-Original Message-
I would like to say the one thing I have not heard through this
discussion is the real reason why the C standards comittee decided
signed overflow as being undefined. All I can think of is they were
thinking of target that do saturation for plus/minus but wrapping
-Original Message-
I would like to say the one thing I have not heard through this
discussion is the real reason why the C standards comittee decided
signed overflow as being undefined. All I can think of is they were
thinking of target that do saturation for plus/minus but wrapping
Paolo Bonzini [EMAIL PROTECTED] writes:
I think this will only confuse the matter.
I think it's worth mentioning that integer overflow, in theory at
least (we hope not in practice) actually allows that sort of
aggressive optimization. However, I've tried to reword the example to
make it less
Andrew Pinski [EMAIL PROTECTED] writes:
the one thing I have not heard through this
discussion is the real reason why the C standards comittee decided
signed overflow as being undefined.
I wasn't there, but my impression is that many of the optimization
issues we've talked about in this
The C Standard says that if a program has signed integer overflow its
behavior is undefined, and the undefined behavior can even precede the
overflow. To take an extreme example:
@c Inspired by Robert Dewar's example in
@c http://gcc.gnu.org/ml/gcc/2007-01/msg00038.html (2007-01-01).
@example
The C Standard says that if a program has signed integer overflow its
behavior is undefined, and the undefined behavior can even precede the
overflow. To take an extreme example:
@c Inspired by Robert Dewar's example in
@c http://gcc.gnu.org/ml/gcc/2007-01/msg00038.html (2007-01-01).
@example
Andrew Pinski [EMAIL PROTECTED] writes:
|
| [EMAIL PROTECTED] (Richard Kenner) writes:
|
|Many portable C programs assume that signed integer overflow wraps
around
|reliably using two's complement arithmetic.
|
|
| I was looking for an adjective that mean the programs work on
Paul Eggert [EMAIL PROTECTED] writes:
| Here are further patches I checked into the Autoconf documentation to
| reflect today's comments (some of which I received privately). Thanks
| to all of you. The trickiest bit was documenting one simple way to
| reliably detect overflow without
Andrew Pinski writes:
This will always cause a trap on x86, even with -fwrapv so really
-fwrapv has a bug on x86. I will file this bug sometime later
tomorrow. Oh and fixing this bug will actually slow down users
of -fwrapv even more than what it is currently does because
you can no
Andrew Pinski [EMAIL PROTECTED] writes:
the one thing I have not heard through this
discussion is the real reason why the C standards comittee decided
signed overflow as being undefined.
I wasn't there, but my impression is that many of the optimization
issues we've talked about in this
Andrew Pinski [EMAIL PROTECTED] writes:
|
| [EMAIL PROTECTED] (Richard Kenner) writes:
|
|Many portable C programs assume that signed integer overflow wraps
around
|reliably using two's complement arithmetic.
|
|
| I was looking for an adjective that mean the programs work on
Paul Eggert [EMAIL PROTECTED] writes:
| Here are further patches I checked into the Autoconf documentation to
| reflect today's comments (some of which I received privately). Thanks
| to all of you. The trickiest bit was documenting one simple way to
| reliably detect overflow without
Andrew Pinski writes:
This will always cause a trap on x86, even with -fwrapv so really
-fwrapv has a bug on x86. I will file this bug sometime later
tomorrow. Oh and fixing this bug will actually slow down users
of -fwrapv even more than what it is currently does because
you can no
Andrew Pinski [EMAIL PROTECTED] writes:
the one thing I have not heard through this
discussion is the real reason why the C standards comittee decided
signed overflow as being undefined.
I wasn't there, but my impression is that many of the optimization
issues we've talked about in this
A few comments:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I'd replace portable C programs with widely-used C programs. The normal
use of portable means that it conforms to the standard.
Conversely, in at least one
Today I updated the Autoconf manual to contain the following
description of the current situation with signed integer overflow.
This section of the manual is intended to advise programmers what to
do about portable C programs in this area.
I think some discussion along these lines also
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems more appropriate than
widely-used.
Maybe just say what you
[EMAIL PROTECTED] (Richard Kenner) writes:
Many portable C programs assume that signed integer overflow wraps
around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems
Here are further patches I checked into the Autoconf documentation to
reflect today's comments (some of which I received privately). Thanks
to all of you. The trickiest bit was documenting one simple way to
reliably detect overflow without converting to unsigned and back.
(At least, I
Andrew Pinski [EMAIL PROTECTED] writes:
|
| [EMAIL PROTECTED] (Richard Kenner) writes:
|
|Many portable C programs assume that signed integer overflow wraps
around
|reliably using two's complement arithmetic.
|
|
| I was looking for an adjective that mean the programs work on
Paul Eggert [EMAIL PROTECTED] writes:
| Here are further patches I checked into the Autoconf documentation to
| reflect today's comments (some of which I received privately). Thanks
| to all of you. The trickiest bit was documenting one simple way to
| reliably detect overflow without
Andrew Pinski writes:
This will always cause a trap on x86, even with -fwrapv so really
-fwrapv has a bug on x86. I will file this bug sometime later
tomorrow. Oh and fixing this bug will actually slow down users
of -fwrapv even more than what it is currently does because
you can no
A few comments:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I'd replace portable C programs with widely-used C programs. The normal
use of portable means that it conforms to the standard.
Conversely, in at least one
[EMAIL PROTECTED] (Richard Kenner) writes:
A few comments:
Thanks for the quick review.
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I'd replace portable C programs with widely-used C programs. The normal
use of
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems more appropriate than
widely-used.
Maybe just say what you
Richard Kenner wrote:
A few comments:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I'd replace portable C programs with widely-used C programs. The normal
use of portable means that it conforms to the standard.
Andrew Pinski [EMAIL PROTECTED] writes:
Let me make the point that signed overflow has been undefined since
before the C standard was finialized and in fact there is a nice
paper/book called C Traps and Pitfalls[2] which mentions all of this
back in 1988.
C Traps and Pitfalls, like KRv2, is
[EMAIL PROTECTED] (Richard Kenner) writes:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems more appropriate
Here are further patches I checked into the Autoconf documentation to
reflect today's comments (some of which I received privately). Thanks
to all of you. The trickiest bit was documenting one simple way to
reliably detect overflow without converting to unsigned and back.
(At least, I hope it's
[EMAIL PROTECTED] (Richard Kenner) writes:
Many portable C programs assume that signed integer overflow wraps
around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems
Today I updated the Autoconf manual to contain the following
description of the current situation with signed integer overflow.
This section of the manual is intended to advise programmers what to
do about portable C programs in this area.
I think some discussion along these lines also belongs in
[EMAIL PROTECTED] (Richard Kenner) writes:
A few comments:
Thanks for the quick review.
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I'd replace portable C programs with widely-used C programs. The normal
use of
A few comments:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I'd replace portable C programs with widely-used C programs. The normal
use of portable means that it conforms to the standard.
Conversely, in at least one
Today I updated the Autoconf manual to contain the following
description of the current situation with signed integer overflow.
This section of the manual is intended to advise programmers what to
do about portable C programs in this area.
I think some discussion along these lines also
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems more appropriate than
widely-used.
Maybe just say what you
Richard Kenner wrote:
A few comments:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I'd replace portable C programs with widely-used C programs. The normal
use of portable means that it conforms to the standard.
Richard Kenner wrote:
A few comments:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I'd replace portable C programs with widely-used C programs. The normal
use of portable means that it conforms to the standard.
Wrap-around is very useful for digital signal processing.
Saturation is also very useful for DSPs.
This is why for embedded C[1], they are adding types which the
user is able to decide which behavior they want instead of
just being undefined.
Let me make the point that signed overflow has
Andrew Pinski [EMAIL PROTECTED] writes:
Let me make the point that signed overflow has been undefined since
before the C standard was finialized and in fact there is a nice
paper/book called C Traps and Pitfalls[2] which mentions all of this
back in 1988.
C Traps and Pitfalls, like KRv2, is
[EMAIL PROTECTED] (Richard Kenner) writes:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems more appropriate
Here are further patches I checked into the Autoconf documentation to
reflect today's comments (some of which I received privately). Thanks
to all of you. The trickiest bit was documenting one simple way to
reliably detect overflow without converting to unsigned and back.
(At least, I hope it's
[EMAIL PROTECTED] (Richard Kenner) writes:
Many portable C programs assume that signed integer overflow wraps
around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems
Here are further patches I checked into the Autoconf documentation to
reflect today's comments (some of which I received privately). Thanks
to all of you. The trickiest bit was documenting one simple way to
reliably detect overflow without converting to unsigned and back.
(At least, I
Today I updated the Autoconf manual to contain the following
description of the current situation with signed integer overflow.
This section of the manual is intended to advise programmers what to
do about portable C programs in this area.
I think some discussion along these lines also belongs in
[EMAIL PROTECTED] (Richard Kenner) writes:
Many portable C programs assume that signed integer overflow wraps around
reliably using two's complement arithmetic.
I was looking for an adjective that mean the programs work on a wide
variety of platforms, and portable seems more appropriate
Here are further patches I checked into the Autoconf documentation to
reflect today's comments (some of which I received privately). Thanks
to all of you. The trickiest bit was documenting one simple way to
reliably detect overflow without converting to unsigned and back.
(At least, I hope it's
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