llvm-nm: Implement --special-syms.

Differential Revision: https://reviews.llvm.org/D82251

lld/test/ELF/aarch64-thunk-script.s

@@ -6,7 +6,7 @@
 // RUN:       } " > %t.script
 // RUN: ld.lld --script %t.script %t.o -o %t
 // RUN: llvm-objdump -d --no-show-raw-insn --print-imm-hex %t | FileCheck %s
-// RUN: llvm-nm --no-sort %t | FileCheck --check-prefix=NM %s
+// RUN: llvm-nm --no-sort --special-syms %t | FileCheck --check-prefix=NM %s
 
 // Check that we have the out of branch range calculation right. The immediate
 // field is signed so we have a slightly higher negative displacement.

llvm/docs/CommandGuide/llvm-nm.rst

@@ -219,7 +219,7 @@ OPTIONS
 
 .. option:: --special-syms
 
- Ignored. For GNU compatibility only.
+ Do not filter special symbols from the output.
 
 .. option:: --undefined-only, -u
 

llvm/docs/ReleaseNotes.rst

@@ -187,6 +187,10 @@ Changes to the LLVM tools
 * Added an option (--show-section-sizes) to llvm-dwarfdump to show the sizes
   of all debug sections within a file.
 
+* llvm-nm now implements the flag ``--special-syms`` and will filter out special
+  symbols, i.e. mapping symbols on ARM and AArch64, by default. This matches
+  the GNU nm behavior.
+
 Changes to LLDB
 ===============
 

llvm/test/MC/AArch64/mapping-within-section.s

@@ -1,4 +1,4 @@
-// RUN: llvm-mc -triple=aarch64-none-linux-gnu -filetype=obj %s | llvm-nm - | FileCheck %s
+// RUN: llvm-mc -triple=aarch64-none-linux-gnu -filetype=obj %s | llvm-nm --special-syms - | FileCheck %s
 
     .text
 // $x at 0x0000

llvm/test/tools/llvm-nm/AArch64/special-syms.test

@@ -1,10 +1,7 @@
+## Test --special-syms flag.
 # RUN: yaml2obj %s -o %t
-# Test --special-syms flag. Currently this flag is a no-op, so outputs with and without
-# this flag should be identical. GNU nm doesn't show ARM and AArch64 special symbols
-# without --special-syms, so this test is to be changed when/if we decide to implement
-# GNU nm-like behavior in llvm-nm
 
-# RUN: llvm-nm %t | FileCheck %s
+# RUN: llvm-nm %t | count 0
 # RUN: llvm-nm %t --special-syms | FileCheck %s
 
 !ELF

llvm/test/tools/llvm-nm/debug-syms.test

@@ -1,6 +1,6 @@
 # RUN: yaml2obj %s -o %t.o
-# RUN: llvm-nm --debug-syms %t.o | FileCheck %s --implicit-check-not {{.}} --check-prefix SYMBOL
-# RUN: llvm-nm -a %t.o | FileCheck %s --implicit-check-not {{.}} --check-prefix SYMBOL
+# RUN: llvm-nm --special-syms --debug-syms %t.o | FileCheck %s --implicit-check-not {{.}} --check-prefix SYMBOL
+# RUN: llvm-nm --special-syms -a %t.o | FileCheck %s --implicit-check-not {{.}} --check-prefix SYMBOL
 
 # SYMBOL:      0000000000000000 n $a
 # SYMBOL-NEXT: 0000000000000000 n $d

llvm/tools/llvm-nm/llvm-nm.cpp

@@ -182,8 +182,10 @@ cl::opt<bool> JustSymbolName("just-symbol-name",
 cl::alias JustSymbolNames("j", cl::desc("Alias for --just-symbol-name"),
                           cl::aliasopt(JustSymbolName), cl::Grouping);
 
-cl::opt<bool> SpecialSyms("special-syms",
-                          cl::desc("No-op. Used for GNU compatibility only"));
+cl::opt<bool>
+    SpecialSyms("special-syms",
+                cl::desc("Do not filter special symbols from the output"),
+                cl::cat(NMCat));
 
 cl::list<std::string> SegSect("s", cl::multi_val(2), cl::ZeroOrMore,
                               cl::value_desc("segment section"), cl::Hidden,
@@ -733,6 +735,16 @@ static void writeFileName(raw_ostream &S, StringRef ArchiveName,
   }
 }
 
+static bool isSpecialSym(SymbolicFile &Obj, StringRef Name) {
+  auto *ELFObj = dyn_cast<ELFObjectFileBase>(&Obj);
+  if (!ELFObj)
+    return false;
+  uint16_t EMachine = ELFObj->getEMachine();
+  if (EMachine != ELF::EM_ARM && EMachine != ELF::EM_AARCH64)
+    return false;
+  return !Name.empty() && Name[0] == '$';
+}
+
 static void sortAndPrintSymbolList(SymbolicFile &Obj, bool printName,
                                    StringRef ArchiveName,
                                    StringRef ArchitectureName) {
@@ -822,7 +834,8 @@ static void sortAndPrintSymbolList(SymbolicFile &Obj, bool printName,
     bool Global = SymFlags & SymbolRef::SF_Global;
     bool Weak = SymFlags & SymbolRef::SF_Weak;
     if ((!Undefined && UndefinedOnly) || (Undefined && DefinedOnly) ||
-        (!Global && ExternalOnly) || (Weak && NoWeakSymbols))
+        (!Global && ExternalOnly) || (Weak && NoWeakSymbols) ||
+        (!SpecialSyms && isSpecialSym(Obj, Name)))
       continue;
     if (PrintFileName)
       writeFileName(outs(), ArchiveName, ArchitectureName);