[PATCH] Improve comments about returning floats in i386-tdep.c

Mark Kettenis
2001-07-12T11:52:00Z

Since it's a bit tricky, more comments don't hurt.

Is this better Eli?

Mark

Index: ChangeLog
from Mark Kettenis <kettenis@gnu.org>

* i386-tdep.c (i386_extract_return_value): Undo 2001-07-11 changes
to comment.
(i386_store_return_value): Improve comments about storing
floating-point return values.

Index: i386-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/i386-tdep.c,v
retrieving revision 1.32
diff -u -p -r1.32 i386-tdep.c
--- i386-tdep.c 2001/07/11 08:43:41 1.32
+++ i386-tdep.c 2001/07/12 18:47:39
@@ -803,8 +803,7 @@ i386_extract_return_value (struct type *
return;
}

- /* Floating-point return values can be found in %st(0).
- FIXME: Does %st(0) always correspond to FP0? */
+ /* Floating-point return values can be found in %st(0). */
if (len == TARGET_LONG_DOUBLE_BIT / TARGET_CHAR_BIT
&& TARGET_LONG_DOUBLE_FORMAT == &floatformat_i387_ext)
{
@@ -870,7 +869,10 @@ i386_store_return_value (struct type *ty
return;
}

- /* Floating-point return values can be found in %st(0). */
+ /* Returning floating-point values is a bit tricky. Apart from
+ storing the return value in %st(0), we have to simulate the
+ state of the FPU at function return point. */
+
if (len == TARGET_LONG_DOUBLE_BIT / TARGET_CHAR_BIT
&& TARGET_LONG_DOUBLE_FORMAT == &floatformat_i387_ext)
{
@@ -884,7 +886,7 @@ i386_store_return_value (struct type *ty
DOUBLEST val;

/* Convert the value found in VALBUF to the extended
- floating point format used by the FPU. This is probably
+ floating-point format used by the FPU. This is probably
not exactly how it would happen on the target itself, but
it is the best we can do. */
val = extract_floating (valbuf, TYPE_LENGTH (type));
@@ -893,14 +895,17 @@ i386_store_return_value (struct type *ty
FPU_REG_RAW_SIZE);
}

- /* Set the top of the floating point register stack to 7. That
- makes sure that FP0 (which we set above) is indeed %st(0).
- FIXME: Perhaps we should completely reset the status word? */
+ /* Set the top of the floating-point register stack to 7. The
+ actual value doesn't really matter, but 7 is what a normal
+ function return would end up with if the program started out
+ with a freshly initialized FPU. */
fstat = read_register (FSTAT_REGNUM);
fstat |= (7 << 11);
write_register (FSTAT_REGNUM, fstat);

- /* Mark %st(1) through %st(7) as empty. */
+ /* Mark %st(1) through %st(7) as empty. Since we set the top of
+ the floating-point register stack to 7, the appropriate value
+ for the tag word is 0x3fff. */
write_register (FTAG_REGNUM, 0x3fff);
}
else