Post #2409965

2025-09-09 05:37 UTC

@JamesWidman@mastodon.social @steve@discuss.systems @regehr@mastodon.social Now wide decode specifically (backend less so, that's neither here nor there) is actually appreciably easier for ARM AArch64 because it's fixed-width 32-bit instructions. That was something that I was actually wondering whether it might screw over x86 one day. But Tremont and later with their clustered decode approach (that is, getting 6 or 9 insns/cycle by doing, essentially, out-of-order decode using 2 or 3 separate 3-wide decoders) are a game changer.

Replies (2)

@JamesWidman@mastodon.social 2025-09-09 05:39

@rygorous@mastodon.gamedev.place @steve@discuss.systems @regehr@mastodon.social > [x86] out-of-order decode using 2 or 3 separate 3-wide decoders wat. (i was not paying attention when this dropped (in 2020? 2021?); i'll have to go read about it)

Open ##2409966
@rygorous@mastodon.gamedev.place 2025-09-09 05:41

@JamesWidman@mastodon.social The key point being that it definitely easily scales to 9-wide (I mean, the product has been on shelves for a year) and probably much further, lets you stack 3-wide decoders (which are at a reasonably sweet spot for x86) and doesn't have any scary super-linear stuff (that I can tell) on the branch pred/fetch/merge sides either. Might hit a wall at some point, but a reasonable (and low-power!) way to get to at least 15-wide decode or so is a lot of runway.

Open ##2409972