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[AsmPrinter] Split up .gcc_except_table
MC currently produces monolithic .gcc_except_table section. GCC can split up .gcc_except_table: * if comdat: `.section .gcc_except_table._Z6comdatv,"aG",@progbits,_Z6comdatv,comdat` * otherwise, if -ffunction-sections: `.section .gcc_except_table._Z3fooi,"a",@progbits` This ensures that (a) non-prevailing copies are discarded and (b) .gcc_except_table associated to discarded text sections can be discarded by a .gcc_except_table-aware linker (GNU ld, but not gold or LLD) This patches matches the GCC behavior. If -fno-unique-section-names is specified, we don't append the suffix. If -ffunction-sections is additionally specified, use `.section ...,unique`. Note, if clang driver communicates that the linker is LLD and we know it is new (11.0.0 or later) we can use SHF_LINK_ORDER to avoid string table costs, at least in the -fno-unique-section-names case. We cannot use it on GNU ld because as of binutils 2.35 it does not support mixed SHF_LINK_ORDER & non-SHF_LINK_ORDER components in an output section https://sourceware.org/bugzilla/show_bug.cgi?id=26256 For RISC-V -mrelax, this patch additionally fixes an assembler-linker interaction problem: because a section is shrinkable, the length of a call-site code range is not a constant. Relocations referencing the associated text section (STT_SECTION) are needed. However, a STB_LOCAL relocation referencing a discarded section group member from outside the group is disallowed by the ELF specification (PR46675): ``` // a.cc inline int comdat() { try { throw 1; } catch (int) { return 1; } return 0; } int main() { return comdat(); } // b.cc inline int comdat() { try { throw 1; } catch (int) { return 1; } return 0; } int foo() { return comdat(); } clang -target riscv64-linux -c a.cc b.cc -fPIC -mno-relax ld.lld -shared a.o b.o => ld.lld: error: relocation refers to a symbol in a discarded section: ``` -fbasic-block-sections= is similar to RISC-V -mrelax: there are outstanding relocations. Reviewed By: jrtc27, rahmanl Differential Revision: https://reviews.llvm.org/D83655
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