[ELF] Reword --no-allow-shlib-undefined diagnostic

[MaskRay]

Summary

Use a format more similar to unresolved references from regular object
files.

[ikudrin]

LGTM, thanks!

[MaskRay]

An internal user reported that the wording isn't so clear. While I don't fully agree, I wonder whether the new one is clearer...

[peter.smith]

No objections to the new format. It is probably easier for people less familiar, or not associating a linker error with clang's convention of putting command line options for warnings.

[MaskRay]

Thanks for the feedback!

lld/ELF/Writer.cpp

for (SharedFile *file : ctx->sharedFiles) {
       bool allNeededIsKnown =
           llvm::all_of(file->dtNeeded, [&](StringRef needed) {
             return symtab->soNames.count(CachedHashStringRef(needed));
           });
       if (!allNeededIsKnown)
         continue;
       for (Symbol *sym : file->requiredSymbols)
         if (sym->isUndefined() && !sym->isWeak())
-          diagnose(toString(file) + ": undefined reference to " +
-                   toString(*sym) + " [--no-allow-shlib-undefined]");
+          diagnose("undefined reference due to --no-allow-shlib-undefined: " +
+                   toString(*sym) + "\n>>> referenced by " + toString(file));
     }
   }
 
   {
     llvm::TimeTraceScope timeScope("Add symbols to symtabs");
     // Now that we have defined all possible global symbols including linker-
     // synthesized ones. Visit all symbols to give the finishing touches.
     for (Symbol *sym : symtab->symbols()) {

lld/test/ELF/allow-shlib-undefined.s

 # RUN: cp %t.so %t2.so
 # RUN: not ld.lld %t.o %t.so %t2.so -o /dev/null 2>&1 | \
 # RUN:   FileCheck %s --check-prefixes=CHECK,CHECK2
 
 .globl _start
 _start:
   callq _shared@PLT
 
-# CHECK: error: {{.*}}.so: undefined reference to _unresolved [--no-allow-shlib-undefined]
-# CHECK2: error: {{.*}}2.so: undefined reference to _unresolved [--no-allow-shlib-undefined]
-# WARN: warning: {{.*}}.so: undefined reference to _unresolved [--no-allow-shlib-undefined]
+# CHECK:       error: undefined reference due to --no-allow-shlib-undefined: _unresolved
+# CHECK-NEXT:  >>> referenced by {{.*}}.so
+# CHECK2:      error: undefined reference due to --no-allow-shlib-undefined: _unresolved
+# CHECK2-NEXT: >>> referenced by {{.*}}2.so
+# WARN:        warning: undefined reference due to --no-allow-shlib-undefined: _unresolved
+# WARN-NEXT:   >>> referenced by {{.*}}.so

lld/test/ELF/unresolved-symbols.s

 # RUN: llvm-readobj %t1_3 > /dev/null 2>&1
 
 ## Ignoring undefines in objects should not produce error for symbol from object.
 # RUN: ld.lld %t1.o %t2.o -o %t2 --unresolved-symbols=ignore-in-object-files
 # RUN: llvm-readobj %t2 > /dev/null 2>&1
 ## --unresolved-symbols overrides a previous --allow-shlib-undefined.
 # RUN: not ld.lld %t1.o %t.so -o /dev/null --allow-shlib-undefined --unresolved-symbols=ignore-in-object-files 2>&1 | FileCheck %s --check-prefix=SHLIB
 
-# SHLIB: error: {{.*}}.so: undefined reference to undef [--no-allow-shlib-undefined]
+# SHLIB: error: undefined reference due to --no-allow-shlib-undefined: undef
 
 ## Ignoring undefines in shared should produce error for symbol from object.
 # RUN: not ld.lld %t2.o -o /dev/null --unresolved-symbols=ignore-in-shared-libs 2>&1 | \
 # RUN:   FileCheck -check-prefix=ERRUND %s
 ## And should not produce errors for symbols from DSO.
 # RUN: ld.lld %t1.o %t.so -o %t3_1 --allow-shlib-undefined --unresolved-symbols=ignore-in-shared-libs
 # RUN: llvm-readobj %t3_1 > /dev/null 2>&1
 

lld/test/ELF/wrap-shlib-undefined.s

 # RUN: ld.lld -shared %t/main.o %t/call-foo.o --wrap foo -o %t1.so
 # RUN: llvm-readelf -r %t1.so | FileCheck %s --check-prefix=CHECK1
 
 # CHECK1:      R_X86_64_JUMP_SLOT 0000000000000000 bar + 0
 # CHECK1-NEXT: R_X86_64_JUMP_SLOT 0000000000000000 __wrap_foo + 0
 
 ## --no-allow-shlib-undefined errors because __real_foo is not defined.
 # RUN: not ld.lld %t/main.o %t/bar.so -o /dev/null 2>&1 | FileCheck --check-prefix=ERR %s
-# ERR: {{.*}}.so: undefined reference to __real_foo [--no-allow-shlib-undefined]
+# ERR: error: undefined reference due to --no-allow-shlib-undefined: __real_foo
 
 ## --wrap=foo defines __real_foo.
 # RUN: ld.lld %t/main.o %t/bar.so --wrap=foo -o %t2
 # RUN: llvm-readelf --dyn-syms %t2 | FileCheck %s --check-prefix=CHECK2
 
 ## See wrap-plt2.s why __wrap_foo is retained.
 # CHECK2:      Symbol table '.dynsym' contains 3 entries:
 # CHECK2:      NOTYPE  LOCAL  DEFAULT  UND