Pass only mangled function names to __cxa_demangle

[emaste]

Summary

The Itanium C++ ABI states the string passed to __cxa_demnagle "may be either an external name, i.e. with a "_Z" prefix, or an internal NTBS mangling, e.g. of a type for type_info."

llvm-symbolizer passes all names to cxa_demangle, which fails for many names on FreeBSD. Our cxa_demangle is currently not very strict about rejecting invalid names, and returns "unsigned int" for a function "main". On other platforms I believe llvm-symbolizer would produce incorrect output for functions with the same name as a mangled type - for example, returning "float" for a function "f".

This change checks that the name starts with the _Z prefix before passing it to __cxa_demangle.

http://llvm.org/pr18431

[emaste]

I missed that the _Z heuristic already exists, but was applied to global data only. Move the heuristic into DemangleName so that it is applied to function names too.

Add a new .c test specifically for llvm-symbolizer. I was going to modify dwarfdump-test.cc to make function f() extern "C", but don't want to have to deal with the binary objects that were already generated from it.

[samsonov]

LGTM (modulo comment below).

test/DebugInfo/Inputs/llvm-symbolizer-test.c

14 ↗

(On Diff #6478)

Update the build instructions.

[emaste]

Committed as r199404

test/DebugInfo/llvm-symbolizer.test

 RUN: llvm-symbolizer < %t.input3 | FileCheck %s --check-prefix=UNKNOWN-ARCH
 
 UNKNOWN-ARCH-NOT: main
 UNKNOWN-ARCH: ??
 UNKNOWN-ARCH-NOT: main
 
 RUN: echo "0x400559" > %t.input4
 RUN: echo "0x400436" >> %t.input4
+RUN: echo "0x400510" >> %t.input4
 RUN: llvm-symbolizer --obj %p/Inputs/dwarfdump-test.elf-x86-64 < %t.input4 \
 RUN:   | FileCheck %s --check-prefix=BINARY
 
 BINARY:       main
 BINARY-NEXT: /tmp/dbginfo{{[/\\]}}dwarfdump-test.cc:16
 BINARY:      _start
+BINARY:      f

tools/llvm-symbolizer/LLVMSymbolize.cpp

 #if !defined(_MSC_VER)
 // Assume that __cxa_demangle is provided by libcxxabi (except for Windows).
 extern "C" char *__cxa_demangle(const char *mangled_name, char *output_buffer,
                                 size_t *length, int *status);
 #endif
 
 std::string LLVMSymbolizer::DemangleName(const std::string &Name) {
 #if !defined(_MSC_VER)
+  if (Name.substr(0, 2) != "_Z")
+    return Name;
   int status = 0;
   char *DemangledName = __cxa_demangle(Name.c_str(), 0, 0, &status);
   if (status != 0)
     return Name;
   std::string Result = DemangledName;
   free(DemangledName);
   return Result;
 #else