[PATCH] D65727: [LLD] [COFF] Omit automatically imported symbols from the symbol table

[Martin Storsjö via Phabricator via llvm-commits]

mstorsjo created this revision.
mstorsjo added reviewers: ruiu, rnk.
Herald added a project: LLVM.

For these symbols, they actually point to the symbol's IAT entry instead, which obviously is different from the symbol itself (which is imported from a different module and doesn't exist in this one).

Omitting this symbol helps gdb inspect automatically imported symbols, see https://sourceware.org/bugzilla/show_bug.cgi?id=24574 for discussion on the matter.

Surprisingly, those extra symbols don't seem to be an issue for gdb when the sources have been built with clang, only with gcc. The actual logic in gdb that this depends on still is unknown, but omitting these symbols from the symbol table is the right thing to do in any case.

While this isn't a normal bugfix, it fixes one issue reported to me by a user, and the fix is very straightforward, so maybe it still could qualify for the release branch @hans?


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D65727

Files:
  COFF/Writer.cpp
  test/COFF/autoimport-gnu-implib.s
  test/COFF/autoimport-x86.s


Index: test/COFF/autoimport-x86.s
===================================================================
--- test/COFF/autoimport-x86.s
+++ test/COFF/autoimport-x86.s
@@ -5,11 +5,12 @@
 # RUN: lld-link -out:%t-lib.dll -dll -entry:DllMainCRTStartup %t-lib.obj -lldmingw -implib:%t-lib.lib
 
 # RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.obj
-# RUN: lld-link -lldmingw -out:%t.exe -entry:main %t.obj %t-lib.lib -verbose
+# RUN: lld-link -lldmingw -debug:symtab -out:%t.exe -entry:main %t.obj %t-lib.lib -verbose
 
 # RUN: llvm-readobj --coff-imports %t.exe | FileCheck -check-prefix=IMPORTS %s
 # RUN: llvm-objdump -d %t.exe | FileCheck -check-prefix=DISASM %s
 # RUN: llvm-objdump -s %t.exe | FileCheck -check-prefix=CONTENTS %s
+# RUN: llvm-nm %t.exe | FileCheck -check-prefix=SYMBOLS %s
 
 # IMPORTS: Import {
 # IMPORTS-NEXT: Name: autoimport-x86.s.tmp-lib.dll
@@ -20,7 +21,7 @@
 
 # DISASM: Disassembly of section .text:
 # DISASM-EMPTY:
-# DISASM: .text:
+# DISASM: main:
 # Relative offset at 0x1002 pointing at the IAT at 0x2080.
 # DISASM: 140001000:      8b 05 7a 10 00 00       movl    4218(%rip), %eax
 # DISASM: 140001006:      c3      retq
@@ -41,6 +42,10 @@
 # CONTENTS:  140003000 80200040 01000000 00200040 01000000
 # CONTENTS:  140003010 24200040 01000000
 
+# Check that the automatically imported symbol "variable" is not listed in
+# the symbol table.
+# SYMBOLS-NOT: variable
+
     .global main
     .text
 main:
Index: test/COFF/autoimport-gnu-implib.s
===================================================================
--- test/COFF/autoimport-gnu-implib.s
+++ test/COFF/autoimport-gnu-implib.s
@@ -7,9 +7,10 @@
 # RUN: llvm-ar rcs %t-implib.a %t-dabcdh.o %t-dabcds00000.o %t-dabcdt.o
 
 # RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.obj
-# RUN: lld-link -lldmingw -out:%t.exe -entry:main %t.obj %t-implib.a -verbose
+# RUN: lld-link -lldmingw -debug:symtab -out:%t.exe -entry:main %t.obj %t-implib.a -verbose
 
 # RUN: llvm-readobj --coff-imports %t.exe | FileCheck -check-prefix=IMPORTS %s
+# RUN: llvm-nm %t.exe | FileCheck -check-prefix=SYMBOLS %s
 
 # IMPORTS: Import {
 # IMPORTS-NEXT: Name: foo.dll
@@ -18,6 +19,10 @@
 # IMPORTS-NEXT: Symbol: data (0)
 # IMPORTS-NEXT: }
 
+# Check that the automatically imported symbol "data" is not listed in
+# the symbol table.
+# SYMBOLS-NOT: {{ }}data
+
     .global main
     .text
 main:
Index: COFF/Writer.cpp
===================================================================
--- COFF/Writer.cpp
+++ COFF/Writer.cpp
@@ -1095,6 +1095,13 @@
   }
   }
 
+  // Symbols that are runtime pseudo relocations don't point to the actual
+  // symbol data itself (as they are imported), but points to the IAT entry
+  // instead. Avoid emitting them to the symbol table, as they can confuse
+  // debuggers.
+  if (def->isRuntimePseudoReloc)
+    return None;
+
   StringRef name = def->getName();
   if (name.size() > COFF::NameSize) {
     sym.Name.Offset.Zeroes = 0;


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