Add special cases in the x64 emit_cmp helper

[elliottt]

Feature

The x64 backend has a helper named emit_cmp in inst.isle that is used when emitting comparisons. Currently it will always use x64_cmp to emit the comparison, but for cases of equality comparisons with 0, x64_test would be a better choice. Additionally, the select lowering does not use emit_cmp right now, and could be refactored to do so with this change in place.

Benefit

This will generate better code for cases where emit_cmp is used to compile a use of icmp in the x64 backend.

[jameysharp]

The first part of this issue (using test for comparisons with 0) was finished in #6086, but there's still work to do with simplifying the lowering for Cranelift's select instruction on x86.

[Solo-steven]

Hi @jameysharp I am interested in this issue, I am a beginner to both compiler backend and isle, so I would like to discuss my solution there before I open PR ~

I think the second part of the issue can be solved by adding a helper for lowering select inst.

(decl lower_select_icmp (Type IcmpCondResult Value Value) InstOutput)
(rule (lower_select_icmp ty (IcmpCondResult.Condition flags cc) x y)
      (with_flags flags (cmove_from_values ty cc x y)))

I think we can change the existing rule to use the helper above, form existed code

wasmtime/cranelift/codegen/src/isa/x64/lower.isle

Line 2084 in ed68661

(rule (lower (has_type ty (select (maybe_uextend (icmp cc a @ (value_type (fits_in_64 a_ty)) b)) x y)))

to use helper above :

(rule (lower (has_type ty (select (maybe_uextend (icmp cc a @ (value_type (fits_in_64 a_ty)) b)) x y)))
      (lower_select_icmp ty (emit_cmp cc a b ) x y))

[cfallin]

@Solo-steven your refactor looks reasonable -- we'd be happy to review a PR with this. Thanks!

[sudoHackIn]

Hi, @cfallin is this issue still actual or should be closed after Solo-steven's pr is merged?

[cfallin]

@sudoHackIn the PR above addresed the second bit (select using the helper) but not the first (using test rather than cmp), so this issue should remain open, I think.