On Sun, Dec 09, 2018 at 10:38:10AM +0100, Georg Sauthoff wrote: > Hello, > > during packaging of a new python-img2pdf version I noticed that 2 > unit-test cases fail on aarch64. > > This is caused by zlib.compress() yielding different output on aarch64 > and x86-64. Which triggers the failure because the unit-test case just > compares the result after compression (it's a small PDF that contains a > compressed image - and that PDF is compared against a reference PDF). > > I checked the aarch64 compress output and it's valid zlib data, i.e. I can > decompress it on both aarch64 and x86-64 and get the same decompression > result (i.e. the input image data). > > As far as I understand RFC 1951, it allows for different implementations > to yield differing compress output bit-streams - as long as other > requirements are met and uncompress(compress(x)) == x (in any > combination of implementations). > > Funnily, the img2pdf author uses Debian, and Debian aarch64 > zlib.compress() *does* yield bit-identical output (to x86-64). Perhaps > the deviating Fedora aarch64 behaviour is due to different compile > options/patches (e.g. some Neon optimization or something like that). IMO there's two bugs that should be independently fixed: - img2pdf should not assume bit-indentical output since zlib guarantee that. It *would* be convenient to assume that, but it clearly doesn't work. - More of an RFE, but I'd really try to make zlib output identical on all architectures, to make things more predictable and repeatable. From the bug report, the compression is more efficient by one byte sometimes, so that sounds like an optimization opportunity. Zbyszek > The img2pdf author isn't really convinced that zlib may produce > different output on different architectures: > > https://gitlab.mister-muffin.de/josch/img2pdf/issues/51 > > Any thoughts? > > Best regards > Georg > -- > 'I have long really held the opinion that the amount of noise > which any one can bear undisturbed stands in inverse proportion > to his mental capacity, and therefore may be regarded as a pretty > fair measure of it.' (Arthur Schopenhauer, The World As Will And > Idea. Vol. 2, p.199, 1883) > _______________________________________________ > devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx > To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx > Fedora Code of Conduct: https://getfedora.org/code-of-conduct.html > List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines > List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://getfedora.org/code-of-conduct.html List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx
