[llvm-objcopy] [COFF] Remove unreferenced undefined externals with --strip-unneeded.

[mstorsjo]

Summary

Unreferenced undefined external symbols are removed by --strip-unneeded, but not by --discard-all.

[jhenderson]

LGTM.

test/tools/llvm-objcopy/COFF/Inputs/discard-locals.yaml

 --- !COFF
 header:          
   Machine:         IMAGE_FILE_MACHINE_AMD64
   Characteristics: [  ]
 sections:        
   - Name:            .text
     Characteristics: [  ]
     Alignment:       4
-    SectionData:     E800000000C3C3C3
+    SectionData:     E800000000E800000000C3C3C3
     Relocations:     
       - VirtualAddress:  1
         SymbolName:      local_referenced
         Type:            IMAGE_REL_AMD64_REL32
+      - VirtualAddress:  5
+        SymbolName:      external_undefined
+        Type:            IMAGE_REL_AMD64_REL32
 symbols:         
   - Name:            external
     Value:           0
     SectionNumber:   1
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_NULL
     StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
   - Name:            external_undefined
     Value:           0
     SectionNumber:   0
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_NULL
     StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            external_undefined_unreferenced
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
   - Name:            local_unreferenced
     Value:           6
     SectionNumber:   1
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_NULL
     StorageClass:    IMAGE_SYM_CLASS_STATIC
   - Name:            local_referenced
     Value:           7

test/tools/llvm-objcopy/COFF/discard-all.test

 
 RUN: cp %t.in.o %t.strip-discard-all.o
 RUN: llvm-strip --discard-all %t.strip-discard-all.o
 RUN: cmp %t.out.o %t.strip-discard-all.o
 
 SYMBOLS: SYMBOL TABLE:
 SYMBOLS-NEXT: external
 SYMBOLS-NEXT: external_undefined
+SYMBOLS-NEXT: external_undefined_unreferenced
 SYMBOLS-PRE-NEXT: local_unreferenced
 SYMBOLS-NEXT: local_referenced
 SYMBOLS-NEXT: local_undefined_unreferenced
 SYMBOLS-PRE-NEXT: @feat.00
 SYMBOLS-EMPTY:

test/tools/llvm-objcopy/COFF/strip-unneeded.test

 RUN: yaml2obj %p/Inputs/discard-locals.yaml > %t.in.o
 
 RUN: llvm-objdump -t %t.in.o | FileCheck %s --check-prefixes=SYMBOLS,SYMBOLS-PRE
 
 RUN: llvm-objcopy --strip-unneeded %t.in.o %t.out.o
 RUN: llvm-objdump -t %t.out.o | FileCheck %s --check-prefix=SYMBOLS
 
 SYMBOLS: SYMBOL TABLE:
 SYMBOLS-NEXT: external
 SYMBOLS-NEXT: external_undefined
+SYMBOLS-PRE-NEXT: external_undefined_unreferenced
 SYMBOLS-PRE-NEXT: local_unreferenced
 SYMBOLS-NEXT: local_referenced
 SYMBOLS-PRE-NEXT: local_undefined_unreferenced
 SYMBOLS-PRE-NEXT: @feat.00
 SYMBOLS-EMPTY:

tools/llvm-objcopy/COFF/COFFObjcopy.cpp

if (is_contained(Config.SymbolsToRemove, Sym.Name)) {
         reportError(Config.OutputFilename,
                     make_error<StringError>(
                         "not stripping symbol '" + Sym.Name +
                             "' because it is named in a relocation.",
                         llvm::errc::invalid_argument));
       return true;
     }
 
-    if (!Sym.Referenced && Sym.Sym.StorageClass == IMAGE_SYM_CLASS_STATIC) {
-      if (Config.StripUnneeded)
+    if (!Sym.Referenced) {
+      // With --strip-unneeded, GNU objcopy removes all unreferenced local
+      // symbols, and any unreferenced undefined external.
+      if (Config.StripUnneeded &&
+          (Sym.Sym.StorageClass == IMAGE_SYM_CLASS_STATIC ||
+           Sym.Sym.SectionNumber == 0))
         return true;
+
       // GNU objcopy keeps referenced local symbols and external symbols
       // if --discard-all is set, similar to what --strip-unneeded does,
       // but undefined local symbols are kept when --discard-all is set.
-      if (Config.DiscardAll && Sym.Sym.SectionNumber != 0)
+      if (Config.DiscardAll && Sym.Sym.StorageClass == IMAGE_SYM_CLASS_STATIC &&
+          Sym.Sym.SectionNumber != 0)
         return true;
     }
 
     return false;
   });
   return Error::success();
 }