[cfe-dev] [RFC] Path mappings for reproducable builds and debugging hacks

[Reid Kleckner via cfe-dev]

I haven't looked deeply into this issue, but I thought
-no-canonical-prefixes was the flag for this use case. Can you elaborate on
what makes it unsuitable?

On Thu, Sep 28, 2017 at 6:43 AM, Joerg Sonnenberger via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hello all,
> I've been discussing this topic on #llvm with some of the regulars, but
> this merrits a wider audience. As you some of you might know, NetBSD
> allows doing a full release build with GCC in a reproducable way,
> including variations of the source locations. This is currently not
> possible with clang and I want to fix that. There are four identified
> primary points where absolute path names leak into the output:
>
> (1) .file
> (2) DWARF
> (3) __FILE__
> (4) __PRETTY_FUNCTION__ for lambdas etc
>
> We have -fdebug-prefix-map [-fdpm from here] for (2) with some
> limitations. I've created -iremap for GCC for (3) years ago, the patch
> is still in review limbo.  We don't have anything for (1) and (4) right
> now in clangland. I've started to write patches for that, but this is a
> bit messy as it tends to duplicate code. This made me want to step back
> and review whether we need/want many different switches in first place.
> I couldn't come up with a very good reason, but it has been mentioned
> that Facebook is using -fdpm for speakable hacks to get space into the
> binaries for patching in real patches. That seems to be abusive for me,
> even when I can somewhat understand the motivation.
>
> My proposal forward is:
> (1) Tighten the definition of -fdpm to mean prefix paths, i.e. the next
> character must be a path separator:
>   -fdebug-prefix-map=/foo=/bar should not change /foobar into /barbar
> (2) Introduce a new option -frewrite-path=src=dst and make -fdpm an
> alias of it. The translation applies to all four points above.
> (3) Introduce a new -gdwarf-path-padding=$n option to prefix all path
> names encoded in DWARF with $n path separators.
>
> The goal forward is that path references in output files generated by
> clang (except dependency files) should easy adjustable to a canonical
> location, independent of where the sources are located.
>
> Joerg
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