[LLVMdev] LTO question

[Philip Reames]

On 12/26/2014 03:46 PM, Xinliang David Li wrote:
> Vikram, LLVM can handle Firefox size programs. Honza wrote two good
> articles about LTO.
>
> http://hubicka.blogspot.com/2014/04/linktime-optimization-in-gcc-1-brief.html
> http://hubicka.blogspot.com/2014/04/linktime-optimization-in-gcc-2-firefox.html
>
> Comparison with LLVM is described in the second article. It took about
> 40min to finish building Firefox with llvm using lto and -g. The
> following is a quote:
>
> "This graph shows issues with debug info memory use. LLVM goes up to
> 35GB. LLVM developers are also working on debug info merging
> improvements (equivalent to what GCC's type merging is) and the
> situation has improved in last two releases until the current shape.
> Older LLVM checkouts happily run out of 60GB memory & 60GB swap on my
> machine.".
One thing I want to highlight in the above is that this is a build with 
debug information.  My understanding from listening in on conversations 
at the developers meeting is that LTO w/o debug information scales 
substantially better.  Most of the discussion of memory usage is 
specific to retaining full debug info while doing LTO.  Either compiling 
without debug info at all, or compiling with only line table information 
results in dramatically reduced memory usage.

Note: I have no direct experience here; I'm just repeating what I've heard.

Philip



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