On 2020-06-22, Nick Desaulniers wrote:
On Wed, Jun 17, 2020 at 2:27 PM Fāng-ruì Sòng maskray@google.com wrote:
On 2020-06-17, Nick Desaulniers wrote:
ld.bfd's internal linker script considers .text.hot AND .text.hot.* to be part of .text, as well as .text.unlikely and .text.unlikely.*.
ld.lld will produce .text.hot.*/.text.unlikely.* sections.
Correction to this sentence. lld is not relevant here.
-ffunction-sections combined with profile-guided optimization can produce .text.hot.* .text.unlikely.* sections. Newer clang may produce .text.hot. .text.unlikely. (without suffix, but with a trailing dot) when -fno-unique-section-names is specified, as an optimization to make .strtab smaller.
Then why was the bug report reporting https://reviews.llvm.org/D79600 as the result of a bisection, if LLD is not relevant? Was the bisection wrong?
https://reviews.llvm.org/D79600 is an LLVM codegen change, unrelated to LLD.. (As described in the patch, LLD's -z keep-text-section-prefix only recognizes ".text.exit.*", not ".text.exit")
The upstream report wasn't initially public, for no good reason. So I didn't include it, but if we end up taking v1, this should have
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760
Thanks for making it public.
The kernel doesn't use -fno-unique-section-names; is that another flag that's added by CrOS' compiler wrapper? https://source.chromium.org/chromiumos/chromiumos/codesearch/+/master:src/th... Looks like no. It doesn't use `-fno-unique-section-names` or `-ffunction-sections`.
-fno-unique-section-names is a very rare option. It is not supported by GCC (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95095 ). clang users use it very rarely, probably because not many people care about additional strings taken by section names ".text.hot.a" ".text.hot.b" ".text.hot.c" in the string table ".strtab" (clang since some point of 2018 uses .strtab instead of .shstrtab which enables more string sharing).
We've already seen that GCC can place main in .text.startup without -ffunction-sections. There may be other non -ffunction-sections cases for .text.hot.* or .text.unlikely.*. So it is definitely a good idea to be more specific even if we don't care about -ffunction-sections for now.
Make sure to group these together. Otherwise these orphan sections may be placed outside of the the _stext/_etext boundaries.
Cc: stable@vger.kernel.org Link: https://sourceware.org/git/?p=binutils-gdb.git%3Ba=commitdiff%3Bh=add44f8d5c... Link: https://sourceware.org/git/?p=binutils-gdb.git%3Ba=commitdiff%3Bh=1de778ed23... Link: https://reviews.llvm.org/D79600 Reported-by: Jian Cai jiancai@google.com Debugged-by: Luis Lozano llozano@google.com Suggested-by: Fāng-ruì Sòng maskray@google.com Tested-by: Luis Lozano llozano@google.com Tested-by: Manoj Gupta manojgupta@google.com Signed-off-by: Nick Desaulniers ndesaulniers@google.com
include/asm-generic/vmlinux.lds.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h index d7c7c7f36c4a..fe5aaef169e3 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -560,7 +560,9 @@ */ #define TEXT_TEXT \ ALIGN_FUNCTION(); \
*(.text.hot TEXT_MAIN .text.fixup .text.unlikely) \
*(.text.hot .text.hot.*) \*(TEXT_MAIN .text.fixup) \*(.text.unlikely .text.unlikely.*) \ NOINSTR_TEXT \ *(.text..refcount) \ *(.ref.text) \-- 2.27.0.290.gba653c62da-goog
-- Thanks, ~Nick Desaulniers