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Re: [2/5] C-SKY port: Backend implementation



> 在 2018年8月3日,06:27,Jeff Law <law@redhat.com> 写道:
> 
> On 07/26/2018 12:06 AM, 瞿仙淼 wrote:
>> 
>>> 在 2018年7月25日,上午5:24,Jeff Law <law@redhat.com> 写道:
>>> 
>>> On 07/24/2018 12:18 PM, Sandra Loosemore wrote:
>>>> On 07/24/2018 09:45 AM, Jeff Law wrote:
>>>>> On 07/23/2018 10:21 PM, Sandra Loosemore wrote:
>>>>> I'm not a big fan of more awk code, but I'm not going to object to it :-)
>>>>> 
>>>>> Why does the port have its own little pass for condition code
>>>>> optimization (cse_cc)?  What is it doing that can't be done with our
>>>>> generic optimizers?
>>>> 
>>>> This pass was included in the initial patch set we got from C-SKY, and
>>>> as it didn't seem to break anything I left it in.  Perhaps C-SKY can
>>>> provide a testcase that demonstrates why it's still useful in the
>>>> current version of GCC; otherwise we can remove this from the initial
>>>> port submission and restore it later if some performance analysis shows
>>>> it is still worthwhile.
>>> FWIW it looks like we model CC setting on just a few insns, (add,
>>> subtract) so I'd be surprised if this little mini pass found much.  I'd
>>> definitely like to hear from the csky authors here.
>>> 
>>> Alternately, you could do add some instrumentation to flag when it
>>> triggers, take a test or two that does, reduce it and we can then look
>>> at the key RTL sequences and see what the pass is really doing.
>>> 
>> 
>> I wrote a case to reproduce this problem on C-SKY. C code is as follows:
>> -----------------------------------------------------------------------
>> int e1, e2;
>> 
>> void func (int a, int b, int c, int d, int f, int g)
>> {
>>  e1 = a > b ? f : g;
>>  e2 = a > b ? c : d;
>> 
>>  return;
>> }
>> -----------------------------------------------------------------------
>> 
>> compile to assembler with option “-O3 -S” :
>> -----------------------------------------------------------------------
>> func:
>>  cmplt a1, a0
>>  ld.w  t1, (sp, 0)
>>  ld.w  t0, (sp, 4)
>>  movt  t0, t1
>>  cmplt a1, a0
>>  movt  a3, a2
>>  lrw a1, e2
>>  lrw a2, e1
>>  st.w  a3, (a1, 0)
>>  st.w  t0, (a2, 0)
>>  rts
>> -----------------------------------------------------------------------
>> There is an extra “cmplt a1, a0" in the above code without cse_cc. This situation mainly occurs when a relatively short branch jump is converted into a conditional execution instruction. And the CSE pass can not reduce the same conditional comparison instruction . Here is the rtx sequence after “cse2” pass.
>> 
>> -----------------------------------------------------------------------
>> (insn 28 13 29 2 (set (reg:CC 33 c)
>>        (gt:CC (reg/v:SI 77 [ a ])
>>            (reg/v:SI 78 [ b ]))) func.c:5 1099 {*cmpgtsi}
>>     (nil))
>> (insn 29 28 30 2 (set (reg/v:SI 82 [ g ])
>>        (if_then_else:SI (eq (reg:CC 33 c)
>>                (const_int 0 [0]))
>>            (reg/v:SI 82 [ g ])
>>            (reg/v:SI 81 [ f ]))) func.c:5 983 {movf}
>>     (expr_list:REG_DEAD (reg/v:SI 81 [ f ])
>>        (expr_list:REG_DEAD (reg:CC 33 c)
>>            (nil))))
>> (insn 30 29 31 2 (set (reg:CC 33 c)
>>        (gt:CC (reg/v:SI 77 [ a ])
>>            (reg/v:SI 78 [ b ]))) func.c:5 1099 {*cmpgtsi}
>>     (expr_list:REG_DEAD (reg/v:SI 78 [ b ])
>>        (expr_list:REG_DEAD (reg/v:SI 77 [ a ])
>>            (nil))))
>> (insn 31 30 18 2 (set (reg/v:SI 80 [ d ])
>>        (if_then_else:SI (eq (reg:CC 33 c)
>>                (const_int 0 [0]))
>>            (reg/v:SI 80 [ d ])
>>            (reg/v:SI 79 [ c ]))) func.c:5 983 {movf}
>>     (expr_list:REG_DEAD (reg/v:SI 79 [ c ])
>>        (expr_list:REG_DEAD (reg:CC 33 c)
>>            (nil))))
>> -----------------------------------------------------------------------
>> 
>> It doesn't seem to check the same conditional comparison instruction .So I wrote this to solve this problem, but I am not sure if this is the best way : )
>> 
>> PS, the same conditional comparison instruction cannot be reduced with the latest version gcc with C-SKY because I just insert the “cse_cc” after “cse1”, when I insert after “cse2”, this problem can be solved very well.
> I think the cse_cc pass is really just working around one or more bugs
> in CSE and/or a backend bug.  The RTL above clearly shows a common
> subexpression that is not eliminated by CSE.
> 
> What CSE should be trying to do is changing the second and third
> occurrences of (gt:CC (reg 77) (reg 78)) with (reg 33) which would
> create nop-sets which would be subsequently deleted.  I suspect you do
> not have an insn which matches that nop set of the CC register.  If you
> fix that I suspect CSE will work better and eliminate the need for your
> cse_cc pass.

Thanks you for your suggestions, we will try this method.

> 
> jeff


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