Em Fri, Apr 02, 2021 at 11:07:10AM -0700, Nick Desaulniers escreveu: > On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song <yhs@xxxxxx> wrote: > > Currently, clang LTO built vmlinux won't work with pahole. > > LTO introduced cross-cu dwarf tag references and broke > > current pahole model which handles one cu as a time. > > The solution is to merge all cu's as one pahole cu as in [1]. > > We would like to do this merging only if cross-cu dwarf > > references happens. The LTO build mode is a pretty good > > indication for that. > > In earlier version of this patch ([2]), clang flag > > -grecord-gcc-switches is proposed to add to compilation flags > > so pahole could detect "-flto" and then merging cu's. > > This will increate the binary size of 1% without LTO though. > > Arnaldo suggested to use a note to indicate the vmlinux > > is built with LTO. Such a cheap way to get whether the vmlinux > > is built with LTO or not helps pahole but is also useful > > for tracing as LTO may inline/delete/demote global functions, > > promote static functions, etc. > > So this patch added an elfnote with a new type LINUX_ELFNOTE_LTO_INFO. > > The owner of the note is "Linux". > > With gcc 8.4.1 and clang trunk, without LTO, I got > > $ readelf -n vmlinux > > Displaying notes found in: .notes > > Owner Data size Description > > ... > > Linux 0x00000004 func > > description data: 00 00 00 00 > > ... > > With "readelf -x ".notes" vmlinux", I can verify the above "func" > > with type code 0x101. > > > > With clang thin-LTO, I got the same as above except the following: > > description data: 01 00 00 00 > > which indicates the vmlinux is built with LTO. > > > > [1] https://lore.kernel.org/bpf/20210325065316.3121287-1-yhs@xxxxxx/ > > [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@xxxxxx/ > > > > Suggested-by: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx> > > Signed-off-by: Yonghong Song <yhs@xxxxxx> > > LGTM thanks Yonghong! > Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Tested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> Thanks! - Arnaldo
