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* [Bug c/36402] New: -0x80000000 (INT_MIN) erroneously treated as unsigned
@ 2008-05-31 19:39 gcczilla at achurch dot org
2008-05-31 19:50 ` [Bug c/36402] " ebotcazou at gcc dot gnu dot org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: gcczilla at achurch dot org @ 2008-05-31 19:39 UTC (permalink / raw)
To: gcc-bugs
GCC seems to treat the 32-bit integer constant -0x80000000 (INT_MIN) as an
unsigned value, when it should be signed. (I don't think this is a duplicate
of bug 25329, since I'm not trying to negate the constant.) For example:
int a = 1;
int foo(void)
{
return -0x80000000 < a;
}
improperly returns 0, even though -0x80000000 is less than any positive value.
>From the assembly output, it seems as though GCC is treating -0x80000000 as an
unsigned value (using the unsigned "seta" instruction to interpret the
comparison result):
movl a, %eax
cmpl $-2147483648, %eax
seta %al
Changing the comparison to:
return (int)-0x80000000 < a; // Cast to signed
succeeds, as expected (interestingly using "setne" rather than "setg", though
certainly either works).
--
Summary: -0x80000000 (INT_MIN) erroneously treated as unsigned
Product: gcc
Version: 4.2.4
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: gcczilla at achurch dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36402
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/36402] -0x80000000 (INT_MIN) erroneously treated as unsigned
2008-05-31 19:39 [Bug c/36402] New: -0x80000000 (INT_MIN) erroneously treated as unsigned gcczilla at achurch dot org
@ 2008-05-31 19:50 ` ebotcazou at gcc dot gnu dot org
2008-06-01 4:17 ` gcczilla at achurch dot org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: ebotcazou at gcc dot gnu dot org @ 2008-05-31 19:50 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from ebotcazou at gcc dot gnu dot org 2008-05-31 19:50 -------
> GCC seems to treat the 32-bit integer constant -0x80000000 (INT_MIN) as an
> unsigned value, when it should be signed.
Incorrect. -0x80000000 is not an integer constant, it's the negation of the
integer constant 0x80000000, which is unsigned (C99 6.4.4.1).
--
ebotcazou at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |ebotcazou at gcc dot gnu dot
| |org
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36402
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/36402] -0x80000000 (INT_MIN) erroneously treated as unsigned
2008-05-31 19:39 [Bug c/36402] New: -0x80000000 (INT_MIN) erroneously treated as unsigned gcczilla at achurch dot org
2008-05-31 19:50 ` [Bug c/36402] " ebotcazou at gcc dot gnu dot org
@ 2008-06-01 4:17 ` gcczilla at achurch dot org
2008-06-01 7:26 ` ebotcazou at gcc dot gnu dot org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: gcczilla at achurch dot org @ 2008-06-01 4:17 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from gcczilla at achurch dot org 2008-06-01 04:17 -------
Fair enough, but GCC's documentation explicitly says (gcc.info section 4.5):
* `Whether signed integer types are represented using sign and
magnitude, two's complement, or one's complement, and whether the
extraordinary value is a trap representation or an ordinary value
(C99 6.2.6.2).'
GCC supports only two's complement integer types, and all bit
patterns are ordinary values.
Given that, I think a typical user would assume (as I have) that GCC would
treat -0x80000000 as the signed value -2^31; otherwise there would seem to be
no way to write a single constant to represent that valid value. So I'm going
to have to argue that this is still a bug, whether in the documentation or in
the compiler itself.
