Re: Floating point rounding modes: we should restrict them slightly
Rainer Deyke wrote: Don wrote: Ah. The important thing that makes this work is that the global FP state has a clearly defined default. (round-to-nearest, full precision, no floating point exceptions enabled). Part of the implicit contract of calling a 'memoisable pure' function is that the global FP state is in the default state. Then I'm not sure it makes to treat memoisable as a property of a (pure) function. It seems more like a property of the context from which the function is called. *All* pure function are memoisable if you always call them with the floating point rounding mode set to the default. Conversely, all pure functions can be called with an arbitrary rounding mode if you treat them an unmemoisable. That is correct. Some function - those that don't perform any floating point calculation, directly or indirectly - are independent from floating point state, and could even memoised even when called with an arbitrary floating point state. But there is no good way to automatically detect these functions. It sounds as though you never saw my proposal. My proposal was that, by analogy to module(system), specific modules should be marked, for example: module (advancedfloatingpoint, system) std.math; or: module (nondefaultfloat, system) std.math; All pure functions are memoisable except for pure advancedfloatingpoint functions when called from a function (pure or not) which is also in an advancedfloatingpoint module. This is a very tiny hole in the 'purity' rules, and is a tiny addition to the language, but it's enough to cover the practical uses of floating-point rounding and exception flags.
Re: Floating point rounding modes: we should restrict them slightly
Don wrote: Ah. The important thing that makes this work is that the global FP state has a clearly defined default. (round-to-nearest, full precision, no floating point exceptions enabled). Part of the implicit contract of calling a 'memoisable pure' function is that the global FP state is in the default state. Then I'm not sure it makes to treat memoisable as a property of a (pure) function. It seems more like a property of the context from which the function is called. *All* pure function are memoisable if you always call them with the floating point rounding mode set to the default. Conversely, all pure functions can be called with an arbitrary rounding mode if you treat them an unmemoisable. Some function - those that don't perform any floating point calculation, directly or indirectly - are independent from floating point state, and could even memoised even when called with an arbitrary floating point state. But there is no good way to automatically detect these functions. -- Rainer Deyke - rain...@eldwood.com
Re: Floating point rounding modes: we should restrict them slightly
Rainer Deyke wrote:
Don wrote:
Rainer Deyke wrote:
Don't forget that uses floating point is a transitive property. Any
pure function that calls a pure-but-unmemoisable function is itself
pure-but-unmemoisable.
This is incorrect.
Then I must have misunderstood your proposal. Either you did a poor job
of explaining it, or I did a poor job of trying to understand it, or
it's not as simple as you make it out to be.
Since I'm not really interested in discussing the details of floating
point rounding modes and memoisation, I'll just leave it at that.
The description I just gave of it was
The thing you missed is that the non-memoisable pure functions can only
read the global state.
Is there any real need to treat floating point state differently from
other global state in regard to pure?
Yes. Because it's all-pervasive.
It only affects floating point operations. Other pieces of global state
(e.g. the global locale) affect other operations.
But you ALWAYS have a choice about that. You can rewrite functions which
are independent of locale, or you can pass the locale as a parameter.
With floating point, you don't have that option.
BTW, global locales suck, big time. The idea that you can specify the
formatting in the form of a locale is clearly the creation of someone
who had never worked in an international environment. (And any locale
which includes 'currency' is clearly the creation of an idiot).
Once you make a
special case for floating point mode, it's easy to argue that the
special case should be extended to other pieces of global data.
No, because this is a hardware special case. All the other cases can be
dealt with in software.
Or maybe I'm completely overthinking this.
Unless you treat it specially, you
cannot use floating point in any pure function. That creates a
restriction which is so severe that 'pure' becomes useless. Something
like the following template function could not be pure:
T add(T a, T b) { return a + b; }
In the presence of structs and class executing arbitrary code in opPlus,
you can't make this pure anyway. Unless you force opPlus itself to be
pure, which is almost certainly too restrictive for the current
definition of pure.
You're right. Bad example. But seriously, people give sqrt(x) as the
classic example of a pure function.
Re: Floating point rounding modes: we should restrict them slightly
Walter Bright wrote: Brad Roberts wrote: Walter Bright wrote: strlen() is safe, while strcpy() and printf() are not. You sure? Does running beyond the bounds of the array if there's no null termination count as safe some how? :) Yes. Memory safety is defined as being free of memory corruption errors. Simply reading memory out of bounds does not corrupt memory. It does result in undefined behaviour, though. I don't see much difference. (Corrupting memory is a problem only because you read it again afterwards...) Note that it is ok for a memory safe program to generate a seg fault. It'd be OK if it was guaranteed to generate a seg fault. But I don't think that's true here. printf() is not memory safe because of the %n format.
Re: Floating point rounding modes: we should restrict them slightly
Don wrote:
The thing you missed is that the non-memoisable pure functions can only
read the global state.
