[all-commits] [llvm/llvm-project] ee5d1a: [AsmPrinter] Split up .gcc_except_table

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: ee5d1a04494390ab8dbece1e32cb00312ecce4b1
      https://github.com/llvm/llvm-project/commit/ee5d1a04494390ab8dbece1e32cb00312ecce4b1
  Author: Fangrui Song <maskray at google.com>
  Date:   2020-11-02 (Mon, 02 Nov 2020)

  Changed paths:
    M llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
    M llvm/include/llvm/MC/MCObjectFileInfo.h
    M llvm/include/llvm/Target/TargetLoweringObjectFile.h
    M llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp
    M llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
    A llvm/test/CodeGen/X86/gcc_except_table-multi.ll

  Log Message:
  -----------
  [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", at progbits,_Z6comdatv,comdat`
* otherwise, if -ffunction-sections: `.section .gcc_except_table._Z3fooi,"a", at 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