Re: "Mini" tags to reduce the number of op codes

MitchAlsup1 wrote:

Terje Mathisen wrote:
 

MitchAlsup1 wrote:

>

 

In the non-OoO (i.e Pentium) days, I would have inverted the loop in order to hide the latencies as much as possible, resulting in an inner loop something like this:

 

  next:
   adc eax,ebx
   mov ebx,[edx+ecx*4]    ; First cycle

 

   mov [edi+ecx*4],eax
   mov eax,[esi+ecx*4]    ; Second cycle

 

   inc ecx
   jnz next        ; Third cycle

 

Terje

 As opposed to::
      .global mpn_add_n
mpn_add_n:
     MOV   R5,#0     // c
     MOV   R6,#0     // i
      VEC   R7,{}
     LDD   R8,[R2,Ri<<3]       // Load 128-to-512 bits
     LDD   R9,[R3,Ri<<3]       // Load 128-to-512 bits
     CARRY R5,{{IO}}
     ADD   R10,R8,R9           // Add pair to add octal
     STD   R10,[R1,Ri<<3]      // Store 128-to-512 bits
     LOOP  LT,R6,#1,R4         // increment 2-to-8 times
     RET
 --------------------------------------------------------
      LDD   R8,[R2,Ri<<3]       // AGEN cycle 1
     LDD   R9,[R3,Ri<<3]       // AGEN cycle 2 data cycle 4
     CARRY R5,{{IO}}
     ADD   R10,R8,R9           // cycle 4
     STD   R10,[R1,Ri<<3]      // AGEN cycle 3 write cycle 5
     LOOP  LT,R6,#1,R4         // cycle 3
 OR
      LDD       LDd
          LDD       LDd                    ADD
               ST        STd
               LOOP
                    LDD       LDd
                         LDD       LDd                                   ADD
                              ST        STd
                              LOOP
 10 instructions (2 iterations) in 4 clocks on a 64-bit 1-wide VVM machine !!
without code scheduling heroics.
 40 instructions (8 iterations) in 4 clocks on a 512 wide SIMD VVM machine !!

It all comes down to the carry propagation, right?
The way I understood the original code, you are doing a very wide unsigned add, so you need a carry to propagate from each and every block to the next, right?
If you can do that at half a clock cycle per 64 bit ADD, then consider me very impressed!
Terje
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"