On Sun, May 8, 2022 at 12:10 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote: > > When CONFIG_LTO_CLANG=y, additional intermediate *.prelink.o is created > for each module. Also, objtool is postponed until LLVM bitcode is > converted to ELF. > > CONFIG_X86_KERNEL_IBT works in a similar way to postpone objtool until > objects are merged together. > > This commit stops generating *.prelink.o, so the build flow will look > the same with/without LTO. > > The following figures show how the LTO build currently works, and > how this commit is changing it. > > Current build flow > ================== > > [1] single-object module > > $(LD) > $(CC) +objtool $(LD) > foo.c --------------------> foo.o -----> foo.prelink.o -----> foo.ko > (LLVM bitcode) (ELF) | > | > foo.mod.o --/ > > [2] multi-object module > $(LD) > $(CC) $(AR) +objtool $(LD) > foo1.c -----> foo1.o -----> foo.o -----> foo.prelink.o -----> foo.ko > | (archive) (ELF) | > foo2.c -----> foo2.o --/ | > (LLVM bitcode) foo.mod.o --/ > > One confusion is foo.o in multi-object module is an archive despite of > its suffix. > > New build flow > ============== > > [1] single-object module > > Since there is only one object, we do not need to have the LLVM > bitcode stage. Use $(CC)+$(LD) to generate an ELF object in one > build rule. When LTO is disabled, $(LD) is unneeded because $(CC) > produces an ELF object. > > $(CC)+$(LD)+objtool $(LD) > foo.c ------------------------> foo.o -------> foo.ko > (ELF) | > | > foo.mod.o --/ > > [2] multi-object module > > Previously, $(AR) was used to combine LLVM bitcode into an archive, > but there was no technical reason to do so. > This commit just uses $(LD) to combine and convert them into a single > ELF object. > > $(LD) > $(CC) +objtool $(LD) > foo1.c -------> foo1.o -------> foo.o -------> foo.ko > | (ELF) | > foo2.c -------> foo2.o ---/ | > (LLVM bitcode) foo.mod.o --/ > > Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx> > Reviewed-by: Nicolas Schier <nicolas@xxxxxxxxx> > Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx> Looks good, thanks for cleaning this up! Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx> Sami
