Luke Lau
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lukel97.bsky.social
Luke Lau
@lukel97.bsky.social
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LLVM at Igalia
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lukel97.bsky.social
Trying to find the slowest possible RISC-V instruction. This single vlse8.v with a stride of 65536 bytes takes 66 million cycles on a Banana Pi F3. That's 0.04 seconds @1.6GHz
#risc-v
A screenshot of a terminal: luke@bananapif3:~/slowest-instr$ cat main.S .section .rodata str: .asciz "Cycles: %d\n" foo: .zero 256 * STRIDE .section .text .global main main: addi sp, sp, -8 sd ra, 0(sp) rdcycle s1 rdcycle s2 sub s3, s2, s1 # rdcycle overhead la a0, foo li a1, STRIDE vsetvli t0, zero, e8, m8, tu, mu rdcycle s1 vlse8.v v8, (a0), a1 rdcycle s2 sub s1, s2, s1 sub s1, s1, s3 la a0, str mv a1, s1 call printf ld ra, 0(sp) addi sp, sp, 8 ret luke@bananapif3:~/slowest-instr$ clang main.S -DSTRIDE=65536 -march=rv64gv luke@bananapif3:~/slowest-instr$ perf stat -e cycles:u ./a.out Cycles: 66640979 Performance counter stats for './a.out': 78,064,581 cycles:u 0.049648957 seconds time elapsed 0.000000000 seconds user 0.049907000 seconds sys
December 11, 2024 at 9:40 AM