No, I saw that. But if a (pure) function calls another (pure) function
that depends on global state, then the first (pure) function also
depends on global state.
int global_state;
pure int f() {
return global_state;
}
pure int g() {
return f();
}
int h() {
global_state = 5;
return g();
}
But you ALWAYS have a choice about that. You can rewrite functions which
are independent of locale, or you can pass the locale as a parameter.
With floating point, you don't have that option.
True. Unless you add functions/operators that use a fixed rounding mode
to the language.
BTW, global locales suck, big time. The idea that you can specify the
formatting in the form of a locale is clearly the creation of someone
who had never worked in an international environment. (And any locale
which includes 'currency' is clearly the creation of an idiot).
No argument there.
--
Rainer Deyke - rain...@eldwood.com
Re: Floating point rounding modes: we should restrict them slightly
Stewart Gordon wrote: Don wrote: snip Floating point settings are just another case of the same thing, except that currently violations in relation to the former are allowed. There's a fundamental difference between them: the floating point settings are a hardware feature and it is IMPOSSIBLE to avoid them. Actually, you _can_ avoid changing the floating point settings in the course of an average program. But still The hardware dependency is still there, in every function which uses floating point. And the compiler must assume that dependency. You can choose not to use locale settings. Or, you can pass them as a parameter, which doesn't work for floating point settings. As long as it's the built-in arithmetic operators you're concerned about. Of course. There's nothing else to floating point, other than the built-in operators. And this is the problem with unconstrained rounding modes: they influence EVERYTHING. But that said, wrapping every arithmetic operation in a function to purify it of dependence on floating point settings wouldn't be ideal. It's worse than that. Every function which might call a function which might use floating point would have to pass the parameter. It doesn't work. Please do not get sidetracked on pure functions, it is orthogonal to this issue. I'm not sure about this. ISTM arithmetic operators are essentially pure functions, and so the compiler may treat them as such. No, they always depend on the FP settings, and assuming they are pure functions is not permitted in C, C++, or D. C allows the rounding mode to change at any time. This proposal: Rounding mode changes must only affect callees, not callers. The pure function memoisation issue: Certain functions may be marked as 'pure', even though they use floating point (and are therefore dependent on the FP state). The compiler must be able to prevent memoisation of those functions in cases where the rounding mode may have changed. (Or, equivalently, the memoisation must include the FP state). I've provided very simple solutions for both of these. I'm getting pretty irritated that people are acting as though these are difficult problems and trying to come up with convoluted solutions. Impose minimal semantics and it becomes trivial.
Re: Floating point rounding modes: we should restrict them slightly
Walter Bright wrote: bearophile wrote: An important purpose of a not bug-prone language is remove as many undefined situations as possible. That's right. If you add that to D2 specs, can the compiler catch at compile time all cases of violations to that rule? Unlikely. But that has to be weighed against the former behavior being defined in a useless way. We could get close, I think. We could catch all violations in SafeD. Disallow calling core.stdc.fenv functions from inside SafeD modules, and provide an RAII object in std.math for changing the rounding mode. Actually there's probably a few other C functions we want to prohibit in SafeD -- the infamous string functions, for example. I think the goal should be try to remove undefined behaviour from SafeD. Long term goal: If a program uses no system modules other than those provided in the standard library, it should not be exposed to undefined behaviour. ? D has something for the actions that must be done before the function exits, the scope(exit). Can something like that be used to automatically avoid undefined rounding situations? Using an RAII object to do it might be better. I agree. I'd like to do something similar with the floating point exception state, too. Haven't worked out the details yet.
Re: Floating point rounding modes: we should restrict them slightly
Don wrote: The pure function memoisation issue: Certain functions may be marked as 'pure', even though they use floating point (and are therefore dependent on the FP state). The compiler must be able to prevent memoisation of those functions in cases where the rounding mode may have changed. (Or, equivalently, the memoisation must include the FP state). Don't forget that uses floating point is a transitive property. Any pure function that calls a pure-but-unmemoisable function is itself pure-but-unmemoisable. The pure annotation is now used for two related but different categories of functions with different properties, and the compiler needs some way to keep track of which is which across module boundaries. If the compiler can do that, then it can use the same mechanism to differentiate between pure and non-pure functions, and the pure annotation becomes redundant. Is there any real need to treat floating point state differently from other global state in regard to pure? -- Rainer Deyke - rain...@eldwood.com
Re: Floating point rounding modes: we should restrict them slightly
Don wrote: Walter Bright wrote: bearophile wrote: An important purpose of a not bug-prone language is remove as many undefined situations as possible. That's right. If you add that to D2 specs, can the compiler catch at compile time all cases of violations to that rule? Unlikely. But that has to be weighed against the former behavior being defined in a useless way. We could get close, I think. We could catch all violations in SafeD. Disallow calling core.stdc.fenv functions from inside SafeD modules, and provide an RAII object in std.math for changing the rounding mode. Actually there's probably a few other C functions we want to prohibit in SafeD -- the infamous string functions, for example. Yes, all the C functions will have to be gone through and separated. strlen() is safe, while strcpy() and printf() are not. I think the goal should be try to remove undefined behaviour from SafeD. Long term goal: If a program uses no system modules other than those provided in the standard library, it should not be exposed to undefined behaviour. ? Right!
