gdb/180: printing expression gives different result to executing equivalent statement

[dcoutts at cray dot com]

>Number:         180
>Category:       gdb
>Synopsis:       printing expression gives different result to executing equivalent statement
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jul 24 09:08:02 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     dcoutts@cray.com
>Release:        gdb 5.0 (mips-sgi-irix6.5, and others)
>Organization:
>Environment:
native mips-sgi-irix6.5x
gdb build both with system cc, and gcc 2.95.3
>Description:
Printing the value of an expression on the gdb command line
can have a different answer from when the same expression
is evaluated by the machine, even on native targets.

The example I use is long lx = -(1 << 63)

I came across this piece of code when debugging gdb with gdb.
On a 64bit target, this code crops up in get_discrete_bounds
when called with a TYPE_CODE_INT argument:
*lowp = -(1 << (TYPE_LENGTH (type) * TARGET_CHAR_BIT - 1))

As a side issue this code does not have the expected effect 
on a 64bit target, it gives the result 0, which is not the
most negative number avaliable in the representation, (which
was the intent of the code, right?)
>How-To-Repeat:
when debugging the attached C program, if we print the value
of the expression we get one answer:

(gdb) p (long) -(1 << 63)
$1 = 0

however, executing the statement and checking the result:
6        lx = -(1 << 63);
(gdb) n
(gdb) p lx
$2 = -2147483648

here lx is defined as long, so I would expect that -(1 << 63)
cast to a long would have the same value as assigning the 
same expression to a varialbe of type long.

We get the same behaviour with llx, which is a long long.
On the mips-sgi-irix6.5x target, long is 32bit and long long
is 64bit.
>Fix:
Don't trust the expression evaluation too much.
>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: application/octet-stream; name="gdb_bug.c"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="gdb_bug.c"

dm9pZCBtYWluICgpCnsKICBsb25nIGx4OwkgLyogMzJiaXQgb24gbWlwcy1zZ2ktaXJpeDYuNSAq
LwogIGxvbmcgbG9uZyBsbHg7IC8qIDY0Yml0IG9uIG1pcHMtc2dpLWlyaXg2LjUgKi8KCiAgbHgg
PSAtKDEgPDwgNjMpOwogIGxseCA9IC0oMSA8PCA2Myk7Cn0K