https://sourceware.org/bugzilla/show_bug.cgi?id=33885
Bug ID: 33885
Summary: gas: Add an option to disable section symbol
adjustment
Product: binutils
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: gas
Assignee: unassigned at sourceware dot org
Reporter: maskray at sourceware dot org
Target Milestone: ---
When the assembler generates an unresolved fixup for a local symbol, it can
convert the relocation to reference the section symbol (`STT_SECTION`) instead,
folding the original symbol's offset within the section into the addend.
This allows the original local symbol to be omitted from `.symtab`.
The tradeoff is that the `STT_SECTION` symbol itself must be present, so the
conversion saves `.symtab` entries only when a section has more than one local
symbol referenced by relocations.
This is common in practice:
* Text sections often contain labels for jump targets or C++ exception
handling.
* DWARF `.debug_*` sections contain labels referenced by other `.debug_*`
sections.
* `SHF_STRINGS` sections (`.rodata.str1.1`, `.debug_str`, `.debug_line_str`)
have a label for each string literal.
(
Not all relocations are eligible for this conversion.
PLT-generating and GOT-generating relocations, for example, may require dynamic
relocations where the symbol identity is significant, so they must reference
the original symbol.
In GNU Assembler, the backend hook `tc_fix_adjustable` controls which
relocation types are excluded from the conversion.)
In certain scenarios, disabling the STT_SECTION adjustment is preferable.
For instance, when range extension thunks for x86-64 are available, retaining
the original foo0 and foo1 symbols in R_X86_64_PLT32 relocations allows the
linker to generate descriptive thunk names like __X86_64CallThunk_foo0
With the unnamed section symbol, the linker cannot easily generate a
descriptive name (if clang is used with -ffunction-sections and
-fno-unique-section-names, the section will be .text instead of .text.foo0).
call foo0
call foo1
.section .text.foo0,"ax"
foo0:
.section .text.foo1,"ax"
foo1:
(In lld/ELF's --branch-to-branch optimization
https://github.com/llvm/llvm-project/pull/145579/ , it would be simpler if no
assembler generated R_X86_64_PLT32 referencing STT_SECTION symbol.)
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