Re: [PATCH] [gdb/exp] Fix cast handling for indirection

[Pedro Alves <pedro@palves.net>]

On 2024-05-03 18:30, Kevin Buettner wrote:
> On Fri, 3 May 2024 17:04:41 +0100
> Pedro Alves <pedro@palves.net> wrote:
> 
>>> I was wondering if this patch causes gdb to accept some weird things
>>> that might have been rejected in the past, by introducing a hidden cast.
>>> Maybe print (char) *85732 does something surprising now.  I'm not
>>> entirely sure if that's bad.  
>>
>> I am totally surprised that:
>>
>>  +# Regression test for PR31693.
>>  +gdb_test "p (char)*a_loc ()" " = 97 'a'"
>>
>> this actually works, instead of telling the user:
>>
>>   "'a_loc' has unknown return type; cast the call to its declared return type"
>>
>> It seems like a misfeature to me to assume that "char *" is the right type.
>>
>> Thus, I don't agree with the patch.
> 
> Using a GDB built with Tom de Vries's patch, I see:
> 
> (gdb) p *a_loc()
> 'a_loc' has unknown return type; cast the call to its declared return type
> 
> This is the same as the pre-patch behavior.
> 
> With Tom's patch, GDB now infers the function's return type, based
> on the cast:
> 
> (gdb) p (char)*a_loc()
> $1 = 97 'a'
> 

But that is not what GDB told you to do.  It told you to cast the _call_,
not the result of de-referencing the result of the call.  
It is telling cast to the declared return type, which is "char *".  I.e.,
it is telling you to write:

 (gdb) p *(char *)a_loc()

See 7022349d5c86 ("Stop assuming no-debug-info functions return int").

This cast here:

 (gdb) p (char)*a_loc()

... should not affect the call's return type.  That is decided before
the * operator is involved.  In the same way, this:

 (gdb) p (long long)*a_loc()

should not result in gdb assuming that a_loc() returns a "long long *",
that it wrong.  It should still error out with

  'a_loc' has unknown return type; cast the call to its declared return type

and so the user should write:

 (gdb) p (long long) *(char *)a_loc()

and then with this last expression a proper sign extension is applied when
char is converted to long long, if char is signed.  I.e., in steps:

 1: (char *)a_loc() -> call, and get char * return value
 2: *(char *)a_loc() -> deref, and get char value
 3: (long long) *(char *)a_loc() -> sign extend char value -> long long value

> I like this behavior and certainly find it preferable to the behavior
> without his patch:
> 
> (gdb) p (char)*a_loc()
> Cannot access memory at address 0x4

That's not what I am suggesting.

Pedro Alves