On Thu, Aug 3, 2017 at 12:45 PM, Ximin Luo <infini...@pwned.gg> wrote: > Yury Gribov: >> On 03.08.2017 3:06, Ximin Luo wrote: >>> Jeff Law: >>>> On 07/21/2017 10:15 AM, Ximin Luo wrote: >>>>> (Please keep me on CC, I am not subscribed) >>>>> >>>>> >>>>> Proposal >>>>> ======== >>>>> >>>>> This patch series adds a new environment variable BUILD_PATH_PREFIX_MAP. >>>>> When >>>>> this is set, GCC will treat this as extra implicit >>>>> "-fdebug-prefix-map=$value" >>>>> command-line arguments that precede any explicit ones. This makes the >>>>> final >>>>> binary output reproducible, and also hides the unreproducible value (the >>>>> source >>>>> path prefixes) from CFLAGS et. al. which many build tools (understandably) >>>>> embed as-is into their build output. >>>> I'd *really* avoid doing this with magic environment variables. Make it >>>> a first class option to the compiler. Yes, it means projects that want >>>> this behavior have to arrange to pass that flag to their compiler, but >>>> IMHO that's much preferred over environment variables. >>>> >>>> Jeff >>>> >>> >>> Hi Jeff, >>> >>> 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. >> >> 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. > 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)? -Y
