Support configuring the set of codegen backends to build per host triple

[bjorn3]

This allows building the compiler itself with one backend while using another backend at runtime. For example this allows compiling rustc to wasm using LLVM, while using Cranelift at runtime to produce actual code. Cranelift can't compile to wasm, but is perfectly capable of running on wasm. LLVM can compile to wasm, but can't run on wasm. ¹

Footnotes

1. The prototype of this still requires a couple of other patches. ↩

[rustbot]

r? @albertlarsan68

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[rustbot]

This PR modifies config.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[onur-ozkan]

Can you add a change info for this field? Similar to

rust/src/bootstrap/src/utils/change_tracker.rs

Lines 74 to 78 in 04521fd

	ChangeInfo {
	change_id: 117435,
	severity: ChangeSeverity::Info,
	summary: "New option `rust.parallel-compiler` added to config.toml.",
	},

[bjorn3]

Done

[albertlarsan68]

How does this interact with stages ?

[bjorn3]

If the host triple matches the build triple, then setting it in target.<triple>.codegen-backends will apply to all stages. If they don't match, it will apply to rustc compiled for the respective target.

[albertlarsan68]

Thanks for the PR!
@bors r+

[bors]

📌 Commit f4e279f has been approved by albertlarsan68

It is now in the queue for this repository.

[bors]

⌛ Testing commit f4e279f with merge 93a65c6...

[bors]

☀️ Test successful - checks-actions
Approved by: albertlarsan68
Pushing 93a65c6 to master...

[rust-timer]

Finished benchmarking commit (93a65c6): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.3%	[-4.3%, -4.3%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.1%	[2.1%, 2.1%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 639.468s -> 638.862s (-0.09%)
Artifact size: 306.35 MiB -> 306.32 MiB (-0.01%)