Kyrill Tkachov <kyrylo.tkac...@foss.arm.com> writes: > On 05/03/16 05:52, Jeff Law wrote: >> On 03/04/2016 09:33 AM, Kyrill Tkachov wrote: >>> >>> On 04/03/16 16:21, Jeff Law wrote: >>>> On 03/04/2016 08:05 AM, Richard Biener wrote: >>>>>> does that mean that the shift amount should be DImode? >>>>>> Seems like a more flexible approach would be for the midend to be able >>>>>> to handle these things... >>>>> >>>>> Or macroize for all integer modes? >>>> That's probably worth exploring. I wouldn't be at all surprised if it >>>> that turns out to be better than any individual mode, not just for >>>> arm & aarch64, but would help a variety of targets. >>>> >>> >>> What do you mean by 'macroize' here? Do you mean use iterators to create >>> multple variants of patterns with different >>> modes on the shift amount? >>> I believe we'd still run into the issue at >>> https://gcc.gnu.org/ml/gcc/2016-03/msg00036.html. >> We might, but I would expect the the number of incidences to be fewer. >> >> Essentially we're giving the compiler multiple options when it comes >> to representation of the shift amount -- allowing the compiler >> (combine in particular) to use the shift amount in whatever mode is >> most natural. ie, if the count is >> sitting in a QI, HI, SI or possibly even a DI register, then it can be >> used as-is. No subregs, no zero/sign extensions, or and-imm masking. >> > > The RTL documentation for ASHIFT and friends says that the shift amount must > be: > "a fixed-point mode or be a constant with mode @code{VOIDmode}; which > mode is determined by the mode called for in the machine description > entry for the left-shift instruction". For example, on the VAX, the mode > of @var{c} is @code{QImode} regardless of @var{m}. > > From what I understand the "ashl" standard name should expand to an > ASHIFT with a particular mode for the shift amount. Currently that's > QImode for aarch64. So whenever combine tries to propagate anything > into the shift amount it has to force it into QImode.
Yeah. It'd be nice to retain a predictable mode if possible. One of the things on my todo list for GCC 7 is to automatically generate a function that gives you the correct shift amount mode for a particular shift value mode, so that there's a lot less guessing. (It's one of the main blockers to having modes on CONST_INTs.) Thanks, Richard
