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| author | Christian Krinitsin <mail@krinitsin.com> | 2025-07-17 09:10:43 +0200 |
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| committer | Christian Krinitsin <mail@krinitsin.com> | 2025-07-17 09:10:43 +0200 |
| commit | f2ec263023649e596c5076df32c2d328bc9393d2 (patch) | |
| tree | 5dd86caab46e552bd2e62bf9c4fb1a7504a44db4 /results/scraper/fex/2563 | |
| parent | 63d2e9d409831aa8582787234cae4741847504b7 (diff) | |
| download | qemu-analysis-main.tar.gz qemu-analysis-main.zip | |
Diffstat (limited to 'results/scraper/fex/2563')
| -rw-r--r-- | results/scraper/fex/2563 | 175 |
1 files changed, 175 insertions, 0 deletions
diff --git a/results/scraper/fex/2563 b/results/scraper/fex/2563 new file mode 100644 index 000000000..80126a5e0 --- /dev/null +++ b/results/scraper/fex/2563 @@ -0,0 +1,175 @@ +Scalar vector performance issues with FMOD +FMOD burns a CPU core, specifically in Hollow Knight's title screen. Seems to be a little loop that spins really hot that we don't optimize well. It's bad enough that a ton of Unity+FMOD games will stutter their audio quite hard. + +This is due to how the code is using a loop-unrolled scalar FMA to process audio blocks and FEX converting the code badly. + +Original code. +```asm +01aaf061 89ce mov esi, ecx +01aaf063 4889ea mov rdx, rbp +01aaf066 4889d8 mov rax, rbx + +01aaf069 f30f1012 movss xmm2, dword [rdx] +01aaf06d 4883c020 add rax, 0x20 +01aaf071 f30f59d0 mulss xmm2, xmm0 +01aaf075 4883c220 add rdx, 0x20 +01aaf079 f30f5850e0 addss xmm2, dword [rax-0x20] +01aaf07e f30f1150e0 movss dword [rax-0x20], xmm2 +01aaf083 f30f1052e4 movss xmm2, dword [rdx-0x1c] +01aaf088 f30f59d1 mulss xmm2, xmm1 +01aaf08c f30f5850e4 addss xmm2, dword [rax-0x1c] +01aaf091 f30f1150e4 movss dword [rax-0x1c], xmm2 +01aaf096 f30f1052e8 movss xmm2, dword [rdx-0x18] +01aaf09b f30f59d0 mulss xmm2, xmm0 +01aaf09f f30f5850e8 addss xmm2, dword [rax-0x18] +01aaf0a4 f30f1150e8 movss dword [rax-0x18], xmm2 +01aaf0a9 f30f1052ec movss xmm2, dword [rdx-0x14] +01aaf0ae f30f59d1 mulss xmm2, xmm1 +01aaf0b2 f30f5850ec addss xmm2, dword [rax-0x14] +01aaf0b7 f30f1150ec movss dword [rax-0x14], xmm2 +01aaf0bc f30f1052f0 movss xmm2, dword [rdx-0x10] +01aaf0c1 f30f59d0 mulss xmm2, xmm0 +01aaf0c5 f30f5850f0 addss xmm2, dword [rax-0x10] +01aaf0ca f30f1150f0 movss dword [rax-0x10], xmm2 +01aaf0cf f30f1052f4 movss xmm2, dword [rdx-0xc] +01aaf0d4 f30f59d1 mulss xmm2, xmm1 +01aaf0d8 f30f5850f4 addss xmm2, dword [rax-0xc] +01aaf0dd f30f1150f4 movss dword [rax-0xc], xmm2 +01aaf0e2 f30f1052f8 movss xmm2, dword [rdx-0x8] +01aaf0e7 f30f59d0 mulss xmm2, xmm0 +01aaf0eb f30f5850f8 addss xmm2, dword [rax-0x8] +01aaf0f0 f30f1150f8 movss dword [rax-0x8], xmm2 +01aaf0f5 f30f1052fc movss xmm2, dword [rdx-0x4] +01aaf0fa f30f59d1 mulss xmm2, xmm1 +01aaf0fe f30f5850fc addss xmm2, dword [rax-0x4] +01aaf103 f30f1150fc movss dword [rax-0x4], xmm2 +01aaf108 83ee01 sub esi, 0x1 +01aaf10b 0f8558ffffff jne 0x1aaf069 +``` +Single step of the job: +``` asm +00010029 f30f1052e0 movss xmm2, dword [rdx-0x20] +0001002e f30f59d0 mulss xmm2, xmm0 +00010032 f30f5850e0 addss xmm2, dword [rax-0x20] +00010037 f30f1150e0 movss dword [rax-0x20], xmm2 +``` + +Resulting AArch64 asm +```asm + 0x0000ffffe2d000bc: ldur s18, [x6, #-32] + 0x0000ffffe2d000c0: fmul s4, s18, s16 + 0x0000ffffe2d000c4: mov v18.s[0], v4.s[0] + 0x0000ffffe2d000c8: ldur s4, [x4, #-32] + 0x0000ffffe2d000cc: fadd s4, s18, s4 + 0x0000ffffe2d000d0: mov v18.s[0], v4.s[0] + 0x0000ffffe2d000d4: eor v0.16b, v0.16b, v0.16b + 0x0000ffffe2d000d8: mov v0.s[0], v18.s[0] + 0x0000ffffe2d000dc: mov v4.16b, v0.16b + 0x0000ffffe2d000e0: stur s4, [x4, #-32] +``` + +Ideally the AArch64 asm should be: +```asm +ldur s18, [x6, #-32] +fmul s18, s18, s16 +ldur s4, [x4, #-32] +fadd s18, s18, s4 +stur s18, [x4, #-32] +``` + +IPC numbers are trash according to llvm-mca: +```bash +ryanh@ubuntu-linux-20-04-desktop:~$ llvm-mca -mcpu=cortex-a57 test.s +Iterations: 100 +Instructions: 1000 +Total Cycles: 3214 +Total uOps: 1300 + +Dispatch Width: 3 +uOps Per Cycle: 0.40 +IPC: 0.31 +Block RThroughput: 5.0 +``` + +Theoretical improvements: +```bash +ryanh@ubuntu-linux-20-04-desktop:~$ llvm-mca -mcpu=cortex-a57 test_opt.s +Iterations: 100 +Instructions: 500 +Total Cycles: 217 +Total uOps: 500 + +Dispatch Width: 3 +uOps Per Cycle: 2.30 +IPC: 2.30 +Block RThroughput: 2.0 +``` + + +Sample FEX ASM unit test that can see the issue looking at the generated code. +```asm + +; This is based on the hottest block of Unity+FMOD's audio code from Hollow Knight +; This is doing some sort of unrolled scalar FMA that works on a full block at a time + +; xmm0 contains one channel multiplication value +; xmm1 contains the other channel? +; These come from arguments +movss xmm0, dword [rel scalar_val] +movss xmm1, dword [rel scalar_val2] + +; Break the block to be closer to original block +jmp local +local: + +; RAX contains the source block of data. Each block containing 32-bytes of data +; The data is modified in-place +; Data format +; struct Data { +; float data; +; float adder; +; } +mov rax, 0xe8000000 +; ESI contains the number of blocks to operate on, Only one block in this test +mov esi, 1 +; RDX contains the source value to multiply by +mov rdx, 0xe8000020 + +top: + +add rax, 0x20 ; Increment Source block +add rdx, 0x20 ; Increment multiplier stereo blocks + +; Do the remainder in a repeating macro +%macro OneChannelUnroll 2 +; Load chan1 + movss xmm2, dword [rdx - %1] +; Multiply chan1 by channel multiplier + mulss xmm2, %2 +; Add to data source and store back to memory + addss xmm2, dword [rax - %1] + movss dword [rax - %1], xmm2 +%endmacro + +; Channel 1 +OneChannelUnroll 0x20, xmm0 +; Channel 2 +OneChannelUnroll 0x1c, xmm1 + +;OneChannelUnroll 0x18, xmm0 +;OneChannelUnroll 0x14, xmm1 +;OneChannelUnroll 0x10, xmm0 +;OneChannelUnroll 0x0c, xmm1 +;OneChannelUnroll 0x08, xmm0 +;OneChannelUnroll 0x04, xmm1 + +sub esi, 1 +jne top + +hlt + +scalar_val: +dd 1.0 +scalar_val2: +dd 1.0 +``` \ No newline at end of file |