Re: Building Emacs on MacOS with clang+LTO fails

[Paul Eggert]

dhruva wrote:
> I am trying to build emacs using clang 8 (from home-brew) with link time
> optimization enabled.

As INSTALL says, link time optimization seems to make Emacs slower and/or 
crashier even when Emacs builds. Plus as you've discovered, Emacs tends not to 
build. So I don't recommend LTO unless your job is to fix LTO bugs.

> Emacs configured as:
> LD=lld CC=clang CFLAGS="-flto" ../../git/emacs/configure --with-mailutils
> --with-ns

I don't think LD matters.

I suggest using the --enable-link-time-optimization flag of 'configure' instead.

For what it's worth, I just now tried building Emacs master on Fedora 30 as 
follows:

./configure --enable-link-time-optimization CC=clang

and found a recently-introduced bug with fingerprinting that is a different bug 
from your problem. I installed the attached patch to work around it. As this 
patch suggests, LTO is not often tested with Emacs.

>    CCLD     make-fingerprint
> ld: reference to bitcode symbol '_rpl_getopt' which LTO has not compiled in
> '_main' from /var/folders/5c/5w9bqxts6gl2pgfgdpzq94n0nhqj5f/T/cc-ebc71c.o
> for architecture x86_64
> clang-8: error: linker command failed with exit code 1 (use -v to see
> invocation)

If make-fingerprint links without LTO and fails to link with LTO, it's most 
likely a bug in the way Emacs is using the linker (or a bug in the compiler or 
linker), which surely would need to be investigated further on your platform.

> -  while (0 <= (c = getopt (argc, argv, "rh")))
> +  while (0 <= (c = getopt_long (argc, argv, "rh", NULL, NULL)))

This doesn't look right, for a couple of reasons. First, getopt_long doesn't 
document what it does wit a null 4th arg. More importantly, why is this change 
needed, and not similar changes in other getopt calls? It'd be better to debug 
the actual linking problem than to try to paper over it.

0001-Port-new-fingerprinting-scheme-to-clang-LTO.patch
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