[2/5] C-SKY port: Backend implementation
Sandra Loosemore
sandra@codesourcery.com
Sat Jul 28 01:49:00 GMT 2018
On 07/26/2018 12:06 AM, ç¿ä»æ·¼ 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 â-O3 -Sâ :
> -----------------------------------------------------------------------
> 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 âcmplt 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 âcse2â 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 âcse_ccâ after âcse1â, when I insert after âcse2â, 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?
-Sandra
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