[llvm-cxxfilt][macOS] Don't strip underscores on macOS by default (#106233)

Currently, `llvm-cxxfilt` will strip the leading underscore of its input
on macOS. Historically MachO symbols were prefixed with an extra
underscore and this is why this default exists. However, nowadays, the
`ItaniumDemangler` supports all of the following mangling prefixes:
`_Z`, `__Z`, `___Z`, `____Z`. So really `llvm-cxxfilt` can simply
forward the mangled name to the demangler and let the library decide
whether it's a valid encoding.

Compiling C++ on macOS nowadays will generate symbols with `_Z` and
`___Z` prefixes. So users trying to demangle these symbols will have to
know that they need to add the `-n` prefix. This routinely catches
people off-guard.

This patch removes the `-n` default for macOS and allows calling into
the `ItaniumDemangler` with all the `_Z` prefixes that the demangler
supports (1-4 underscores).

rdar://132714940

Author: Michael137

llvm/lib/Demangle/Demangle.cpp

@@ -38,8 +38,9 @@ std::string llvm::demangle(std::string_view MangledName) {
 }
 
 static bool isItaniumEncoding(std::string_view S) {
-  // Itanium encoding requires 1 or 3 leading underscores, followed by 'Z'.
-  return starts_with(S, "_Z") || starts_with(S, "___Z");
+  // Itanium demangler supports prefixes with 1-4 underscores.
+  const size_t Pos = S.find_first_not_of('_');
+  return Pos > 0 && Pos <= 4 && S[Pos] == 'Z';
 }
 
 static bool isRustEncoding(std::string_view S) { return starts_with(S, "_R"); }

llvm/test/tools/llvm-cxxfilt/invalid.test

@@ -1,6 +1,8 @@
-RUN: llvm-cxxfilt -n _Z1fi __Z1fi f ___ZSt1ff_block_invoke | FileCheck %s
+RUN: llvm-cxxfilt -n _Z1fi __Z1fi _____Z1fi f ___ZSt1ff_block_invoke ____ZSt1ff_block_invoke | FileCheck %s
 
 CHECK: f(int)
-CHECK-NEXT: __Z1fi
+CHECK-NEXT: f(int)
+CHECK-NEXT: _____Z1fi
 CHECK-NEXT: f
 CHECK-NEXT: invocation function for block in std::f(float)
+CHECK-NEXT: invocation function for block in std::f(float)

llvm/test/tools/llvm-cxxfilt/strip-underscore-default-darwin.test

@@ -1,17 +1,23 @@
-## Show the behaviour of --[no-]strip-underscore. This test does not test
-## the platform-specific default behaviour. This is tested elsewhere.
+## Show the behaviour of --[no-]strip-underscore.
 
-RUN: llvm-cxxfilt -_ __ZN2ns1fE _ZSt1f _f _._Z3f.0v | FileCheck %s -check-prefix CHECK-STRIPPED
-RUN: llvm-cxxfilt --strip-underscore __ZN2ns1fE _ZSt1f _f _._Z3f.0v | FileCheck %s -check-prefix CHECK-STRIPPED
-RUN: llvm-cxxfilt -n __ZN2ns1fE _ZSt1f _f _._Z3f.0v | FileCheck %s -check-prefix CHECK-UNSTRIPPED
-RUN: llvm-cxxfilt --no-strip-underscore __ZN2ns1fE _ZSt1f _f _._Z3f.0v | FileCheck %s -check-prefix CHECK-UNSTRIPPED
+RUN: llvm-cxxfilt -_ ___ZN2ns1fE _____Z1fi_block_invoke _ZSt1f _f _._Z3f.0v | FileCheck %s -check-prefix CHECK-STRIPPED
+RUN: llvm-cxxfilt --strip-underscore __ZN2ns1fE _____Z1fi_block_invoke _ZSt1f _f _._Z3f.0v | FileCheck %s -check-prefix CHECK-STRIPPED
+RUN: llvm-cxxfilt -n ___ZN2ns1fE _____Z1fi_block_invoke _ZSt1f _f _._Z3f.0v | FileCheck %s -check-prefix CHECK-UNSTRIPPED
+RUN: llvm-cxxfilt --no-strip-underscore ___ZN2ns1fE _____Z1fi_block_invoke _ZSt1f _f _._Z3f.0v | FileCheck %s -check-prefix CHECK-UNSTRIPPED
+RUN: llvm-cxxfilt -n -_ _ZSt1f | FileCheck %s -check-prefix OVERRIDE-STRIPPED
+RUN: llvm-cxxfilt -_ -n _ZSt1f | FileCheck %s -check-prefix OVERRIDE-UNSTRIPPED
 
 CHECK-STRIPPED: ns::f
+CHECK-STRIPPED: invocation function for block in f(int)
 CHECK-STRIPPED: _ZSt1f
 CHECK-STRIPPED: _f
 CHECK-STRIPPED: ._Z3f.0v
 
-CHECK-UNSTRIPPED: __ZN2ns1fE
+CHECK-UNSTRIPPED: ___ZN2ns1fE
+CHECK-UNSTRIPPED: _____Z1fi_block_invoke
 CHECK-UNSTRIPPED: std::f
 CHECK-UNSTRIPPED: _f
 CHECK-UNSTRIPPED: _._Z3f.0v
+
+OVERRIDE-STRIPPED:   _ZSt1f
+OVERRIDE-UNSTRIPPED: std::f

llvm/test/tools/llvm-cxxfilt/strip-underscore-default.test

@@ -165,13 +165,8 @@ int llvm_cxxfilt_main(int argc, char **argv, const llvm::ToolContext &) {
     return 0;
   }
 
-  // The default value depends on the default triple. Mach-O has symbols
-  // prefixed with "_", so strip by default.
-  if (opt::Arg *A =
-          Args.getLastArg(OPT_strip_underscore, OPT_no_strip_underscore))
-    StripUnderscore = A->getOption().matches(OPT_strip_underscore);
-  else
-    StripUnderscore = Triple(sys::getProcessTriple()).isOSBinFormatMachO();
+  StripUnderscore =
+      Args.hasFlag(OPT_strip_underscore, OPT_no_strip_underscore, false);
 
   ParseParams = !Args.hasArg(OPT_no_params);