[lld] [lld][BP] Avoid ordering ICF'ed sections (PR #126327)

[Ellis Hoag via llvm-commits]

ellishg wrote:

> Thanks for updating the ELF test! If you want to update the ELF code, you could do the following.
> 
> ```
> --- i/lld/ELF/BPSectionOrderer.cpp
> +++ w/lld/ELF/BPSectionOrderer.cpp
> @@ -75,8 +75,11 @@ DenseMap<const InputSectionBase *, int> elf::runBalancedPartitioning(
>      auto *d = dyn_cast<Defined>(&sym);
>      if (!d)
>        return;
> -    auto *sec = dyn_cast_or_null<InputSectionBase>(d->section);
> -    if (!sec || sec->size == 0 || !orderer.secToSym.try_emplace(sec, d).second)
> +    auto *sec = dyn_cast_or_null<InputSection>(d->section);
> +    // Skip empty, discarded, ICF folded sections. Skipping ICF folded sections
> +    // reduces duplicate detection work in BPSectionOrderer.
> +    if (!sec || sec->size == 0 || !sec->isLive() || sec->repl != sec ||
> +        !orderer.secToSym.try_emplace(sec, d).second)
>        return;
>      rootSymbolToSectionIdxs[CachedHashStringRef(getRootSymbol(sym.getName()))]
>          .insert(sections.size());
> ```
> 
> In ELF, `InputSectionBase` can be InputSection (regular sections from .o), EhInputSection (.eh_frame, already merged), MergeInputSection (mergeable string sections like .rodata.str1 .debug.str, similar to Mach-O `__TEXT,__cstring`; already merged into a MergeSyntheticSection). We only need to handle InputSection.
> 
> `!sec->isLive()` skips unreachable input sections due to --gc-sections (like -dead_strip).
> 
> While writing this comment, I realize that the tests probably should be enhanced to test section garbage collection (dead stripping). Since many functions are actually unreachable from main, you will probably need to add this attribute
> 
> // used is to suppress compiler garbage collection in ELF; retain is to suppress linker garbage collection; used is not needed for non-internal linkage symbols
> 
> // used is for both compiler/linker GC in Mach-O; retain is ignored for Mach-O
> 
> ```c
> #define RETAIN [[gnu::used,gnu::retain]]
> 
> RETAIN void A() {}
> RETAIN void B() {}
> ```

Thanks for the useful info! I had assumed that ELF was skipping dead sections already because the test I added already passes.

https://github.com/llvm/llvm-project/pull/126327