[MC] Rename temporary symbols of empty name to ".L0 " (#89693)

Temporary symbols generated for .eh_frame and .debug_line have an empty
name, which appear in .symtab in the presence of RISC-V style linker
relaxation and will not be discarded by ld/objcopy --discard-locals
(-X).

In contrast, GNU assembler's riscv port assigns a fake name ".L0 " (with
a trailing space) to these symbols so that will be discarded by
ld/objcopy --discard-locals.

This patch matches the GNU behavior. Since Clang's RISC-V targets pass
-X to ld, and GNU ld defaults to -X for RISC-V targets, these ".L0 "
symbols will be discarded after linking by default, as expected by
users.

The llvm-symbolizer special case for RISC-V `SF_FormatSpecific` symbols
https://reviews.llvm.org/D98669 needs to be adjusted.

Note: `"":` in assembly currently crashes.

Note: bolt tests used /usr/bin/clang before
llvmorg-19-init-9532-g59bfc3106874.
The revert llvmorg-19-init-9531-g28b55342e1a8 actually broke
bolt/test/RISCV/fake-label-no-entry.c

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 005521ba..b8ef265 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -725,7 +725,13 @@ void ELFWriter::computeSymbolTable(
         HasLargeSectionIndex = true;
     }
 
+    // Temporary symbols generated for certain assembler features (.eh_frame,
+    // .debug_line) of an empty name may be referenced by relocations due to
+    // linker relaxation. Rename them to ".L0 " to match the gas fake label name
+    // and allow ld/objcopy --discard-locals to discard such symbols.
     StringRef Name = Symbol.getName();
+    if (Name.empty())
+      Name = ".L0 ";
 
     // Sections have their own string table
     if (Symbol.getType() != ELF::STT_SECTION) {