On Tue, Mar 30, 2021 at 6:48 PM Yonghong Song <yhs@xxxxxx> wrote: > > > > On 3/30/21 5:25 PM, Fangrui Song wrote: > > On 2021-03-30, 'Yonghong Song' via Clang Built Linux wrote: > >> > >> > >> On 3/29/21 3:52 PM, Nick Desaulniers wrote: > >>> (replying to > >>> https://lore.kernel.org/bpf/20210328064121.2062927-1-yhs@xxxxxx/) > >>> > >>> Thanks for the patch! > >>> > >>>> +# gcc emits compilation flags in dwarf DW_AT_producer by default > >>>> +# while clang needs explicit flag. Add this flag explicitly. > >>>> +ifdef CONFIG_CC_IS_CLANG > >>>> +DEBUG_CFLAGS += -grecord-gcc-switches > >>>> +endif > >>>> + > > > > Yes, gcc defaults to -grecord-gcc-switches. Clang doesn't. > > Could you know why? dwarf size concern? > > > > >>> This adds ~5MB/1% to vmlinux of an x86_64 defconfig built with clang. > >>> Do we > >>> want to add additional guards for CONFIG_DEBUG_INFO_BTF, so that we > >>> don't have > >>> to pay that cost if that config is not set? > >> > >> Since this patch is mostly motivated to detect whether the kernel is > >> built with clang lto or not. Let me add the flag only if lto is > >> enabled. My measurement shows 0.5% increase to thinlto-vmlinux. > >> The smaller percentage is due to larger .debug_info section > >> (almost double) for thinlto vs. no lto. > >> > >> ifdef CONFIG_LTO_CLANG > >> DEBUG_CFLAGS += -grecord-gcc-switches > >> endif > >> > >> This will make pahole with any clang built kernels, lto or non-lto. > > > > I share the same concern about sizes. Can't pahole know it is clang LTO > > via other means? If pahole just needs to know the one-bit information > > (clang LTO vs not), having every compile option seems unnecessary.... > > This is v2 of the patch > https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@xxxxxx/ > The flag will be guarded with CONFIG_LTO_CLANG. > > As mentioned in commit message of v2, the alternative is > to go through every cu to find out whether DW_FORM_ref_addr is used > or not. In other words, check every possible cross-cu references > to find whether cross-cu reference actually happens or not. This > is quite heavy for pahole... > > What we really want to know is whether cross-cu reference happens > or not? If there is an easy way to get it, that will be great. +David Blaikie > > > >> If the maintainer wants further restriction with CONFIG_DEBUG_INFO_BTF, > >> I can do that in another revision. > >> > >> -- > >> You received this message because you are subscribed to the Google > >> Groups "Clang Built Linux" group. > >> To unsubscribe from this group and stop receiving emails from it, send > >> an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > >> To view this discussion on the web visit > >> https://groups.google.com/d/msgid/clang-built-linux/0b8d17be-e015-83c3-88d8-7c218cd01536@xxxxxx > >> . -- 宋方睿
