[Patch] [Aarch64] PR rtl-optimization/87763 - this patch fixes gcc.target/aarch64/lsl_asr_sbfiz.c

Jeff Law law@redhat.com
Fri Apr 26 23:54:00 GMT 2019


On 4/26/19 4:30 PM, Richard Earnshaw (lists) wrote:
>>> It makes most sense if the mode for extv<mode> is the same both in and out
>>> (it has only one mode in the pattern name, to start with), and for
>>> sign_extract to be similar.  The docs aren't quite clear, but defining it
>>> to have multiple modes doesn't really solve anything afaics, subregs work
>>> just fine here?
>> You'd have to scatter them in the MD file.  That's generally frowned upon.
> 
> A subreg on a reg is fine (which is what we'd have in this specific
> case).  It's when the subreg gets left on something else (other than a
> mem) when the problems start.
I've generally found that we're better off avoiding (subreg (reg)) when
we reasonably can inside MD patterns.  But I won't object if you'd
prefer to go with a subreg style solution.

> 
>>
>> My argument is that we have very clear semantics here.  We're taking a
>> field from an object in a mode.  We zero or sign extract it to the mode
>> of the destination.   Two modes actually make reasonable sense here.
>>
>> Think of it like zero_extend or sign_extend, but for a bitfield.
> 
> This certainly makes sense, but the cases are more general since the
> extraction might produce a result that is either wider or narrower than
> the original reg being acted upon.
True, but aarch64 promotes just about everything to at least SImode.  So
supporting QI/HI destinations doesn't seem to make sense.  Supporting
different modes on the source could make sense, though I suspect the
vast majority of the benefit will be from SImode & DImode.

How do you want to proceed?   I'm just trying to move things along and
will happily step aside if you'd prefer to go with something like
Steve's approach.

Jeff



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