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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Jun 22, 2010 at 11:05 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Mon, 2010-06-21 at 12:33 -0700, H.J. Lu wrote:
>> Hi,
>>
>> This patch adds 8bit divmov pattern for x86. X86 8bit divide
>> instructions return result in AX with
>>
>> AL <- Quotient
>> AH <- Remainder
>>
>> This patch models it and properly extends quotient. Tested
>> on Intel64 with -m64 and -m32. ?There are no regressions.
>> OK for trunk?
>>
>> BTW, there is only one divb used in subreg_get_info in
>> gcc compilers. The old code is
>>
>> ? ? ? ? movzbl ?mode_size(%r13), %edi
>> ? ? ? ? movzbl ?mode_size(%r14), %esi
>> ? ? ? ? xorl ? ?%edx, %edx
>> ? ? ? ? movl ? ?%edi, %eax
>> ? ? ? ? divw ? ?%si
>> ? ? ? ? testw ? %dx, %dx
>> ? ? ? ? jne ? ? .L1194
>>
>> The new one is
>>
>> ? ? ? ? movzbl ?mode_size(%r13), %edi
>> ? ? ? ? movl ? ?%edi, %eax
>> ? ? ? ? divb ? ?mode_size(%r14)
>> ? ? ? ? movzbl ?%ah, %eax
>> ? ? ? ? testb ? %al, %al
>> ? ? ? ? jne ? ? .L1194
>>
>
> Hm, something is not combined correctly, I'd say "testb %ah, %ah" is
> optimal in the second case.
>

Here is another update adjusted for mov pattern changes in i386.md.

8bit result is stored in

AL <- Quotient
AH <- Remainder

If we use AX for quotient in 8bit divmod pattern, we have to make
sure that AX is valid for quotient.  We have to extend AL with UNSPEC
since AH isn't the part of quotient,.  Instead, I use AL for quotient and
use UNSPEC_MOVQI_EXTZH to extract remainder from AL. Quotient
access can be optimized very nicely. If remainder is used, we may have
an extract move for UNSPEC_MOVQI_EXTZH. I think this is a reasonable
comprise.


-- 
H.J.
--
gcc/

2010-06-22  H.J. Lu  <hongjiu.lu@intel.com>

	PR target/44588
	* config/i386/i386.md (UNSPEC_MOVQI_EXTZH): New.
	(any_div_code): Likewise.
	(extend_code): Likewise.
	(extract_code): Likewise.
	(*movqi_extzh): Likewise.
	(*movqi_extzh_rex64): Likewise.
	(<u>divmodqi4): Likewise.
	(*<u>divqi3): Likewise.
	(<u>divqi3): Remvoved.

gcc/testsuite/

2010-06-22  H.J. Lu  <hongjiu.lu@intel.com>

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

Attachment: gcc-pr44588-7.patch
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