Use linker optimizations on Linux #10620
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| @@ -1097,6 +1097,10 @@ pub fn link_args(sess: Session, | |||
| // which lives in libm. Similar to above, on some linuxes we | |||
| // have to be explicit about linking to it. See #2510 | |||
| args.push(~"-lm"); | |||
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| // The GNU linker supports optimizations. Use them. gc-sections removes metadata and | |||
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Can you phrase this as GNU-style linkers? It applies to ld.bfd, ld.gold (from binutils) and also lld -flavor gnu.
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I'm a little uncomfortable with this. There is no mention of any concrete improvement in this pull request or the comments, so it's unclear to me why we want this by default. Additionally, from a man page I found:
Which sounds like it's recommending that we do not use this for libraries, but only for binaries. It appears that this pull request is unconditionally running it with optimizations. What's the main reason for adding this? I don't want to just run optimizations to run optimizations. I just want to make sure that we have a clear benefit from this. Additionally, does clang do this at all? Is there precedent for this? |
| // GNU-style linkers will use this to omit linking to libraries which don't actually fulfill | ||
| // any relocations, but only for libraries which follow this flag. Thus, use it before | ||
| // specifing libraries to link to. | ||
| args.push(~"-Wl,--as-needed"); |
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This is a subtle change in linking that no one is able to change without recompiling the compiler. What are the benefits of doing this? The quote I found from the manpage was:
This option affects ELF DT_NEEDED tags for dynamic libraries mentioned on the command line after the --as-needed option. Normally the linker will add a DT_NEEDED tag for each dynamic library mentioned on the command line, regardless of whether the library is actually needed or not. --as-needed causes a DT_NEEDED tag to only be emitted for a library that satisfies an undefined symbol reference from a regular object file or, if the library is not found in the DT_NEEDED lists of other libraries linked up to that point, an undefined symbol reference from another dynamic library. --no-as-needed restores the default behaviour.
Which sounds like a library may not link to a native dynamic library even if I ask the compiler to do so. I don't really know of a use case for doing that, but I don't want to prohibit that from happening just because I don't understand the use case.
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It avoids linking against shared objects on platforms where they don't provide any of the symbols used by the program. For example, clock_gettime was moved from librt.so to libc.so in modern glibc versions but the crate is likely to still specify -lrt if it needs it. I can't think of a valid reason to override it. It really only needs to be a flag for backwards compatibility with broken build systems not linking in the right order.
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@alexcrichton: The precedent for this is every widely used Linux distribution using these flags to build packages. Ubuntu, Debian, Fedora, Arch Linux, OpenSUSE, etc. all use these (and most likely use It currently only optimizes dynamic libraries and not binaries, but it will likely optimize binaries in the future. There is no harm in telling it to optimize a binary and it will transparently start doing it when it's improved in the future. All this part is saying is that there's currently only a binary choice between optimizing and not optimizing, with no distinction between numeric values:
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I don't particularly care about this PR, just cleaning up #6190 from triage. |
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(this week, three months later, same thing...) |
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