Re: [patch/rfc, rfa:doco] gdbarch return value

[Andrew Cagney <cagney at gnu dot org>]

> Hello,
>
> This implements the "return_value" method I proposed earlier this week.  It replaces USE_STRUCT_CONVENTION, EXTRACT_RETURN_VALUE, and STORE_RETURN_VALUE.  The implementation is fallout from me fixing the ppc64 return code and finding that a single "reurn_value" method lead to more robust code.
>
> Several things to note:
>
> -  The doco contains a number of maintainer notes.  The intent is to provide something of a rationale for the current interface.  That way future changes will have a reasonable base from which to work from. They are recommended reading.
>
> - I've modified the "return small_struct" code so that, when an architecture doesn't implement return_value an internal_error and not an error is reported.  With the introduction of return_value, it is possible to handle the "return small_struct" case so failing to do this is an a bug in GDB.

I've checked in the attached, which contains one refinement over the 
original.

The "return" code now issues a warning, and not an error, when it 
realises that it can't handle the value being returned.  Without this, a 
test like:

	(gdb) return small_struct

gets GDB (and I think the user) into a confused state.  Even though the 
return was performed, this fact isn't reported.  Contrast this old behavior:

(gdb) return foo1
Make fun1 return now? (y or n) y
GDB does not support specifying a struct or union return value.
(gdb)

with:

(gdb) return foo3
Make fun3 return now? (y or n) y
warning: Location of return value unknown
#0  main () at 
/home/scratch/PENDING/2003-10-19-ppc-return-value/src/gdb/testsui
te/gdb.base/structs-char.c:234
234       L4  = fun4();
(gdb)

Still comming are:

- a testcase
- a tweak to put the warning before the question

committed,
Andrew

2003-10-20  Andrew Cagney  <cagney@redhat.com>

	* values.c (register_value_being_returned): Update comments.  Use
	"gdbarch_return_value" when available.
	(using_struct_return): Ditto.
	(set_return_value): Ditto.  Use "gdbarch_return_value" when
	available..  Print a warning, and not an error, when an unhandled
	return type is encountered.
	* infcmd.c: Include "gdb_assert.h".
	(print_return_value): When gdbarch_return_value_p, and using
	struct return, assume that the value is not available.
	* defs.h (return_value_convention): Define.
	* gdbarch.sh (gdbarch_return_value): New predicate method.
	* gdbarch.h, gdbarch.c: Re-generate
	* ppc-sysv-tdep.c (return_value_convention): Delete definition.

Index: doc/ChangeLog
2003-10-20  Andrew Cagney  <cagney@redhat.com>

	* gdbint.texinfo (Target Architecture Definition): Document
	gdbarch_return_value.  Add cross references from
	USE_STRUCT_CONVENTION, EXTRACT_RETURN_VALUE, and
	STORE_RETURN_VALUE, and from/to EXTRACT_STRUCT_VALUE_ADDRESS.

Index: defs.h
===================================================================
RCS file: /cvs/src/src/gdb/defs.h,v
retrieving revision 1.131
diff -u -r1.131 defs.h
--- defs.h	19 Sep 2003 16:22:38 -0000	1.131
+++ defs.h	20 Oct 2003 15:35:13 -0000
@@ -230,6 +230,21 @@
   AUTO_BOOLEAN_AUTO
 };
 
+/* Potential ways that a function can return a value of a given type.  */
+enum return_value_convention
+{
+  /* Where the return value has been squeezed into one or more
+     registers.  */
+  RETURN_VALUE_REGISTER_CONVENTION,
+  /* Commonly known as the "struct return convention".  The caller
+     passes an additional hidden first parameter to the caller.  That
+     parameter contains the address at which the value being returned
+     should be stored.  While typically, and historically, used for
+     large structs, this is convention is applied to values of many
+     different types.  */
+  RETURN_VALUE_STRUCT_CONVENTION
+};
+
 /* the cleanup list records things that have to be undone
    if an error happens (descriptors to be closed, memory to be freed, etc.)
    Each link in the chain records a function to call and an
Index: gdbarch.sh
===================================================================
RCS file: /cvs/src/src/gdb/gdbarch.sh,v
retrieving revision 1.278
diff -u -r1.278 gdbarch.sh
--- gdbarch.sh	13 Oct 2003 23:41:40 -0000	1.278
+++ gdbarch.sh	20 Oct 2003 15:35:14 -0000
@@ -593,19 +593,33 @@
 f:2:ADDRESS_TO_POINTER:void:address_to_pointer:struct type *type, void *buf, CORE_ADDR addr:type, buf, addr:::unsigned_address_to_pointer::0
 F:2:INTEGER_TO_ADDRESS:CORE_ADDR:integer_to_address:struct type *type, void *buf:type, buf
 #
-f:2:RETURN_VALUE_ON_STACK:int:return_value_on_stack:struct type *type:type:::generic_return_value_on_stack_not::0
 F:2:DEPRECATED_POP_FRAME:void:deprecated_pop_frame:void:-
 # NOTE: cagney/2003-03-24: Replaced by PUSH_ARGUMENTS.
 F:2:DEPRECATED_STORE_STRUCT_RETURN:void:deprecated_store_struct_return:CORE_ADDR addr, CORE_ADDR sp:addr, sp
-#
+
+# It has been suggested that this, well actually its predecessor,
+# should take the type/value of the function to be called and not the
+# return type.  This is left as an exercise for the reader.
+
+M:::enum return_value_convention:return_value:struct type *valtype, struct regcache *regcache, const void *inval, void *outval:valtype, regcache, inval, outval
+
+# The deprecated methods RETURN_VALUE_ON_STACK, EXTRACT_RETURN_VALUE,
+# STORE_RETURN_VALUE and USE_STRUCT_CONVENTION have all been folded
+# into RETURN_VALUE.  For the moment do not try to fold in
+# EXTRACT_STRUCT_VALUE_ADDRESS as, dependant on the ABI, the debug
+# info, and the level of effort, it may well be possible to find the
+# address of a structure being return on the stack.  Someone else can
+# make that change.
+
+f:2:RETURN_VALUE_ON_STACK:int:return_value_on_stack:struct type *type:type:::generic_return_value_on_stack_not::0
 f:2:EXTRACT_RETURN_VALUE:void:extract_return_value:struct type *type, struct regcache *regcache, void *valbuf:type, regcache, valbuf:::legacy_extract_return_value::0
 f:2:STORE_RETURN_VALUE:void:store_return_value:struct type *type, struct regcache *regcache, const void *valbuf:type, regcache, valbuf:::legacy_store_return_value::0
 f:2:DEPRECATED_EXTRACT_RETURN_VALUE:void:deprecated_extract_return_value:struct type *type, char *regbuf, char *valbuf:type, regbuf, valbuf
 f:2:DEPRECATED_STORE_RETURN_VALUE:void:deprecated_store_return_value:struct type *type, char *valbuf:type, valbuf
-#
+f:2:USE_STRUCT_CONVENTION:int:use_struct_convention:int gcc_p, struct type *value_type:gcc_p, value_type:::generic_use_struct_convention::0
+
 F:2:EXTRACT_STRUCT_VALUE_ADDRESS:CORE_ADDR:extract_struct_value_address:struct regcache *regcache:regcache
 F:2:DEPRECATED_EXTRACT_STRUCT_VALUE_ADDRESS:CORE_ADDR:deprecated_extract_struct_value_address:char *regbuf:regbuf
-f:2:USE_STRUCT_CONVENTION:int:use_struct_convention:int gcc_p, struct type *value_type:gcc_p, value_type:::generic_use_struct_convention::0
 #
 F:2:DEPRECATED_FRAME_INIT_SAVED_REGS:void:deprecated_frame_init_saved_regs:struct frame_info *frame:frame
 F:2:DEPRECATED_INIT_EXTRA_FRAME_INFO:void:deprecated_init_extra_frame_info:int fromleaf, struct frame_info *frame:fromleaf, frame
Index: infcmd.c
===================================================================
RCS file: /cvs/src/src/gdb/infcmd.c,v
retrieving revision 1.96
diff -u -r1.96 infcmd.c
--- infcmd.c	2 Oct 2003 20:28:29 -0000	1.96
+++ infcmd.c	20 Oct 2003 15:35:15 -0000
@@ -44,6 +44,7 @@
 #include "reggroups.h"
 #include "block.h"
 #include <ctype.h>
+#include "gdb_assert.h"
 
 /* Functions exported for general use, in inferior.h: */
 
@@ -1079,25 +1080,34 @@
       ui_out_field_stream (uiout, "return-value", stb);
       ui_out_text (uiout, "\n");
     }
-  else
-    {
-      /* FIXME: 2003-09-27: When returning from a nested inferior
-         function call, it's possible (with no help from the
-         architecture vector) to locate and return/print a "struct
-         return" value.  This is just a more complicated case of what
-         is already being done in in the inferior function call code.
-         In fact, when inferior function calls are made async, this
-         will likely be made the norm.  */
-      /* We cannot determine the contents of the structure because
-	 it is on the stack, and we don't know where, since we did not
-	 initiate the call, as opposed to the call_function_by_hand case */
+  /* FIXME: 2003-09-27: When returning from a nested inferior function
+     call, it's possible (with no help from the architecture vector)
+     to locate and return/print a "struct return" value.  This is just
+     a more complicated case of what is already being done in in the
+     inferior function call code.  In fact, when inferior function
+     calls are made async, this will likely be made the norm.  */
 #ifdef DEPRECATED_VALUE_RETURNED_FROM_STACK
-      value = 0;
+#define DEPRECATED_VALUE_RETURNED_FROM_STACK_P 1
+#else
+#define DEPRECATED_VALUE_RETURNED_FROM_STACK_P 0
+#endif
+  else if (gdbarch_return_value_p (current_gdbarch)
+	   || DEPRECATED_VALUE_RETURNED_FROM_STACK_P)
+    /* We cannot determine the contents of the structure because it is
+       on the stack, and we don't know where, since we did not
+       initiate the call, as opposed to the call_function_by_hand
+       case.  */
+    {
+      gdb_assert (gdbarch_return_value (current_gdbarch, value_type, NULL, NULL, NULL)
+		  == RETURN_VALUE_STRUCT_CONVENTION);
       ui_out_text (uiout, "Value returned has type: ");
       ui_out_field_string (uiout, "return-type", TYPE_NAME (value_type));
       ui_out_text (uiout, ".");
       ui_out_text (uiout, " Cannot determine contents\n");
-#else
+      return;
+    }
+  else
+    {
       if (EXTRACT_STRUCT_VALUE_ADDRESS_P ())
 	{
 	  CORE_ADDR addr = EXTRACT_STRUCT_VALUE_ADDRESS (stop_registers);
@@ -1133,7 +1143,6 @@
       value_print (value, stb->stream, 0, Val_no_prettyprint);
       ui_out_field_stream (uiout, "return-value", stb);
       ui_out_text (uiout, "\n");
-#endif
     }
 }
 
Index: ppc-sysv-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/ppc-sysv-tdep.c,v
retrieving revision 1.16
diff -u -r1.16 ppc-sysv-tdep.c
--- ppc-sysv-tdep.c	10 Oct 2003 21:32:47 -0000	1.16
+++ ppc-sysv-tdep.c	20 Oct 2003 15:35:15 -0000
@@ -305,21 +305,6 @@
   return sp;
 }
 
-/* Potential ways that a function can return a value of a given type.  */
-enum return_value_convention
-{
-  /* Where the return value has been squeezed into one or more
-     registers.  */
-  RETURN_VALUE_REGISTER_CONVENTION,
-  /* Commonly known as the "struct return convention".  The caller
-     passes an additional hidden first parameter to the caller.  That
-     parameter contains the address at which the value being returned
-     should be stored.  While typically, and historically, used for
-     large structs, this is convention is applied to values of many
-     different types.  */
-  RETURN_VALUE_STRUCT_CONVENTION
-};
-
 /* Handle the return-value conventions specified by the SysV 32-bit
    PowerPC ABI (including all the supplements):
 
Index: values.c
===================================================================
RCS file: /cvs/src/src/gdb/values.c,v
retrieving revision 1.59
diff -u -r1.59 values.c
--- values.c	2 Oct 2003 04:40:58 -0000	1.59
+++ values.c	20 Oct 2003 15:35:15 -0000
@@ -1202,25 +1202,48 @@
   return val;
 }
 
-/* Deal with the value that is "about to be returned".
+/* Deal with the return-value of a function that has "just returned".
 
-   Return the value that a function, using the register convention,
-   returning now would be returning to its caller, assuming its type
-   is VALTYPE.  RETBUF is where we look for what ought to be the
-   contents of the registers (in raw form).  This is because it is
-   often desirable to restore old values to those registers after
-   saving the contents of interest, and then call this function using
-   the saved values.  */
+   Extract the return-value (as a "struct value") that a function,
+   using register convention, has just returned to its caller.  Assume
+   that the type of the function is VALTYPE, and that the "just
+   returned" register state is found in RETBUF.
+
+   The function has "just returned" because GDB halts a returning
+   function by setting a breakpoint at the return address (in the
+   caller), and not the return instruction (in the callee).
+
+   Because, in the case of a return from an inferior function call,
+   GDB needs to restore the inferiors registers, RETBUF is normally a
+   copy of the inferior's registers.  */
 
