On Wed, Jun 23, 2010 at 4:50 PM, Paolo Bonzini <bonzini@gnu.org> wrote:
> 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.

I am learning new things all the time. Thanks for the pointer.

Here is a new patch without UNSPEC.  OK for trunk?

Thanks.

-- 
H.J.
---
gcc/

2010-06-24  H.J. Lu  <hongjiu.lu@intel.com>

	PR target/44588
	* config/i386/i386.md (extract_code): New.
	(<u>divmodqi4): Likewise.
	(divmodhiqi3): Likewise.
	(udivmodhiqi3): Likewise.
	(<u>divqi3): Remvoved.

gcc/testsuite/

2010-06-24  H.J. Lu  <hongjiu.lu@intel.com>

	PR target/44588
	* gcc.target/i386/mod-1.c: New.
	* gcc.target/i386/umod-1.c: Likewise.
	* gcc.target/i386/umod-2.c: Likewise.
	* gcc.target/i386/umod-3.c: Likewise.