[LLVMdev] Disable loop unroll pass

[Shuxin Yang]

Hi, Gang:

    I remember there were different voices when you check-in the code.
I agree with them although I didn't reply your mail in open64's mailing 
list.

   In the transformation you illustrate, it involves two operations:
   1) promote WHILE-loop into DO-loop (i.e noncountable loop to 
countable loop)
   2) get rid of trip-count dec/inc and compare.

  1) is irrelevant to HW loop. Any scalar optimizer should handle 1).
It is not difficult at all to handle 2) in CodeGen and it is unnecessary to
to introduce a Operator just for that purpose.

Shuxin

On 11/22/2012 06:03 AM, Gang Yu wrote:
> I am the designer for open64 hwloop structure, but I am not a student.
>
> Hope the following helps:
>
> To transform a loop into hwloop, we need the help from optimizer. For 
> example,
> |
>     while(k3>=10){
>       sum+=k1;
>       k3 --;
>     }
> |
>
> into the form:||
>
> |
>     zdl_loop(k3-9) {
>        sum+=k1;
>     }
> |
>
> So, we introduce a new ZDLBR whirl(open64 optimizer intermediate) 
> operator, which represents the loop in whirl as:||
>
> |
> LABEL L2050 0 {line: 0}
> LOOP_INFO 0 1 1
>     I4I4LDID 73 <1,2,.preg_I4> T<4,.predef_I4,4> # k3
>     I4I4LDID 77 <1,2,.preg_I4> T<4,.predef_I4,4> # <preg>
>   END_LOOP_INFO
>     I4I4LDID 74 <1,2,.preg_I4> T<4,.predef_I4,4> # k1
>     I4I4LDID 75 <1,2,.preg_I4> T<4,.predef_I4,4> # sum
>    I4ADD
>   I4STID 75 <1,2,.preg_I4> T<4,.predef_I4,4> # sum {line: 5}
>   ZDLBR L2050 {line: 0}
> Then, we let cg do things. Such a design abstract the general 
> operations in optimizer, while target specific part in cg, still a 
> simulated op, until cg loop optimization finished. We implement a 
> multi nested level hwloop by this approach. Gcc's 3 doloop expand 
> names do the same, we believe.
> |
>
> More details, please take a look at
>
> http://wiki.open64.net/index.php/Zero_Delay_Loop
>
> Thanks
> Gang
>

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