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

Keith Seitz
2025-10-16T15:49:43Z

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.

I've added some tests to start testing string-based maps, e.g.,
std::map<string, int>, but that is quite a bit more complicated to
deal with -- we need to not only allocate memory in the inferior,
we need to initialize the string and its contents. Perhaps this can
be done, but I have not attempted that in this patch.

I uncovered one small hiccup while implementing this change. It
appears that Fortran compilers use a hidden first argument which
is used to pass results from functions back to callers, and these
are implemented behind the scenes as references, marked in the debug
info as DW_TAG_reference. These should not be subject to this logic,
and I have therefore limited this to types defined in C++.

On the plus side, the last remaining failure in c++/15372 is now
fixed, and that bug could 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
---
gdb/infcall.c | 2 +-
gdb/testsuite/gdb.cp/ref-params.cc | 78 +++++++++++++-
gdb/testsuite/gdb.cp/ref-params.exp | 106 ++++++++++++++++++-
gdb/testsuite/gdb.cp/rvalue-ref-overload.exp | 1 -
gdb/valops.c | 25 +++--
gdb/value.h | 3 +-
6 files changed, 202 insertions(+), 13 deletions(-)

diff --git a/gdb/infcall.c b/gdb/infcall.c
index c4b4c8f0bea..823a2b96591 100644
--- a/gdb/infcall.c
+++ b/gdb/infcall.c
@@ -263,7 +263,7 @@ value_arg_coerce (struct gdbarch *gdbarch, struct value *arg,
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);
+ arg = value_coerce_to_target (arg, type);

switch (type->code ())
{
diff --git a/gdb/testsuite/gdb.cp/ref-params.cc b/gdb/testsuite/gdb.cp/ref-params.cc
index cd53adb834f..827b0a1ccf9 100644
--- a/gdb/testsuite/gdb.cp/ref-params.cc
+++ b/gdb/testsuite/gdb.cp/ref-params.cc
@@ -17,6 +17,9 @@

/* Author: Paul N. Hilfinger, AdaCore Inc. */

+#include
+#include <string>
+
struct Parent {
Parent (int id0) : id(id0) { }
int id;
@@ -60,7 +63,53 @@ int mf2(MultiChild& C)
return mf1(C);
}

-int main(void)
+/* For C++/25957 (const reference parameters and std::map operator[]). */
+std::map<int, int> int_map;
+std::map<std::string, int> string_map;
+int global_int_key = 42;
+std::string global_string_key = "test";
+
+/* Helper functions to test reference parameter behavior. */
+int
+const_ref_func (const int &x)
+{
+ return x * 2;
+}
+
+int
+ref_func (int &x)
+{
+ x++;
+ return x;
+}
+
+std::string
+const_string_ref_func (const std::string &s)
+{
+ return s + "_modified";
+}
+
+class TestClass
+{
+public:
+ int value;
+
+ TestClass (int v) : value (v) {}
+
+ int
+ const_ref_method (const int &x) const
+ {
+ return value + x;
+ }
+
+ int
+ ref_method (int &x)
+ {
+ return value + 2 * x;
+ }
+};
+
+int main(void)
{
Child Q(42);
Child& QR = Q;
@@ -76,5 +125,30 @@ int main(void)

mf2(MQ); /* Set breakpoint MQ here. */

- return 0;
+ /* c++/25957 test setup - Initialize maps with some data. */
+ int_map[1] = 10;
+ int_map[2] = 20;
+ int_map[global_int_key] = 100;
+
+ string_map["hello"] = 1;
+ string_map["world"] = 2;
+ string_map[global_string_key] = 99;
+
+ TestClass obj (5);
+ int local_var = 15;
+
+ /* Prevent compiler from optimizing away the method calls. */
+ (void) int_map.size ();
+ (void) int_map.empty ();
+ (void) int_map.find (1);
+ (void) int_map.count (1);
+ int dummy_int = 99;
+ (void) const_ref_func (dummy_int);
+ (void) ref_func (dummy_int);
+ (void) const_string_ref_func (global_string_key);
+ (void) obj.const_ref_method (dummy_int);
+ (void) obj.ref_method (dummy_int);
+
+ /* Breakpoint here for c++/25957 testing. */
+ return 0; /* breakpoint-here */
}
diff --git a/gdb/testsuite/gdb.cp/ref-params.exp b/gdb/testsuite/gdb.cp/ref-params.exp
index 4f2cbb5dace..b1b7ef93317 100644
--- a/gdb/testsuite/gdb.cp/ref-params.exp
+++ b/gdb/testsuite/gdb.cp/ref-params.exp
@@ -24,7 +24,7 @@ require allow_cplus_tests

standard_testfile .cc

-if {[build_executable $testfile.exp $testfile $srcfile {debug c++}] == -1} {
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}]} {
return -1
}