--
gcczilla at achurch dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|INVALID |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36402
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/36402] -0x80000000 (INT_MIN) erroneously treated as unsigned
2008-05-31 19:39 [Bug c/36402] New: -0x80000000 (INT_MIN) erroneously treated as unsigned gcczilla at achurch dot org
2008-05-31 19:50 ` [Bug c/36402] " ebotcazou at gcc dot gnu dot org
2008-06-01 4:17 ` gcczilla at achurch dot org
@ 2008-06-01 7:26 ` ebotcazou at gcc dot gnu dot org
2008-06-01 12:22 ` joseph at codesourcery dot com
2008-06-01 14:00 ` [Bug c/36402] -0x80000000 (INT_MIN, -2147483648) " gcczilla at achurch dot org
4 siblings, 0 replies; 6+ messages in thread
From: ebotcazou at gcc dot gnu dot org @ 2008-06-01 7:26 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from ebotcazou at gcc dot gnu dot org 2008-06-01 07:25 -------
> Given that, I think a typical user would assume (as I have) that GCC would
> treat -0x80000000 as the signed value -2^31; otherwise there would seem to be
> no way to write a single constant to represent that valid value.
You're confusing the internal representation, described by the paragraph you
quoted, and the syntax of literals. There is no bug in this case, it's the
well-know limitation of C whereby you need to write INT_MIN as (-INT_MAX - 1).
--
ebotcazou at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36402
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/36402] -0x80000000 (INT_MIN) erroneously treated as unsigned
2008-05-31 19:39 [Bug c/36402] New: -0x80000000 (INT_MIN) erroneously treated as unsigned gcczilla at achurch dot org
` (2 preceding siblings ...)
2008-06-01 7:26 ` ebotcazou at gcc dot gnu dot org
@ 2008-06-01 12:22 ` joseph at codesourcery dot com
2008-06-01 14:00 ` [Bug c/36402] -0x80000000 (INT_MIN, -2147483648) " gcczilla at achurch dot org
4 siblings, 0 replies; 6+ messages in thread
From: joseph at codesourcery dot com @ 2008-06-01 12:22 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from joseph at codesourcery dot com 2008-06-01 12:22 -------
Subject: Re: -0x80000000 (INT_MIN) erroneously treated as
unsigned
On Sun, 1 Jun 2008, gcczilla at achurch dot org wrote:
> Fair enough, but GCC's documentation explicitly says (gcc.info section 4.5):
It also explicitly explains, in the section "Incompatibilities", that
-2147483648 is positive, and why. (In C99 mode, 2147483648 becomes of
type long long, but 0x80000000 is still unsigned.)
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36402
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/36402] -0x80000000 (INT_MIN, -2147483648) treated as unsigned
2008-05-31 19:39 [Bug c/36402] New: -0x80000000 (INT_MIN) erroneously treated as unsigned gcczilla at achurch dot org
` (3 preceding siblings ...)
2008-06-01 12:22 ` joseph at codesourcery dot com
@ 2008-06-01 14:00 ` gcczilla at achurch dot org
4 siblings, 0 replies; 6+ messages in thread
From: gcczilla at achurch dot org @ 2008-06-01 14:00 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from gcczilla at achurch dot org 2008-06-01 13:59 -------
Thanks for the clarification. (To be honest, I wouldn't have thought to look
in the "Incompatibilities" section--I haven't touched a non-ANSI compiler for
over a decade--but either way I guess it's my fault for not searching for
"2147483648".)
Touched up the summary line a bit to help future searchers.
--
gcczilla at achurch dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|-0x80000000 (INT_MIN) |-0x80000000 (INT_MIN, -
|erroneously treated as |2147483648) treated as
|unsigned |unsigned
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36402
^ permalink raw reply [flat|nested] 6+ messages in thread
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2008-05-31 19:39 [Bug c/36402] New: -0x80000000 (INT_MIN) erroneously treated as unsigned gcczilla at achurch dot org
2008-05-31 19:50 ` [Bug c/36402] " ebotcazou at gcc dot gnu dot org
2008-06-01 4:17 ` gcczilla at achurch dot org
2008-06-01 7:26 ` ebotcazou at gcc dot gnu dot org
2008-06-01 12:22 ` joseph at codesourcery dot com
2008-06-01 14:00 ` [Bug c/36402] -0x80000000 (INT_MIN, -2147483648) " gcczilla at achurch dot org
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