diff options
Diffstat (limited to 'src/internal/runtime')
| -rw-r--r-- | src/internal/runtime/gc/scan/scan_amd64.s | 19 |
1 files changed, 18 insertions, 1 deletions
diff --git a/src/internal/runtime/gc/scan/scan_amd64.s b/src/internal/runtime/gc/scan/scan_amd64.s index 055995fa38..9b4950a767 100644 --- a/src/internal/runtime/gc/scan/scan_amd64.s +++ b/src/internal/runtime/gc/scan/scan_amd64.s @@ -86,7 +86,24 @@ loop: // Collect just the pointers from the greyed objects into the scan buffer, // i.e., copy the word indices in the mask from Z1 into contiguous memory. - VPCOMPRESSQ Z1, K1, (DI)(DX*8) + // + // N.B. VPCOMPRESSQ supports a memory destination. Unfortunately, on + // AMD Genoa / Zen 4, using VPCOMPRESSQ with a memory destination + // imposes a severe performance penalty of around an order of magnitude + // compared to a register destination. + // + // This workaround is unfortunate on other microarchitectures, where a + // memory destination is slightly faster than adding an additional move + // instruction, but no where near an order of magnitude. It would be + // nice to have a Genoa-only variant here. + // + // AMD Turin / Zen 5 fixes this issue. + // + // See + // https://lemire.me/blog/2025/02/14/avx-512-gotcha-avoid-compressing-words-to-memory-with-amd-zen-4-processors/. + VPCOMPRESSQ Z1, K1, Z2 + VMOVDQU64 Z2, (DI)(DX*8) + // Advance the scan buffer position by the number of pointers. MOVBQZX 128(AX), CX ADDQ CX, DX |