Re: PATCH: PR target/44588: Very inefficient 8bit mod/div

[Paolo Bonzini <bonzini@gnu.org>]

On 06/24/2010 01:13 AM, H.J. Lu wrote:
>> I don't understand exactly what the problem was; clearly it couldn't work as
>> long as this pattern was cheating blatantly:
>
> Upper 8bit registers aren't real registers. You can't
> do RA with them.

Please read what other people write.  I'm saying that you should have 
used a HI destination just like you did with your new UNSPEC, which is 
acceptable.

The pattern I quoted below used a QI destination; then magically you 
attempted to extract bit 8..15 of it with an unspec, or something like 
that.  For anything like that the optimizers are going to bite back 
sooner or later.  And you'd deserve it.

>> (define_insn "*<u>divqi3"
>>   [(set (match_operand:QI 0 "register_operand" "=a")
>>         (any_div:QI
>>           (match_operand:HI 1 "register_operand" "0")
>>           (match_operand:QI 2 "nonimmediate_operand" "qm")))
>>
>> Anyway this patch is IMO much nicer than the first ones, so if Uros is okay
>> I don't think it's useful to pursue a more accurate representation.  Just
>> make sure that REG_EQUAL notes are added to the two extractions.
>>
>
> What are REG_EQUAL notes used for?
> As far as the rest of gcc is concerned, there are no upper 8bit
> registers. But you can access bits 8-15 of HI, SI,
> DI registers via XXX_extract.

Understood.  Using an unspec is fine for me, even though it's not an 
approval.  But that's completely orthogonal to putting a REG_EQUAL note 
_on the two regs that you extract out of AX_.  The notes' value should 
be one of

    (subreg:QI (udiv:HI (...) (zero_extend (...))))
    (subreg:QI (div:HI (...) (sign_extend (...))))
    (subreg:QI (umod:HI (...) (zero_extend (...))))
    (subreg:QI (mod:HI (...) (sign_extend (...))))

But I think that with over ten years of GCC experience you do not need 
anyone to tell you this.  In fact, regarding "what are REG_EQUAL notes 
used for?" my first thought was RTFM, but anyway: they are used by CSE, 
fwprop, GCSE and combine to simplify and eliminate redundant expressions.

Paolo