[lld][WebAssemlby] Fix for string merging of -dwarf-5 sections

[sbc100]

Summary

We were mistakenly treating .debug_str_offsets as a string mergable
section when it is not (it contains integers not strings). This is an
indication that we really should find a way to store flags for custom
sections.

[sbc100]

• add bug links

[sbc100]

• add test

[dschuff]

lld/wasm/InputFiles.cpp

375

I guess there is no .debug_line_str.dwo because the .debug_line section goes into the main executable, instead of a dwo file?

[sbc100]

lld/wasm/InputFiles.cpp

375

Indeed. At least there is no such section marked in initWasmMCObjectFileInfo which is the canonical list of string merge-able sections (which is why I mention it here).

lld/test/wasm/merge-string-debug.s

 # RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/merge-string-debug2.s -o %t2.o
 
 # RUN: wasm-ld %t.o %t2.o -o %t.wasm --no-entry
 # RUN: llvm-readobj -x .debug_str %t.wasm | FileCheck %s --check-prefixes CHECK,CHECK-O1
 
 # Check that we -r/--reclocatable can handle string merging too
 # RUN: wasm-ld --relocatable %t.o %t2.o -o %t3.o
 # RUN: wasm-ld -O1 %t3.o -o %t.wasm --no-entry
 # RUN: llvm-readobj -x .debug_str %t.wasm | FileCheck %s --check-prefixes CHECK,CHECK-O1
 
 # RUN: wasm-ld -O0 %t.o %t2.o -o %tO0.wasm --no-entry
 # RUN: llvm-readobj -x .debug_str %tO0.wasm | FileCheck %s --check-prefixes CHECK,CHECK-O0
+# RUN: llvm-readobj -x .debug_str_offsets %tO0.wasm | FileCheck %s --check-prefixes CHECK-OFFSETS
 
 .section .debug_str,"S",@
 .Linfo_string0:
   .asciz "clang version 13.0.0"
 .Linfo_string1:
   .asciz "foobar"
 
 .section .debug_other,"",@
   .int32 .Linfo_string0
 
+.section .debug_str_offsets,"",@
+  .int32 .Linfo_string0
+  .int32 .Linfo_string0
+  .int32 .Linfo_string0
+
 # CHECK: Hex dump of section '.debug_str':
 
 # CHECK-O0: 0x00000000 636c616e 67207665 7273696f 6e203133 clang version 13
 # CHECK-O0: 0x00000010 2e302e30 00666f6f 62617200 636c616e .0.0.foobar.clan
 # CHECK-O0: 0x00000020 67207665 7273696f 6e203133 2e302e30 g version 13.0.0
 # CHECK-O0: 0x00000030 00626172 00666f6f 00                .bar.foo.
 
 # CHECK-O1: 0x00000000 666f6f62 61720066 6f6f0063 6c616e67 foobar.foo.clang
 # CHECK-O1: 0x00000010 20766572 73696f6e 2031332e 302e3000  version 13.0.0.
+
+# CHECK-OFFSETS: Hex dump of section '.debug_str_offsets':
+# CHECK-OFFSETS: 0x00000000 00000000 00000000 00000000          ............

lld/wasm/InputFiles.cpp

 
 static bool shouldMerge(const WasmSection &sec) {
   if (config->optimize == 0)
     return false;
   // Sadly we don't have section attributes yet for custom sections, so we
   // currently go by the name alone.
   // TODO(sbc): Add ability for wasm sections to carry flags so we don't
   // need to use names here.
-  return sec.Name.startswith(".debug_str") ||
-         sec.Name.startswith(".debug_line_str");
+  // For now, keep in sync with uses of wasm::WASM_SEG_FLAG_STRINGS in
+  // MCObjectFileInfo::initWasmMCObjectFileInfo which creates these custom
+  // sections.
+  return sec.Name == ".debug_str" || sec.Name == ".debug_str.dwo" ||
+         sec.Name == ".debug_line_str";
 }
 
 static bool shouldMerge(const WasmSegment &seg) {
   // As of now we only support merging strings, and only with single byte
   // alignment (2^0).
   if (!(seg.Data.LinkingFlags & WASM_SEG_FLAG_STRINGS) ||
       (seg.Data.Alignment != 0))
     return false;