support transparent LTO on linux

[nlewycky]

Bugzilla Link	948
Resolution	FIXED
Resolved on	Feb 03, 2009 22:03
Version	trunk
OS	Linux
CC	@chandlerc

Extended Description

llvm-gcc should know to run llvm-ld when linking bytecode files. Specifically,
running "llvm-gcc main.bc aux.bc -o myprog" should produce a native "myprog". It
should also parse and interpret the command-line parameters -O0 to turn on/off
optimization.

This is necessary to use llvm-gcc with LTO with an autoconf based package.

[nlewycky]

assigned to @nlewycky

[lattner]

Note that llvm-gcc4 already responds correctly to -O[123] options.

The command you really want is:

llvm-gcc -O4 main.bc aux.bc -o myprog

which will fail with a link error today on linux.

-Chris

[llvmbot]

To do this, we need to get binutils ld accepting byte code, I believe. Chandler
had a patch (or partial patch) to make that happen. Chandler, can you attach it
here?

Reid

[llvmbot]

Is there any progress on this? I would like to have the Ada f-e link
bytecode and native object code into executables without having to do
too many horrible hacks. If you could link bytecode using gcc, as in
this PR, I wouldn't have to do anything. Otherwise I can hack the Ada
linker (yes, it has its own, built on top of the system linker, which
it calls via gcc), but I don't really want to :)

[lattner]

A good intermediate to having gnu LD fully understand liblto would be to enhance collect2 to understand
liblto. At that point, linking with llvm-gcc would work fine, and would cover most of the use cases (linking
with ld would not work obviously). This would also be nice because you don't have to explain to people
how to replace their linker, etc.

This obviously complements gnu ld liblto support, it doesn't replace it.

[lattner]

*** Bug llvm/llvm-bugzilla-archive#1849 has been marked as a duplicate of this bug. ***

[nlewycky]

Done. Set up gold as per tools/gold/README.txt and use the llvm-gcc -use-gold-plugin option!

Problems with the plugin should get their own bugs.