On 07/27/2018 07:49 PM, Sandra Loosemore wrote:
> On 07/26/2018 12:06 AM, 瞿仙淼 wrote:
>>
>> 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.
>
> Thanks, this is very helpful. I've verified this and I'm moving the
> pass as you suggest, adding a more detailed comment to the source to
> explain what the pass is for, and adding your test case to the
> testsuite. This will be included when I resubmit the patches to address
> other review comments too.
>
> Jeff, does that adequately address your concerns about whether the pass
> is useful?
I think the pass is papering over problems elsewhere (see my most other
reply on this thread). I do think it would be useful to take that code
and create a test based on it. I suspect you'll want to verify multiple
GT expressions prior to CSE2 and that after CSE2 you have a single GT
expression. Presumably it'd be in the csky specific test.
jeff
