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Currently, on any remotely modern GNU/Linux system,
gdb.cp/no-dmgl-verbose.exp fails like so:
break 'f(std::string)'
Function "f(std::string)" not defined.
(gdb) FAIL: gdb.cp/no-dmgl-verbose.exp: gdb_breakpoint: set breakpoint at 'f(std::string)'
break 'f(std::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
Function "f(std::basic_string<char, std::char_traits<char>, std::allocator<char> >)" not defined.
(gdb) PASS: gdb.cp/no-dmgl-verbose.exp: DMGL_VERBOSE-demangled f(std::string) is not defined
This testcase was added back in 2011, here:
[patch] Remove DMGL_VERBOSE
https://sourceware.org/pipermail/gdb-patches/2011-June/083081.html
Back then, the testcase would pass cleanly. It turns out that the
reason it fails today is that the testcase is exercising something in
GDB that only makes sense if the program is built for the pre-C++11
libstc++ ABI. Back then the C++11 ABI didn't exist yet, but nowadays,
you need to compile with -D_GLIBCXX_USE_CXX11_ABI=0 to use the old
ABI. See "Dual ABI" in the libstdc++ manual, at
<https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html>.
If we tweak the gdb.cp/no-dmgl-verbose.exp testcase to force the old
ABI with -D_GLIBCXX_USE_CXX11_ABI=0, then it passes cleanly.
So why is it that setting a breakpoint at "f(std::string)" fails with
modern ABI, but passes with old ABI?
This is where libiberty demangler's DMGL_VERBOSE option comes in. The
Itanium ABI mangling scheme has a shorthand form for std::string (and
some other types). See
<https://itanium-cxx-abi.github.io/cxx-abi/abi.html>:
"In addition, the following catalog of abbreviations of the form "Sx" are used:
<substitution> ::= St # ::std::
<substitution> ::= Sa # ::std::allocator
<substitution> ::= Sb # ::std::basic_string
<substitution> ::= Ss # ::std::basic_string < char,
::std::char_traits<char>,
::std::allocator<char> >
<substitution> ::= Si # ::std::basic_istream<char, std::char_traits<char> >
<substitution> ::= So # ::std::basic_ostream<char, std::char_traits<char> >
<substitution> ::= Sd # ::std::basic_iostream<char, std::char_traits<char> >
"
When the libiberty demangler encounters such a abbreviation, by
default, it expands it to the user-friendly typedef "std::string",
"std::iostream", etc.. If OTOH DMGL_VERBOSE is specified, the
abbreviation is expanded to the underlying, non-typedefed fullname
"std::basic_string<char, std::char_traits<char>, std::allocator<char> >"
etc. as documented in the Itanium ABI, and pasted above. You can see
the standard abbreviations/substitutions in
libiberty/cp-demangle.c:standard_subs.
Back before Jan's patch in 2011, there were parts of GDB that used
DMGL_VERBOSE, and others that did not, leading to mismatches. The
solution back then was to stop using DMGL_VERBOSE throughout.
GDB has code in place to let users set a breakpoint at a function with
typedefs in its parameters, like "b f(uint32_t)". Demangled function
names as they appear in the symbol tables almost (more on this is in a
bit) never have typedefs in them, so when processing "b f(uint32_t)"
GDB first replaces "uint32_t" for its underlying type, and then sets a
breakpoint on the resulting prototype, in this case "f(unsigned int)".
Now, if DMGL_VERBOSE is _not_ used, then the demangler demangles the
mangled name of a function such as "void f(std::string)" that was
mangled using the standard abbreviations mentioned above really as:
"void f(std::string)".
For example, the mangled name of "void f(std::string)" if you compile
with old pre-C++11 ABI is "_Z1fSs". That uses the abbreviation "Ss",
so if you demangle that without DMGL_VERBOSE, you get:
$ echo "_Z1fSs" | c++filt --no-verbose
f(std::string)
while with DMGL_VERBOSE you'd get:
$ echo "_Z1fSs" | c++filt
f(std::basic_string<char, std::char_traits<char>, std::allocator<char> >)
If, when the user sets a breakpoint at "f(std::string)", GDB would
replace the std::string typedef for its underlying type using the same
mechanism I mentioned for the "f(uint32_t)" example above, then GDB
would really try to set a breakpoint at "f(std::basic_string<char,
std::char_traits<char>, std::allocator<char> >)", and that would fail,
as the function symbol GDB knows about for that function, given no
DMGL_VERBOSE, is "f(std::string)".
For this reason, the code that expands typedefs in function parameter
names has an exception for std::string (and other standard
abbreviation types), such that "std::string" is never
typedef-expanded.
And here lies the problem when you try to do "b f(std::string)" with a
program compiled with the C++11 ABI. In that case, std::string
expands to a different underlying type, like so:
f(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)
and this symbol naturally mangles differently, as:
_Z1fNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
and then because this doesn't use the shorthand mangling abbreviation
for "std::string" anymore, it always demangles as:
f(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)
Now, when using the C++11 ABI, and you set a breakpoint at
"f(std::string)", GDB's typedefs-in-parameters expansion code hits the
exception for "std::string" and doesn't expand it, so the breakpoint
fails to be inserted, because the symbol that exists is really the
f(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)
one, not "f(std::string)".
So to fix things for C++11 ABI, clearly we need to remove the
"std::string" exception from the typedef-in-parameters expansion
logic. If we do just that, then "b f(std::string)" starts working
with the C++11 ABI.
However, if we do _just_ that, and nothing else, then we break
pre-C++11 ABI...
The solution is then to in addition switch GDB to always use
DMGL_VERBOSE. If we do this, then pre-C++11 ABI symbols works the
same as C++11 ABI symbols overall -- the demangler expands the
standard abbreviation for "std::string" as "std::basic_string<char,
std::char_traits<char>, std::allocator<char> >" and letting GDB expand
the "std::string" typedef (etc.) too is no longer a problem.
To avoid getting in the situation where some parts of GDB use
DMGL_VERBOSE and others not, this patch adds wrappers around the
demangler's entry points that GDB uses, and makes those force
DMGL_VERBOSE.
The point of the gdb.cp/no-dmgl-verbose.exp testcase was to try to
ensure that DMGL_VERBOSE doesn't creep back in:
gdb_test {break 'f(std::basic_string<char, std::char_traits<char>, std::allocator<char> >)'} \
{Function ".*" not defined\.} \
"DMGL_VERBOSE-demangled f(std::string) is not defined"
This obviously no longer makes sense to have, since we now depend on
DMGL_VERBOSE. So the patch replaces gdb.cp/no-dmgl-verbose.exp with a
new gdb.cp/break-f-std-string.exp testcase whose purpose is to make
sure that setting a breakpoint at "f(std::string)" works. It
exercises both pre-C++11 ABI and C++11 ABI.
Change-Id: Ib54fab4cf0fd307bfd55bf1dd5056830096a653b
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