@@ -63,3 +63,107 @@ gdb_test "print f1(MQR)" ".* = 53"
gdb_test "print mf1(MQR)" ".* = 106"
gdb_test "print mf2(MQR)" ".* = 106"
gdb_test "print f3(Q.id)" ".* = 42"
+
+# Inferior function call tests which have reference arguments.
+# https://sourceware.org/bugzilla/show_bug.cgi?id=25957
+gdb_start_again "breakpoint-here"
+
+# Test accessing existing map elements
+gdb_test "print int_map\[1\]" ": 10" \
+ "access existing map element with literal key"
+
+gdb_test "print int_map\[global_int_key\]" ": 100" \
+ "access existing map element with variable key"
+
+# Test accessing non-existing elements with literals
+gdb_test "print int_map\[99\]" \
+ ": 0" \
+ "attempt to access non-existing map element with literal" \
+
+# Test using operator[] syntax explicitly
+gdb_test "print int_map.operator\[\]\(1\)" \
+ ": 10" \
+ "explicit operator[] call with literal"
+
+# Test with string maps
+setup_xfail c++/25957 *-*-*
+gdb_test "print string_map\[\"hello\"\]" \
+ ": 1" \
+ "string map access with literal string"
+
+gdb_test "print string_map\[global_string_key\]" \
+ ": 99" \
+ "string map access with variable string"
+
+# Test calling function with const reference parameter and literal
+gdb_test "print const_ref_func(10)" \
+ "= 20" \
+ "call function with const ref param and literal"
+
+# Test calling function with const reference parameter and variable
+gdb_test "print const_ref_func(global_int_key)" \
+ "= 84" \
+ "call function with const ref param and variable"
+
+# Test with local variable
+gdb_test "print const_ref_func(local_var)" \
+ "= 30" \
+ "call function with const ref param and local var"
+
+# Test string const reference
+setup_xfail c++/25957 *-*-*
+gdb_test "print const_string_ref_func(\"test\")" \
+ {= "test_modified"} \
+ "call function with const string ref and literal"
+
+# Test calling function with non-const reference parameter.
+# This should work with variables but not literals.
+gdb_test "print ref_func(global_int_key)" \
+ " = 43" \
+ "call function with non-const ref param and variable"
+
+gdb_test "print ref_func(local_var)" \
+ " = 16" \
+ "call function with non-const ref param and local var"
+
+# This should fail: literals can't be non-const refs.
+gdb_test "print ref_func(10)" \
+ "Cannot resolve function ref_func to any overloaded instance" \
+ "call function with non-const ref param and literal"
+
+gdb_test "print obj.const_ref_method(5)" \
+ "= 10" \
+ "call const method with const ref param and literal"
+
+gdb_test "print obj.const_ref_method(local_var)" \
+ "= 20" \
+ "call const method with const ref param and variable"
+
+gdb_test "print obj.ref_method(local_var)" \
+ " = 35" \
+ "call method with non-const ref param and variable"
+
+# Test potential workarounds
+gdb_test_no_output {set $temp_key = 99} "create temporary variable"
+gdb_test_no_output {set var int_map[$temp_key] = 1234}
+gdb_test {print int_map[$temp_key]} \
+ ": 1234" \
+ "map access with convenience variable"
+
+gdb_test "print int_map.size()" " = 4"
+
+gdb_test "print int_map.empty()" " = false"
+
+gdb_test "print int_map.find(1)" " = {.*}"
+
+gdb_test "print int_map.count(1)" " = 1"
+
+gdb_test "print int_map.count(1234)" "= 0"
+
+gdb_test "whatis const_ref_func" \
+ {type = int \(const int &\)} \
+ "show function signature with const reference"
+
+gdb_test "whatis ref_func" \
+ {type = int \(int &\)} \
+ "show function signature with reference"
diff --git a/gdb/testsuite/gdb.cp/rvalue-ref-overload.exp b/gdb/testsuite/gdb.cp/rvalue-ref-overload.exp
index f4d55be92ef..8d8fe0bee99 100644
--- a/gdb/testsuite/gdb.cp/rvalue-ref-overload.exp
+++ b/gdb/testsuite/gdb.cp/rvalue-ref-overload.exp
@@ -64,7 +64,6 @@ gdb_test "print f (i)" "1" "lvalue reference overload"

gdb_test "print f (ci)" "2" "lvalue reference to const overload"

-setup_kfail "c++/15372" "*-*-*"
gdb_test "print f (3)" "3" "rvalue reference overload"

gdb_test "print g (i)" \
diff --git a/gdb/valops.c b/gdb/valops.c
index fa87546770a..78f411f7211 100644
--- a/gdb/valops.c
+++ b/gdb/valops.c
@@ -1464,18 +1464,29 @@ value_must_coerce_to_target (struct value *val)
storage, and this function copies them to the target. */

struct value *
-value_coerce_to_target (struct value *val)
+value_coerce_to_target (struct value *val, struct type *param_type)
{
LONGEST length;
CORE_ADDR addr;

- if (!value_must_coerce_to_target (val))
- return val;
+ if (param_type != nullptr && param_type->language () == language_cplus
+ && TYPE_IS_REFERENCE (param_type))
+ {
+ length = check_typedef (val->type ())->length ();
+ addr = allocate_space_in_inferior (length);
+ write_memory (addr, val->contents ().data (), length);
+ return value_ref (value_at_lazy (val->type (), addr),
+ param_type->code ());
+ }
+ else if (value_must_coerce_to_target (val))
+ {
+ length = check_typedef (val->type ())->length ();
+ addr = allocate_space_in_inferior (length);
+ write_memory (addr, val->contents ().data (), length);
+ return value_at_lazy (val->type (), addr);
+ }

- length = check_typedef (val->type ())->length ();
- addr = allocate_space_in_inferior (length);
- write_memory (addr, val->contents ().data (), length);
- return value_at_lazy (val->type (), addr);
+ return val;
}

/* Given a value which is an array, return a value which is a pointer
diff --git a/gdb/value.h b/gdb/value.h
index b9d2809bead..eecfc729714 100644
--- a/gdb/value.h
+++ b/gdb/value.h
@@ -1278,7 +1278,8 @@ extern LONGEST value_ptrdiff (struct value *arg1, struct value *arg2);

extern bool value_must_coerce_to_target (struct value *arg1);

-extern struct value *value_coerce_to_target (struct value *arg1);
+extern struct value *value_coerce_to_target (struct value *arg1,
+ struct type *param_type = nullptr);

extern struct value *value_coerce_array (struct value *arg1);

base-commit: b6753354fbbe7c2c66ae9f452ba7aa049db0fe0c
--
2.51.0