Stream: git-wasmtime

Topic: wasmtime / PR #4088 x64: use constant pool for u64 consta...

Wasmtime GitHub notifications bot (Apr 29 2022 at 05:54):

cfallin opened PR #4088 from x64-u64-constants to main:

Another isel improvement from staring at disassemblies of spidermonkey.wasm compilation results: I noticed quite a few uses of constants of the form

   0xC0DD259:  movabsq $-9223372036854775808,%r9

where r9 is used exactly once. This is a consequence of the "emit constants right before use" logic in the x64 backend that is meant to reduce register pressure (otherwise, we get one movabs at the top of the function and then a spill/reload).

There's a better way though: a bunch of these uses are in reg/reg instructions that could be mem/reg, so we could load directly from a constant pool. So instead of orq %r9, %rax after the above, we could do orq OFFSET(%rip), %rax. This avoids the need to use a temporary register, reducing register pressure, and also deduplicates constants (because our constant pool is deduplicated) vs movabs-per-use-site.

This PR improves performance on meshoptimizer by 1%. Surprisingly, and frustratingly, it does not actually make SpiderMonkey faster. I am starting to realize that SpiderMonkey.wasm on the current compilation is cache-bound, and so further improvements in isel may not translate to further performance improvements (at least at the individual lowering level); we need better mid-end opts to advance further. But its hot blocks are a good source of examples of awkward codegen regardless :-)

Builds on top of #4080.

Also fixes an emission bug in pinsrd where RIP-relative addresses were off by one (due to extra byte at end of instruction, not accounted for in the relocation). We'll never hit this currently because no lowerings use pinsrd with RIP-relative data (only used for the constant pool) but with this PR it's now relevant.

Wasmtime GitHub notifications bot (Apr 29 2022 at 22:27):

cfallin updated PR #4088 from x64-u64-constants to main.

Wasmtime GitHub notifications bot (Apr 29 2022 at 22:29):

cfallin has marked PR #4088 as ready for review.

Wasmtime GitHub notifications bot (May 02 2022 at 23:22):

cfallin updated PR #4088 from x64-u64-constants to main.

Wasmtime GitHub notifications bot (May 10 2022 at 16:19):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (May 10 2022 at 16:21):

cfallin merged PR #4088.

Last updated: Dec 13 2025 at 21:03 UTC