preprocessor for optimizing calls to sprintf, since it is the one of the
bottlenecks of my application.
>>> https://github.com/kazuho/qrintf
>>>
>>> For GCC, it was easy for me to override the default C preprocessor, by
using its `-no-integrated-cpp` and `-wrapper` options (how I have done it
can be found in the `qrinf-gcc` and `qrintf-gcc-wrapper` files included in
the above repository).
>>>
>>> However I have so far been unable to find a similar command-line option
for Clang.  Is there any way to post-process the output of the preprocessor
in Clang?  Or do I need to modify the source code of Clang itself?
>>>
>>> Thank you in advance.
>>>
>>> PS. BTW it would be great if compile-time optimization of sprintf goes
into Clang.  Is there any chance of such possibility?
>>
>>
>> In the general case, it's not possible to optimize sprintf fully,
because its behavior depends on the current locale (this affects %d and %u,
which it looks like you optimize) and it might have libc-specific
extensions, but LLVM does optimize in some simple cases: for instance, a
sprintf containing no % is optimized into memcpy. I expect patches would be
accepted to implement further cases, such as handling %s and %c.
>
>
> Thank you for the response.  I am delighted to hear that.
>
> I had thought that %d and %u weren't affected by the locale (since the
digits 0..9 and non-monetary use of positive/negative signs are IMO not
locale-dependent).

Sorry, yes, I think you're right: they aren't affected by locale by
default, only if you use the ' or I flags.
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