RE: [PATCH v2 1/2] gdb, testsuite: adapt function_range expected name

["Kempke, Nils-Christian via Gdb-patches" <gdb-patches@sourceware.org>]

> -----Original Message-----
> From: Bruno Larsen <blarsen@redhat.com>
> Sent: Wednesday, August 10, 2022 2:32 PM
> To: Kempke, Nils-Christian <nils-christian.kempke@intel.com>; gdb-
> patches@sourceware.org
> Subject: Re: [PATCH v2 1/2] gdb, testsuite: adapt function_range expected
> name
> 
> 
> On 04/08/2022 15:03, Nils-Christian Kempke wrote:
> > When writing a dwarf testcase for some C++ code I wanted to use the
> > MACRO_AT_range which in turn uses the function_range proc in dwarf.exp
> > to extract the bounds of 'main'.
> >
> > However, the macro failed as GDB prints the C++ 'main' with its
> > arguments as 'main(int, char**)' or 'main()'.
> >
> > The reason for this is that in read.c::dwarf2_compute_name we call
> > c_type_print_args on C++ functions and append their arguments to the
> > function name.  This does not only happen for 'main' but also for all
> > other C++ functions.  However, other functions often also have a
> > DW_AT_linkage_name which gets printed over the function name in
> > 'disassemble' and similar functions.  So, I could only really reproduce
> > the fail of MARCRO_AT_rang with the C++ 'main' function.
> 
> Hi Nils, thank you for working on this!
> 
> I found this explanation a bit confusing, maybe you could condense it to
> something like
> 
> The reason for this is that in read.c::dwarf2_compute_name we call
> c_type_print_args on C++ functions and append their arguments to the
> function name.  This happens to all c++ functions, but is only visible when
> it doesn't have a linkage name.
> 
> 
> >
> > An example might make this more clear.  Given the following code
> >
> >    >> cat c.cpp
> >    int foo (int a, float b)
> >    {
> >      return 0;
> >    }
> >
> >    int main (int argc, char **argv)
> >    {
> >      return 0;
> >    }
> >
> > which is legal in both languages, C and C++, and compiling it with
> > e.g. clang or gcc will make the disassemble command look like:
> >
> >    >> clang --version
> >    clang version 10.0.0-4ubuntu1
> >    ...
> >    >> clang -O0 -g ./c.cpp
> >    >> gdb -q ./a.out -ex "start"
> >    ...
> >    (gdb) disassemble main
> >    Dump of assembler code for function main(int, char**):
> >       0x0000000000401120 <+0>:     push   %rbp
> >       0x0000000000401121 <+1>:     mov    %rsp,%rbp
> >    ...
> >       0x0000000000401135 <+21>:    ret
> >    End of assembler dump.
> >    (gdb) disassemble foo
> >    Dump of assembler code for function _Z3fooif:
> >       0x0000000000401110 <+0>:     push   %rbp
> >       0x0000000000401111 <+1>:     mov    %rsp,%rbp
> >    ...
> >       0x000000000040111f <+15>:    ret
> >    End of assembler dump.
> >
> > Note, that main is emitted with its arguments while for foo the linkage
> > name is being printed, as also visible in its DWARF:
> >
> >    >> objdump ./a.out --dwarf=info | grep "foo" -A3 -B3
> >        <2b>   DW_AT_low_pc      : 0x401110
> >        <33>   DW_AT_high_pc     : 0x10
> >        <37>   DW_AT_frame_base  : 1 byte block: 56         (DW_OP_reg6 (rbp))
> >        <39>   DW_AT_linkage_name: (indirect string, offset: 0x39): _Z3fooif
> >        <3d>   DW_AT_name        : (indirect string, offset: 0x42): foo
> >        <41>   DW_AT_decl_file   : 1
> >        <42>   DW_AT_decl_line   : 1
> >        <43>   DW_AT_type        : <0x9a>
> >
> > Now, let's rename the C++ file and compile it as C:
> >
> >    >> mv c.cpp c.c
> >    >> clang -O0 -g ./c.c
> >    >> gdb -q ./a.out -ex "start'
> >    ...
> >    (gdb) disassemble main
> >    Dump of assembler code for function main:
> >       0x0000000000401120 <+0>:     push   %rbp
> >       0x0000000000401121 <+1>:     mov    %rsp,%rbp
> >    ...
> >       0x0000000000401135 <+21>:    ret
> >    End of assembler dump.
> >    (gdb) disassemble foo
> >    Dump of assembler code for function foo:
> >       0x0000000000401110 <+0>:     push   %rbp
> >       0x0000000000401111 <+1>:     mov    %rsp,%rbp
> >    ...
> >       0x000000000040111f <+15>:    ret
> >    End of assembler dump.
> >
> > Note, for foo we did not get a linkage name emitted in DWARF, so
> > it is printed by its name:
> >
> >    >> objdump --dwarf=info ./a.out | grep foo -A3 -B3
> >        <2b>   DW_AT_low_pc      : 0x401110
> >        <33>   DW_AT_high_pc     : 0x10
> >        <37>   DW_AT_frame_base  : 1 byte block: 56         (DW_OP_reg6 (rbp))
> >        <39>   DW_AT_name        : (indirect string, offset: 0x37): foo
> >        <3d>   DW_AT_decl_file   : 1
> >        <3e>   DW_AT_decl_line   : 1
> >        <3f>   DW_AT_prototyped  : 1
> >
> > To make the macro and proc work with C++ as well, an optional argument
> > list was added to the regex matching the function name in the
> > disassemble command in function_range.  This does not change any used
> > behavior as currently, there exists no C++ test using the proc
> > function_range.
> >
> > Signed-off-by: Nils-Christian Kempke <nils-christian.kempke@intel.com>
> > ---
> >   gdb/testsuite/lib/dwarf.exp | 12 ++++++++----
> >   1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
> > index 356451bcaac..2c1c4056346 100644
> > --- a/gdb/testsuite/lib/dwarf.exp
> > +++ b/gdb/testsuite/lib/dwarf.exp
> > @@ -391,10 +391,14 @@ proc function_range { func src {options {debug}} }
> {
> >       }
> >
> >       # Compute the size of the last instruction.
> > -    if { $func_length == 0 } then {
> > -	set func_pattern "$func"
> > -    } else {
> > -	set func_pattern "$func\\+$func_length"
> > +    # For C++ GDB appends arguments to the names of functions.  These
> names
> > +    # will (if no linkage name is present, and, e.g., main generally has none)
> > +    # make 'dissasemble' print main (and possibly others) as 'main()' or
> > +    # 'main(int argc, char **argv)' so we take this into accound here by
> > +    # allowing an optinal argument list after the function name.
> 
> I also think the explanation here could be improved a bit. Maybe
> something like:
> 
> # For C++, GDB appends arguments to the names of functions if they don't
> have
> 
> # a linkage name. For example, asking gdb to disassemble main will print the
> 
> # function name as main() or main(int argc, char **argv). Take this into
> account
> 
> # by optionally allowing an argument list after the function name.
> 
> 
> The code itself LGTM, however I can't approve patches for pushing.
> 
> --
> Cheers,
> Bruno
> 
> > +    set func_pattern "$func\(\?\:\\(\.\*\\)\)?"
> > +    if { $func_length != 0 } {
> > +	set func_pattern "$func_pattern\\+$func_length"
> >       }
> >       set test "x/2i $func+$func_length"
> >       gdb_test_multiple $test $test {


Hi Bruno,

Thanks for reviewing this!  I adapted comment and commit message and
will send version 3 shortly. 

Cheers,
Nils
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