Re: libgcc: strange optimization

[Georg-Johann Lay <avr@gjlay.de>]

Richard Guenther wrote:
> On Tue, Aug 2, 2011 at 10:48 AM, Mikael Pettersson <mikpe@it.uu.se> wrote:
>> Hans-Peter Nilsson writes:
>>  > On Mon, 1 Aug 2011, Richard Henderson wrote:
>>  >
>>  > > On 08/01/2011 01:30 PM, Michael Walle wrote:
>>  > > >  1) function inlining
>>  > > >  2) deferred argument evaluation
>>  > > >  3) because our target has no barrel shifter, (arg >> 10) is emitted as a
>>  > > > function call to libgcc's __ashrsi3 (_in place_!)
>>  > > >  4) BAM! dead code elimination optimizes r8 assignment away because calli
>>  > > > may clobber r1-r10 (callee saved registers on lm32).
>>  > >
>>  > > I'm afraid the only solution I can think of is to force F1 out-of-line.
>>  >
>>  > Or another temporary - but the parameter should already have
>>  > that effect.
>>
>> It should, but doesn't.  See PR48863 for similar breakage on ARM.
> 
> On GIMPLE we don't see either the libcall nor those "dependencies".
> 
> Don't use register vars.

IMO such code is supposed to work, e.g. in order to write an interface
to a non-ABI assembler function.  In general this cannot be expressed
by means of constraints because that would imply plethora of different
constraints for each register/mode.

From the documentation a user will expect that he wrote correct code
and is not supposed to bother with GCC inerts like implicit library
calls or GIMPLE or whatever.

Johann

> Richard.