add -Zforce-intrinsic-fallback flag

[folkertdev]

Add a flag that forces the use of the fallback body (if one exists), so that we can test that these fallback implementations actually work.

cc #150946
cc #t-compiler > testing intrinsic fallback bodies
cc #t-infra > CI for -Zforce-intrinsic-fallback

[rustbot]

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

[rustbot]

r? @adwinwhite

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[adwinwhite]

r? codegen

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[scottmcm]

I'm kinda puzzled here: why does this need assembly output? If it wants to be checking assembly should it be in assembly-llvm instead? For that matter, why does it need minicore?

Could this just be a normal codegen test that calls std::intrinsics::sqrtf32? Looking at llvm-ir should be pretty portable for it, no? (Especially with -C no-prepopulate-passes so it just sees what we emit, not post-optimization IR.)

View changes since the review

[folkertdev]

Right, this was an assembly test originally, but LLVM made more sense.

I've made it generic now, with -C no-prepopulate-passes -Copt-level=3 so hopefully the IR should be stable enough.

[folkertdev]

Also given that you've now looked at this code a bunch, maybe you can review it?

[saethlin]

I got here eventually!

@bors r+

[rust-bors]

📌 Commit 69d7670 has been approved by saethlin

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 10. This pull request will be tested once the tree is reopened.