lto: load bitcode sections by name

[durin42]

Upstream change
llvm/llvm-project@6b539f5 changed isSectionBitcode works and it now only respects .llvm.lto sections instead of also .llvmbc, which it says was never intended to be used for LTO. We instead load sections by name, and sniff for raw bitcode by hand.

This is an alternative approach to #115136, where we tried the same thing using the object crate, but it got too fraught to continue.

r? @nikic
@rustbot label: +llvm-main

[durin42]

This is broken for me on macOS (M1, presumably also x86 but that machine is so slow to compile I didn't bother testing there). We look for __LLVM,__bitcode on macOS, but when I iterate the sections I see __text, __gcc_except_tab, __const, __const, __bitcode, __cmdline, __compact_unwind, __eh_frame. Should we be using __bitcode?

[durin42]

Looking, it looks like the __LLVM prefix here is the "segment name" in the macho file, which is a concept unique to that format. I guess for this use case we'll detect-and-elide the __LLVM, prefix and that'll work for now, even if it's gross.

[nikic]

@bors r+

[bors]

📌 Commit 0db6602 has been approved by nikic

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0db6602 with merge ffc48e3...

[bors]

☀️ Test successful - checks-actions
Approved by: nikic
Pushing ffc48e3 to master...

[rust-timer]

Finished benchmarking commit (ffc48e3): 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)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	1.9%	[1.3%, 2.5%]	9
Improvements ✅ (primary)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-2.1%, 2.0%]	2

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 627.243s -> 630.375s (0.50%)
Artifact size: 318.06 MiB -> 318.12 MiB (0.02%)