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Re: Warnings when building with LTO

From: Paul Eggert
Subject: Re: Warnings when building with LTO
Date: Fri, 10 May 2019 00:23:30 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

On 5/9/19 3:49 PM, Alex Gramiak wrote:
> INSTALL states:
>   Link time optimization is not the default as it tends to cause crashes
>   and to make Emacs slower.
> Is this still true with GCC 9.1?

Not clear. Nobody's really tried it as far as I know. Few people are
using LTO and to some extent it's a chicken-and-egg problem. See, for
example, this recent LTO problem on macOS:


for why I still don't recommend using LTO unless your job is to debug LTO.

> Paul, can you still reproduce the
> bug[2] you mentioned in commit 9b4b2e9fc8 with GCC 9.1?
> [2] https://bugzilla.redhat.com/show_bug.cgi?id=1486455

No, it's fixed for me now (Fedora 30 x86-64, 9.1.1 20190503 (Red Hat
9.1.1-1), current master).

I assume that all the warnings you mention are bogus, though I haven't
checked this. It would not be wise to attempt to pacify GCC on all
platforms, as it (1) would take a lot of time and (2) would clutter the
code and get in the way of real work. So we tend to pacify GCC only for
recent GCC with default compiler flags. If LTO becomes the default or is
otherwise commonly used we should pacify the warnings you mention; until
then, I wouldn't bother.

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