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
