Only allow compiler_builtins to call LLVM intrinsics, not any link_name function

[saethlin]

This is another case of accidental reliance on inline(never) like I rooted out in #118770. Without this PR, attempting to build some large programs with -Zcross-crate-inline-threshold=yes with a sysroot also compiled with that flag will result in linker errors like this:

  = note: /usr/bin/ld: /tmp/cargo-installNrfN4T/x86_64-unknown-linux-gnu/release/deps/libcompiler_builtins-d2a9b69f4e45b883.rlib(compiler_builtins-d2a9b69f4e45b883.compiler_builtins.dbbc6c2ca970faa4-cgu.0.rcgu.o): in function `core::panicking::panic_fmt':
          /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panicking.rs:72:(.text.unlikely._ZN4core9panicking9panic_fmt17ha407cc99e97c942fE+0x31): undefined reference to `rust_begin_unwind'

With -Zcross-crate-inline-threshold=yes we can inline panic_fmt into compiler_builtins. Then we end up with a call to an upstream monomorphization, but one that has a link_name set. But unlike LLVM's magic intrinsic names, this link name is going to make it to the linker, and then we have a problem.

This logic looks scuffed, but also we're doing this in 4 other places. Don't know if that means it's good or bad.

rust/compiler/rustc_codegen_cranelift/src/abi/mod.rs

Line 386 in 1684a75

if fx.tcx.symbol_name(instance).name.starts_with("llvm.") {

rust/compiler/rustc_ast_passes/src/feature_gate.rs

Line 306 in 1684a75

let links_to_llvm = link_name.is_some_and(|val| val.as_str().starts_with("llvm."));

rust/compiler/rustc_codegen_ssa/src/codegen_attrs.rs

Line 609 in 1684a75

if name.as_str().starts_with("llvm.") {

rust/compiler/rustc_codegen_gcc/src/declare.rs

Line 170 in 1684a75

if name.starts_with("llvm.") {

[rustbot]

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[Nadrieril]

r? compiler

[Noratrieb]

@bors r+

[bors]

📌 Commit 748dba2 has been approved by Nilstrieb

It is now in the queue for this repository.

[bors]

⌛ Testing commit 748dba2 with merge 0e682e9...

[bors]

☀️ Test successful - checks-actions
Approved by: Nilstrieb
Pushing 0e682e9 to master...

[rust-timer]

Finished benchmarking commit (0e682e9): 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)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.6%	[-2.6%, -2.6%]	1

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)	-	-	0
Regressions ❌ (secondary)	1.7%	[1.4%, 2.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 666.146s -> 667.665s (0.23%)
Artifact size: 315.65 MiB -> 315.66 MiB (0.00%)