Programming Parallel Computers

Chapter 2: Case study

Version 6: Prefetching [advanced]

Earlier we discussed hardware prefetching: for example, when the CPU detects that the program seems to be doing linear reading, it may automatically start to fetch data that we might need in the future. Instead of relying on hardware prefetching, we can also do software prefetching: we can explicitly add instructions that ask the CPU to fetch data that we might need in the future.

In GCC, we can use the built-in function __builtin_prefetch to give such hints. It is important to keep in mind that these hints will get translated into machine language instructions, and this means that there will be more work for the CPU. However, as we saw in the previous section, the execution ports that are associated with memory accesses are currently underused, so it makes sense to try out if software prefetching would help.


Here is the innermost loop, with the prefetch instructions added. Based on benchmarks, prefetching information that will be needed 20 iterations later seemed to give a good performance.

for (int k = 0; k < n; ++k) {
    constexpr int PF = 20;
    __builtin_prefetch(&vd[n*ia + k + PF]);
    __builtin_prefetch(&vt[n*ja + k + PF]);
    float8_t a000 = vd[n*ia + k];
    float8_t b000 = vt[n*ja + k];
    float8_t a100 = swap4(a000);
    float8_t a010 = swap2(a000);
    float8_t a110 = swap2(a100);
    float8_t b001 = swap1(b000);
    vv000 = min8(vv000, a000 + b000);
    vv001 = min8(vv001, a000 + b001);
    vv010 = min8(vv010, a010 + b000);
    vv011 = min8(vv011, a010 + b001);
    vv100 = min8(vv100, a100 + b000);
    vv101 = min8(vv101, a100 + b001);
    vv110 = min8(vv110, a110 + b000);
    vv111 = min8(vv111, a110 + b001);

Note that we do not need to worry about addressing memory that is out of bounds when we use prefetch hints; it is harmless.

Assembly code

We can see the prefetch instructions also in the assembly code:

           vmovaps    -640(%rdx), %ymm2
           prefetcht0 (%rax)
           addq       $32, %rax
           vmovaps    -672(%rax), %ymm3
           prefetcht0 (%rdx)
           addq       $32, %rdx
           vpermilps  $177, %ymm2, %ymm0
           cmpq       %rcx, %rax
           vperm2f128 $1, %ymm3, %ymm3, %ymm13
           vaddps     %ymm2, %ymm3, %ymm15
           vpermilps  $78, %ymm3, %ymm14
           vaddps     %ymm0, %ymm3, %ymm3
           vpermilps  $78, %ymm13, %ymm1
           vminps     %ymm15, %ymm11, %ymm11
           vminps     %ymm3, %ymm7, %ymm7
           vaddps     %ymm14, %ymm2, %ymm3
           vaddps     %ymm14, %ymm0, %ymm14
           vminps     %ymm3, %ymm10, %ymm10
           vaddps     %ymm13, %ymm2, %ymm3
           vaddps     %ymm13, %ymm0, %ymm13
           vaddps     %ymm1, %ymm2, %ymm2
           vaddps     %ymm1, %ymm0, %ymm0
           vminps     %ymm14, %ymm6, %ymm6
           vminps     %ymm3, %ymm9, %ymm9
           vminps     %ymm13, %ymm5, %ymm5
           vminps     %ymm2, %ymm8, %ymm8
           vminps     %ymm0, %ymm4, %ymm4
           jne        LOOP


We did not have much room left for improvement anymore, but prefetching nevertheless gives a visible performance improvement: