On Fri, Sep 29, 2017 at 04:29:25PM +0100, Maciej W. Rozycki wrote: > On Tue, 19 Sep 2017, James Hogan wrote: > > > Avoid modifying the input by using a temporary variable as an output > > which is modified instead of the input and not otherwise used. The asm > > is always __volatile__ so GCC shouldn't optimise it out. The low > > register of the temporary output is written before the high register of > > the input is read, so we have two constraint alternatives, one where > > both use the same registers (for when the input value isn't subsequently > > used), and one with an early clobber on the output in case the low > > output uses the same register as the high input. This allows the > > resulting assembly to remain mostly unchanged. > > Clever and well-spotted! > > Reviewed-by: Maciej W. Rozycki <macro@xxxxxxxxxx> Thanks Maciej! > > NB you could use DINS on MIPS64r2+ (and `__read_64bit_c0_split' could > use one instruction less; patch posted separately). Yes, that did occur to me, but it seemed simpler for this non-critical case to use the more compatible sequence (whereas for the equivalent VZ guest accessors, which imply R5+, I've used DINS in my not-yet-submitted patches). Cheers James
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