limiting call clobbered registers for library functions

[Yury Gribov]

On 01/29/2015 08:32 PM, Richard Henderson wrote:
> On 01/29/2015 02:08 AM, Paul Shortis wrote:
>> I've ported GCC to a small 16 bit CPU that has single bit shifts. So I've
>> handled variable / multi-bit shifts using a mix of inline shifts and calls to
>> assembler support functions.
>>
>> The calls to the asm library functions clobber only one (by const) or two
>> (variable) registers but of course calling these functions causes all of the
>> standard call clobbered registers to be considered clobbered, thus wasting lots
>> of candidate registers for use in expressions surrounding these shifts and
>> causing unnecessary register saves in the surrounding function prologue/epilogue.
>>
>> I've scrutinized and cloned the actions of other ports that do the same,
>> however I'm unable to convince the various passes that only r1 and r2 can be
>> clobbered by these library calls.
>>
>> Is anyone able to point me in the proper direction for a solution to this
>> problem ?
>
> You wind up writing a pattern that contains a call,
> but isn't represented in rtl as a call.

Could it be useful to provide a pragma for specifying function register 
usage? This would allow e.g. library writer to write a hand-optimized 
assembly version and then inform compiler of it's binary interface.

Currently a surrogate of this can be achieved by putting inline asm code 
in static inline functions in public library headers but this has it's 
own disadvantages (e.g. code bloat).

-Y