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?
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
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`.
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