[RISCV] Make empty name symbols SF_FormatSpecific so that llvm-symbolizer ignores them for symbolization

On RISC-V, clang emits empty name symbols used for label differences. (In GCC the symbols are typically `.L0`)
After D95916, the empty name symbols can show up in llvm-symbolizer's symbolization output.
They have no names and thus not useful. Set `SF_FormatSpecific` so that llvm-symbolizer will ignore them.

`SF_FormatSpecific` is also used in LTO but that case should not matter.

Corresponding addr2line problem: https://sourceware.org/bugzilla/show_bug.cgi?id=27585

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D98669

Author: MaskRay

llvm/include/llvm/Object/ELFObjectFile.h

@@ -728,6 +728,15 @@ Expected<uint32_t> ELFObjectFile<ELFT>::getSymbolFlags(DataRefImpl Sym) const {
     }
     if (ESym->getType() == ELF::STT_FUNC && (ESym->st_value & 1) == 1)
       Result |= SymbolRef::SF_Thumb;
+  } else if (EF.getHeader().e_machine == ELF::EM_RISCV) {
+    if (Expected<StringRef> NameOrErr = getSymbolName(Sym)) {
+      // Mark empty name symbols used for label differences.
+      if (NameOrErr->empty())
+        Result |= SymbolRef::SF_FormatSpecific;
+    } else {
+      // TODO: Actually report errors helpfully.
+      consumeError(NameOrErr.takeError());
+    }
   }
 
   if (ESym->st_shndx == ELF::SHN_UNDEF)

llvm/test/DebugInfo/Symbolize/ELF/riscv-empty-name-symbol.s

@@ -0,0 +1,26 @@
+# REQUIRES: riscv-registered-target
+## Ignore empty name symbols.
+
+# RUN: llvm-mc -filetype=obj -triple=riscv64 %s -o %t
+# RUN: llvm-readelf -s %t | FileCheck %s --check-prefix=SYM
+
+# SYM: 0000000000000004  0 NOTYPE LOCAL  DEFAULT [[#]] {{$}}
+# SYM: 0000000000000000  0 NOTYPE GLOBAL DEFAULT [[#]] foo
+
+## Make sure we test at an address larger than or equal to an empty name symbol.
+# RUN: llvm-symbolizer --obj=%t 0 4 | FileCheck %s
+
+# CHECK:       foo
+# CHECK-NEXT:  ??:0:0
+# CHECK-EMPTY:
+# CHECK-NEXT:  foo
+# CHECK-NEXT:  ??:0:0
+
+.globl foo
+foo:
+  nop
+  .file 1 "/tmp" "a.s"
+  .loc 1 1 0
+  nop
+
+.section .debug_line,"",@progbits