 struct value *
 register_value_being_returned (struct type *valtype, struct regcache *retbuf)
 {
   struct value *val = allocate_value (valtype);
-  CHECK_TYPEDEF (valtype);
+
   /* If the function returns void, don't bother fetching the return
      value.  */
-  if (TYPE_CODE (valtype) != TYPE_CODE_VOID)
-    EXTRACT_RETURN_VALUE (valtype, retbuf, VALUE_CONTENTS_RAW (val));
+  if (TYPE_CODE (valtype) == TYPE_CODE_VOID)
+    return val;
+
+  if (!gdbarch_return_value_p (current_gdbarch))
+    {
+      /* NOTE: cagney/2003-10-20: Unlike "gdbarch_return_value", the
+         EXTRACT_RETURN_VALUE and USE_STRUCT_CONVENTION methods do not
+         handle the edge case of a function returning a small
+         structure / union in registers.  */
+      CHECK_TYPEDEF (valtype);
+      EXTRACT_RETURN_VALUE (valtype, retbuf, VALUE_CONTENTS_RAW (val));
+      return val;
+    }
+
+  /* This function only handles "register convention".  */
+  gdb_assert (gdbarch_return_value (current_gdbarch, valtype,
+				    NULL, NULL, NULL)
+	      == RETURN_VALUE_REGISTER_CONVENTION);
+  gdbarch_return_value (current_gdbarch, valtype, retbuf,
+			NULL, VALUE_CONTENTS_RAW (val));
   return val;
 }
 
@@ -1262,13 +1285,25 @@
   if (code == TYPE_CODE_ERROR)
     error ("Function return type unknown.");
 
-  if (code == TYPE_CODE_STRUCT
-      || code == TYPE_CODE_UNION
-      || code == TYPE_CODE_ARRAY
-      || RETURN_VALUE_ON_STACK (value_type))
-    return USE_STRUCT_CONVENTION (gcc_p, value_type);
-
-  return 0;
+  if (!gdbarch_return_value_p (current_gdbarch))
+    {
+      /* FIXME: cagney/2003-10-01: The below is dead.  Instead an
+	 architecture should implement "gdbarch_return_value".  Using
+	 that new function it is possible to exactly specify the ABIs
+	 "struct return" vs "register return" conventions.  */
+      if (code == TYPE_CODE_STRUCT
+	  || code == TYPE_CODE_UNION
+	  || code == TYPE_CODE_ARRAY
+	  || RETURN_VALUE_ON_STACK (value_type))
+	return USE_STRUCT_CONVENTION (gcc_p, value_type);
+      else
+	return 0;
+    }
+
+  /* Probe the architecture for the return-value convention.  */
+  return (gdbarch_return_value (current_gdbarch, value_type,
+				NULL, NULL, NULL)
+	  == RETURN_VALUE_STRUCT_CONVENTION);
 }
 
 /* Store VAL so it will be returned if a function returns now.
@@ -1284,9 +1319,41 @@
   if (code == TYPE_CODE_ERROR)
     error ("Function return type unknown.");
 
+  if (gdbarch_return_value_p (current_gdbarch))
+    {
+      switch (gdbarch_return_value (current_gdbarch, type, NULL, NULL, NULL))
+	{
+	case RETURN_VALUE_REGISTER_CONVENTION:
+	  /* Success.  The architecture can deal with it, write it to
+             the regcache.  */
+	  gdbarch_return_value (current_gdbarch, type, current_regcache,
+				VALUE_CONTENTS (val), NULL);
+	  return;
+	case RETURN_VALUE_STRUCT_CONVENTION:
+	  /* Failure.  For the moment, assume that it is not possible
+             to find the location, on the stack, at which the "struct
+             return" value should be stored.  Only a warning because
+             an error aborts the "return" command leaving GDB in a
+             weird state.  */
+	  warning ("Location of return value unknown");
+	  return;
+	}
+    }
+
+
   if (code == TYPE_CODE_STRUCT
       || code == TYPE_CODE_UNION)	/* FIXME, implement struct return.  */
-    error ("GDB does not support specifying a struct or union return value.");
+    /* FIXME: cagney/2003-10-20: This should be an internal-warning.
+       The problem is that while GDB's core supports "struct return"
+       using "register convention", many architectures haven't been
+       updated to implement the mechanisms needed to make it work.
+       It's a warning, and not an error, as otherwize it will jump out
+       of the "return" command leaving both GDB and the user in a very
+       confused state.  */
+    {
+      warning ("This architecture does not support specifying a struct or union return-value.");
+      return;
+    }
 
   STORE_RETURN_VALUE (type, current_regcache, VALUE_CONTENTS (val));
 }
Index: doc/gdbint.texinfo
===================================================================
RCS file: /cvs/src/src/gdb/doc/gdbint.texinfo,v
retrieving revision 1.176
diff -u -r1.176 gdbint.texinfo
--- doc/gdbint.texinfo	18 Oct 2003 12:28:05 -0000	1.176
+++ doc/gdbint.texinfo	20 Oct 2003 15:35:17 -0000
@@ -3219,13 +3219,17 @@
 the raw register state @var{regbuf} and copy that, in virtual format,
 into @var{valbuf}.
  
