Commits on Oct 8, 2017

1. rustc: Don't inline in CGUs at -O0 …

   This commit tweaks the behavior of inlining functions into multiple codegen
   units when rustc is compiling in debug mode. Today rustc will unconditionally
   treat `#[inline]` functions by translating them into all codegen units that
   they're needed within, marking the linkage as `internal`. This commit changes
   the behavior so that in debug mode (compiling at `-O0`) rustc will instead only
   translate `#[inline]` functions into *one* codegen unit, forcing all other
   codegen units to reference this one copy.
   
   The goal here is to improve debug compile times by reducing the amount of
   translation that happens on behalf of multiple codegen units. It was discovered
   in rust-lang#44941 that increasing the number of codegen units had the adverse side
   effect of increasing the overal work done by the compiler, and the suspicion
   here was that the compiler was inlining, translating, and codegen'ing more
   functions with more codegen units (for example `String` would be basically
   inlined into all codegen units if used). The strategy in this commit should
   reduce the cost of `#[inline]` functions to being equivalent to one codegen
   unit, which is only translating and codegen'ing inline functions once.
   
   Collected [data] shows that this does indeed improve the situation from [before]
   as the overall cpu-clock time increases at a much slower rate and when pinned to
   one core rustc does not consume significantly more wall clock time than with one
   codegen unit.
   
   One caveat of this commit is that the symbol names for inlined functions that
   are only translated once needed some slight tweaking. These inline functions
   could be translated into multiple crates and we need to make sure the symbols
   don't collideA so the crate name/disambiguator is mixed in to the symbol name
   hash in these situations.
   
   [data]: rust-lang#44941 (comment)
   [before]: rust-lang#44941 (comment)

   

   alexcrichton committed Oct 8, 2017
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