Yury Gribov: > On Thu, Aug 3, 2017 at 12:45 PM, Ximin Luo <infini...@pwned.gg> wrote: >> Yury Gribov: >>> [..] >>> >>> Shouldn't -fdebug-prefix-map be updated to use the same syntax as >>> BUILD_PATH_PREFIX_MAP? >>> >> >> -fdebug-prefix-map is a CLI option and can be given multiple times, each >> flag given is in the form of $from=$to where $from can't contain a '='. >> >> BUILD_PATH_PREFIX_MAP is a single envvar that encodes a list-of-pairs of the >> form $to=$from:$to=$from with some escaping for flexibility and to support >> things like windows paths. Since it's a new envvar, Ian Jackson suggested >> $to=$from to emphasise the reproducible ($to) part. I liked the idea so I >> implemented it like that. (We did a lot of bikeshedding over on the >> rb-general mailing list about the exact format and this is what we settled >> on, I'd like to avoid getting into that again but would nevertheless do it, >> if it's necessary to get this patch accepted.) >> >> Because -fdebug-prefix-map currently only encodes one $from=$to pair, it >> would be a very disruptive and highly backward-incompatible change to make >> it use the same syntax as B_P_P_M. A slightly less disruptive but still >> backward-incompatible change would be to make it encode a single $to=$from >> pair, but I don't really see the advantage to doing so - what were your >> thoughts on this? > > I believe it would much easier to reason about environment variable > behavior when it boils down to "prepend some standard flag to > command-line flags". It would also simplify maintenance of local > compiler patch as core functionality can be merged to mainline GCC > whereas debatable environment variable part stays in the distro. >
[answered in another email together with other related points] >> If by "first class option" you meant a command-line flag, GCC *already has* >> that (-fdebug-prefix-map) and it wasn't enough to achieve reproducibility in >> many cases we tested. >> dpkg-buildflags actually already adds these flags to CFLAGS CXXFLAGS etc on >> Debian. However, with this patch using the environment variable, we are able >> to reproduce 1800 more packages out of 26000. > > Just curious, why -fdebug-prefix-map (maybe modified to support > multiple renames) was not enough for these packages (and why they > can't be fixed instead)? > One important reason is that some packages embed CFLAGS/CXXFLAGS in build output such as pkg-config files or Makefiles to be installed as examples. To fix this, we'd have to add buildsystem-specific logic to strip out -fdebug-prefix-map when it was writing such output. This does not affect all of these 1800 packages, but I saw enough cases that I was convinced that the use of a new envvar was a better approach - I don't think buildsystems should be burdened with having to know "which flag values are reproducible vs not", this is not the case with other CFLAGS. X -- GPG: ed25519/56034877E1F87C35 GPG: rsa4096/1318EFAC5FBBDBCE https://github.com/infinity0/pubkeys.git
