RE: [PATCH v4] infcall: Add support for integer literals as reference function paramters

Aktemur, Tankut Baris
2026-02-02T17:06:34Z

On Wednesday, January 28, 2026 2:55 PM, Andrew Burgess wrote:
> Keith Seitz <keiths@redhat.com> writes:
>
> > This patch attempts to mitigate the shortcomings of passing literals
> > to inferior function calls requiring references. The specific use
> case here
> > is std::map's operator[]:
> >
> > std::map int_map<int, int>;
> > int_map[1] = 10;
> > (gdb) print int_map[1]
> > Attempt to take address of value not located in memory.
> >
> > This is occurring because while value_coerce_to_target understands
> > that some values need to be allocated and copied to the inferior's
> > memory, it only considers the actual parsed type of the argument
> value,
> > ignoring the actual type of the function parameter. That is,
> > in this specific case, the value's parsed type is TYPE_CODE_INT, but
> > the function requires TYPE_CODE_REF. We need to account for the
> > reference.
> >
> > In value_arg_coerce, we have special handling for references, but it
> > has not specifically dealt with this case. It now checks if the
> > reference is in memory, and if it isn't, it copies it, if the type
> > is trivially copyable.
> >
> > As a result of this patch, the last remaining failure in c++/15372 is
> now
> > fixed, and that bug can be closed.
> >
> > With this patch, we can now print map entries with integer keys:
> >
> > (gdb) print int_map[1]
> > $1 = (std::map<int, int, std::less<int>, std::allocator<std::pair<int
> const, int> > >::mapped_type &) @0x41f2d4: 10
> >
> > Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=15372
> > Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=25957
> >
> > Changes in v4
> > - Allocate to stack instead of heap
>
> I'm interested by this change. Within value_arg_coerce, a few lines
> before your change, there's this comment:
>
> /* Force the value to the target if we will need its address. At
> this point, we could allocate arguments on the stack instead of
> calling malloc if we knew that their addresses would not be
> saved by the called function. */
> arg = value_coerce_to_target (arg);
>
> I don't believe there's anything stopping the inferior function taking
> the address of the reference argument and storing it. Could you explain
> this change a little more?

My review comment must have caused this change in v4. I'm sorry about that.

GDB allocates implicit pass-by-reference arguments on stack. It also allocates
struct return values on stack [1]. That is the reason why I had commented so.

Suppose I have a struct S and in the code there is

S *gp;

void func (S &s1) { gp = &s1; }

S bar () { return {99}; }

Then, in GDB if I do

(gdb) print func(bar())

the global pointer `gp` would be pointing to a stack-allocated object,
which is now garbage. I believe we should fix this then, right?
(It's a side topic and not directly part of Keith's patch.)

Regards,
-Baris

[1] infcall.c:

/* Reserve space for the return structure to be written on the
stack, if necessary.

While evaluating expressions, we reserve space on the stack for
return values of class type even if the language ABI and the target
ABI do not require that the return value be passed as a hidden first
argument. This is because we want to store the return value as an
on-stack temporary while the expression is being evaluated. This
enables us to have chained function calls in expressions.

Keeping the return values as on-stack temporaries while the expression
is being evaluated is OK because the thread is stopped until the
expression is completely evaluated. */

if (return_method != return_method_normal
|| (stack_temporaries && class_or_union_p (values_type)))
struct_addr = reserve_stack_space (values_type, sp);

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