+This method has been deprecated in favour of @code{gdbarch_return_value}
+(@pxref{gdbarch_return_value}).
+
 @item EXTRACT_STRUCT_VALUE_ADDRESS(@var{regbuf})
 @findex EXTRACT_STRUCT_VALUE_ADDRESS
+@anchor{EXTRACT_STRUCT_VALUE_ADDRESS}
 When defined, extract from the array @var{regbuf} (containing the raw
 register state) the @code{CORE_ADDR} at which a function should return
 its structure value.
 
-If not defined, @code{EXTRACT_RETURN_VALUE} is used.
+@xref{gdbarch_return_value}.
 
 @item EXTRACT_STRUCT_VALUE_ADDRESS_P()
 @findex EXTRACT_STRUCT_VALUE_ADDRESS_P
@@ -3824,6 +3828,48 @@
 Define this to convert sdb register numbers into @value{GDBN} regnums.  If not
 defined, no conversion will be done.
 
+@item enum return_value_convention gdbarch_return_value (struct gdbarch *@var{gdbarch}, struct type *@var{valtype}, struct regcache *@var{regcache}, const void *@var{inval}, void *@var{outval})
+@findex gdbarch_return_value
+@anchor{gdbarch_return_value} Given a function with a return-value of
+type @var{rettype}, return which return-value convention that function
+would use.
+
+@value{GDBN} currently recognizes two function return-value conventions:
+@code{RETURN_VALUE_REGISTER_CONVENTION} where the return value is found
+in registers; and @code{RETURN_VALUE_STRUCT_CONVENTION} where the return
+value is found in memory and the address of that memory location is
+passed in as the function's first parameter.
+
+If the register convention is being used, and @var{inval} is
+non-@code{NULL}, also copy the return-value in @var{inval} into
+@var{regcache}.
+
+If the register convention is being used, and @var{outval} is
+non-@code{NULL}, also copy the return value from @var{regcache} into
+@var{outval} (@var{regcache} contains a copy of the registers from the
+just returned function).
+
+@xref{EXTRACT_STRUCT_VALUE_ADDRESS}, for a description of how
+return-values that use the struct convention are handled.
+
+@emph{Maintainer note: This method replaces separate predicate, extract,
+store methods.  By having only one method, the logic needed to determine
+the return-value convention need only be implemented in one place.  If
+@value{GDBN} were written in an @sc{oo} language, this method would
+instead return an object that knew how to perform the register
+return-value extract and store.}
+
+@emph{Maintainer note: This method does not take a @var{gcc_p}
+parameter, and such a parameter should not be added.  If an architecture
+that requires per-compiler or per-function information be identified,
+then the replacement of @var{rettype} with @code{struct value}
+@var{function} should be persued.}
+
+@emph{Maintainer note: The @var{regcache} parameter limits this methods
+to the inner most frame.  While replacing @var{regcache} with a
+@code{struct frame_info} @var{frame} parameter would remove that
+limitation there has yet to be a demonstrated need for such a change.}
+
 @item SKIP_PERMANENT_BREAKPOINT
 @findex SKIP_PERMANENT_BREAKPOINT
 Advance the inferior's PC past a permanent breakpoint.  @value{GDBN} normally
@@ -3882,6 +3928,9 @@
 @var{valbuf}, into the @var{regcache}.  @var{type} is the type of the
 value that is to be returned.
 
+This method has been deprecated in favour of @code{gdbarch_return_value}
+(@pxref{gdbarch_return_value}).
+
 @item SUN_FIXED_LBRAC_BUG
 @findex SUN_FIXED_LBRAC_BUG
 (Used only for Sun-3 and Sun-4 targets.)
@@ -4016,6 +4065,9 @@
 being considered is known to have been compiled by GCC; this is helpful
 for systems where GCC is known to use different calling convention than
 other compilers.
+
+This method has been deprecated in favour of @code{gdbarch_return_value}
+(@pxref{gdbarch_return_value}).
 
 @item VALUE_TO_REGISTER(@var{type}, @var{regnum}, @var{from}, @var{to})
 @findex VALUE_TO_REGISTER