Comment by isaacimagine

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Comment by isaacimagine

There are 163 lines of C. Of them, with -O3, 104 lines are present in the assembly output. So the C compiler is able to eliminate an additional ~36.2% of the instructions. It doesn't do anything fancy, like autovectorization.

I profiled just now:

          | instrs (aarch64) | time 100k (s) | conway samples (%) | 
    | -O0 |              606 |        19.10s |             78.50% |
    | -O3 |              135 |          3.45 |             90.52% |
The 3.45s surprises me, because it's faster than the 4.09s I measured earlier. Maybe I had a P core vs an E core. For -O0, the compiler is emitting machine code like:

    0000000100002d6c ldr x8, [sp, #0x4a0]
    0000000100002d70 ldr x9, [sp, #0x488]
    0000000100002d74 orn x8, x8, x9
    0000000100002d78 str x8, [sp, #0x470]
Which is comically bad. If I try with e.g. -Og, I get the same disassembly as -O3. Even -01 gives me the same disassembly as -O3. The assembly (-0g, -01, -03) looks like a pretty direct translation of the C. Better, but also nothing crazy (e.g. no autovectorization):

    0000000100003744 orr x3, x3, x10
    0000000100003748 orn x1, x1, x9
    000000010000374c and x1, x3, x1
    0000000100003750 orr x3, x8, x17
Looking more closely, there's actually surprisingly little register spilling.

I think the real question you're asking is, as I wrote:

> If we assume instruction latency is 1 cycle, we should expect 2,590 fps. But we measure a number nearly 10× higher! What gives?

Part of this is due to counting the instructions in the dissassembly wrong. In the blogpost I used 349 instructions, going off Godbolt, but in reality it's 135. If I redo the calculations with this new numbers, I get 2.11 instructions per bit, 0.553 million instrs per step, dividing out 3.70 gcycles/s gives 6,690 fps. Which is better than 2,590 fps, but still 3.6x slower than 24,400. But I think 3.6x is a factor you can chalk up to instruction-level parallelism,.

Hope that answers your questions. Love your writing Gwern.

Replies isaacimagine · 2025-05-29
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