llvm dev - Feb 2011 - [LLVMdev] Implementing platform specific library call simplification

The newlib C library provides iprintf(), a restricted version of printf 
without support for floating-point formatting. I'd like to add an 
optimization which turns calls to printf() into calls to iprintf() if 
the format string has no floating point specifiers.

At the moment I've got this working locally by adding code to the 
simplify-libcalls pass. However this will break on targets where 
iprintf() isn't available. Is there a sensible place to add information 
about which library functions are available for a particular target?

Thanks,

Richard

-- 
Richard Osborne | XMOS
http://www.xmos.com

On Feb 2, 2011, at 10:11 AM, Richard Osborne wrote:
> The newlib C library provides iprintf(), a restricted version of printf 
> without support for floating-point formatting. I'd like to add an 
> optimization which turns calls to printf() into calls to iprintf() if 
> the format string has no floating point specifiers.
Cool, ok.  I can see how this would be very useful for a variety of embedded
systems.
> At the moment I've got this working locally by adding code to the 
> simplify-libcalls pass. However this will break on targets where 
> iprintf() isn't available. Is there a sensible place to add information
> about which library functions are available for a particular target?
I'd suggest adding a bool argument (HasIPrintf) to the
createSimplifyLibCallsPass function and the pass constructor.  Then clang (or
whatever is setting up the pass manager) can query the appropriate target info
to pass down this flag.

Longer term, I'd like to introduce a simple interface (via TargetRegistry)
for exposing target libcall info.  This would be useful to eliminate the
HAVE_FLOORF etc #defines in simplifylibcalls, which break cross compilation.

-Chris

Chris and all,

IMHO I think SimplifyLibcall and similar might be provided by rather
clang (and frontends), than LLVM itself.
Frontends might know better than LLVM (as backend infrastructure).

...Takumi

On Feb 13, 2011, at 12:24 AM, Chris Lattner wrote:
> On Feb 2, 2011, at 10:11 AM, Richard Osborne wrote:
>> The newlib C library provides iprintf(), a restricted version of printf
>> without support for floating-point formatting. I'd like to add an 
>> optimization which turns calls to printf() into calls to iprintf() if 
>> the format string has no floating point specifiers.
> 
> Cool, ok.  I can see how this would be very useful for a variety of
embedded systems.
> 
>> At the moment I've got this working locally by adding code to the 
>> simplify-libcalls pass. However this will break on targets where 
>> iprintf() isn't available. Is there a sensible place to add
information
>> about which library functions are available for a particular target?
> 
> I'd suggest adding a bool argument (HasIPrintf) to the
createSimplifyLibCallsPass function and the pass constructor.  Then clang (or
whatever is setting up the pass manager) can query the appropriate target info
to pass down this flag.
> 
> Longer term, I'd like to introduce a simple interface (via
TargetRegistry) for exposing target libcall info.  This would be useful to
eliminate the HAVE_FLOORF etc #defines in simplifylibcalls, which break cross
compilation.
Hi Richard,

Just to close the loop on this, I went ahead and added a new
llvm/Target/TargetLibraryInfo.h and added it to simplifylibcalls. 
TargetLibraryInfo doesn't handle all the libcalls that SimplifyLibCalls
handles yet, but it should provide a starting point for iprintf: just add an
iprintf enum to TargetLibraryInfo and make your transformation predicated on
TLI->has(LibFunc::iprintf).

It would be great to see this go into mainline,

-Chris