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RFA: Avoid converting functions to pointers and back
- To: gdb-patches at sources dot redhat dot com
- Subject: RFA: Avoid converting functions to pointers and back
- From: Jim Blandy <jimb at zwingli dot cygnus dot com>
- Date: Thu, 1 Nov 2001 10:05:31 -0500 (EST)
2001-10-25 Jim Blandy <firstname.lastname@example.org>
* values.c (value_as_address): If VAL is a function or method,
just return its address; don't let COERCE_ARRAY convert its
address to a pointer, and then have unpack_long convert it back to
RCS file: /cvs/src/src/gdb/values.c,v
retrieving revision 1.25
diff -c -r1.25 values.c
*** gdb/values.c 2001/10/16 01:58:07 1.25
--- gdb/values.c 2001/10/25 18:15:46
*** 571,576 ****
--- 571,618 ----
for pointers to char, in which the low bits *are* significant. */
return ADDR_BITS_REMOVE (value_as_long (val));
+ /* There are several targets (IA-64, PowerPC, and others) which
+ don't represent pointers to functions as simply the address of
+ the function's entry point. For example, on the IA-64, a
+ function pointer points to a two-word descriptor, generated by
+ the linker, which contains the function's entry point, and the
+ value the IA-64 "global pointer" register should have --- to
+ support position-independent code. The linker generates
+ descriptors only for those functions whose addresses are taken.
+ On such targets, it's difficult for GDB to convert an arbitrary
+ function address into a function pointer; it has to either find
+ an existing descriptor for that function, or call malloc and
+ build its own. On some targets, it is impossible for GDB to
+ build a descriptor at all: the descriptor must contain a jump
+ instruction; data memory cannot be executed; and code memory
+ cannot be modified.
+ Upon entry to this function, if VAL is a value of type `function'
+ (that is, TYPE_CODE (VALUE_TYPE (val)) == TYPE_CODE_FUNC), then
+ VALUE_ADDRESS (val) is the address of the function. This is what
+ you'll get if you evaluate an expression like `main'. The call
+ to COERCE_ARRAY below actually does all the usual unary
+ conversions, which includes converting values of type `function'
+ to `pointer to function'. This is the challenging conversion
+ discussed above. Then, `unpack_long' will convert that pointer
+ back into an address.
+ So, suppose the user types `disassemble foo' on an architecture
+ with a strange function pointer representation, on which GDB
+ cannot build its own descriptors, and suppose further that `foo'
+ has no linker-built descriptor. The address->pointer conversion
+ will signal an error and prevent the command from running, even
+ though the next step would have been to convert the pointer
+ directly back into the same address.
+ The following shortcut avoids this whole mess. If VAL is a
+ function, just return its address directly. */
+ if (TYPE_CODE (VALUE_TYPE (val)) == TYPE_CODE_FUNC
+ || TYPE_CODE (VALUE_TYPE (val)) == TYPE_CODE_METHOD)
+ return VALUE_ADDRESS (val);
/* Some architectures (e.g. Harvard), map instruction and data