Re: Floating point rounding modes: we should restrict them slightly
Rainer Deyke wrote:
Don wrote:
The pure function memoisation issue:
Certain functions may be marked as 'pure', even though they use floating
point (and are therefore dependent on the FP state). The compiler must
be able to prevent memoisation of those functions in cases where the
rounding mode may have changed. (Or, equivalently, the memoisation must
include the FP state).
Don't forget that uses floating point is a transitive property. Any
pure function that calls a pure-but-unmemoisable function is itself
pure-but-unmemoisable.
This is incorrect. You've got it backwards. It's not uses floating
point that's the issue, it's may use non-default floating point. A
memoisable floating point function can only use default floating point.
So, if it calls a non-memoisable pure function, it'll be using it in the
default state (even though that function can accept non-default state).
The transitivity propagates in the reverse direction: memoisability is
transitive, non-memoisability is not. Non-memoisable functions cannot
call memoisable ones without first ensuring that everything is back into
the default state.
The pure annotation is now used for two
related but different categories of functions with different properties,
and the compiler needs some way to keep track of which is which across
module boundaries. If the compiler can do that, then it can use the
same mechanism to differentiate between pure and non-pure functions, and
the pure annotation becomes redundant.
It's true that you could provide a similar loophole for any specific
global variable. But you'd need a *very* good reason.
Is there any real need to treat floating point state differently from
other global state in regard to pure?
Yes. Because it's all-pervasive. Unless you treat it specially, you
cannot use floating point in any pure function. That creates a
restriction which is so severe that 'pure' becomes useless. Something
like the following template function could not be pure:
T add(T a, T b) { return a + b; }
I do not believe that any other global state has an influence which is
in the same league.
Re: Floating point rounding modes: we should restrict them slightly
Walter Bright: Yes, all the C functions will have to be gone through and separated. strlen() is safe, while strcpy() and printf() are not. For some of the removed functions better C-like ones can be provided: http://www.ddj.com/cpp/214502214 Bye, bearophile
Re: Floating point rounding modes: we should restrict them slightly
bearophile wrote: Walter Bright: Yes, all the C functions will have to be gone through and separated. strlen() is safe, while strcpy() and printf() are not. For some of the removed functions better C-like ones can be provided: http://www.ddj.com/cpp/214502214 Bye, bearophile Just use D instead.
Re: Floating point rounding modes: we should restrict them slightly
Don Wrote: I've provided very simple solutions for both of these. I'm getting pretty irritated that people are acting as though these are difficult problems and trying to come up with convoluted solutions. Impose minimal semantics and it becomes trivial. FWIW, I've liked your proposals, but I don't feel qualified to comment. I never adjust rounding modes. I do like how I can be ignorant and get code that is better optimized. You've shown deep knowledge of floating point operation in the past, and I'm inclined to accept reasonable-sounding proposals from those that know way more than me.
Re: Floating point rounding modes: we should restrict them slightly
On Mon, 14 Sep 2009 16:44:48 +0400, Jason House jason.james.ho...@gmail.com wrote: Don Wrote: I've provided very simple solutions for both of these. I'm getting pretty irritated that people are acting as though these are difficult problems and trying to come up with convoluted solutions. Impose minimal semantics and it becomes trivial. FWIW, I've liked your proposals, but I don't feel qualified to comment. I never adjust rounding modes. I do like how I can be ignorant and get code that is better optimized. You've shown deep knowledge of floating point operation in the past, and I'm inclined to accept reasonable-sounding proposals from those that know way more than me. I second that. Nothing to add.
Re: Floating point rounding modes: we should restrict them slightly
Walter Bright wrote: Don wrote: We could get close, I think. We could catch all violations in SafeD. Disallow calling core.stdc.fenv functions from inside SafeD modules, and provide an RAII object in std.math for changing the rounding mode. Actually there's probably a few other C functions we want to prohibit in SafeD -- the infamous string functions, for example. Yes, all the C functions will have to be gone through and separated. strlen() is safe, while strcpy() and printf() are not. You sure? Does running beyond the bounds of the array if there's no null termination count as safe some how? :)
Re: Floating point rounding modes: we should restrict them slightly
Brad Roberts wrote: Walter Bright wrote: strlen() is safe, while strcpy() and printf() are not. You sure? Does running beyond the bounds of the array if there's no null termination count as safe some how? :) Yes. Memory safety is defined as being free of memory corruption errors. Simply reading memory out of bounds does not corrupt memory. Note that it is ok for a memory safe program to generate a seg fault. printf() is not memory safe because of the %n format.
Re: Floating point rounding modes: we should restrict them slightly
Don wrote:
Rainer Deyke wrote:
Don't forget that uses floating point is a transitive property. Any
pure function that calls a pure-but-unmemoisable function is itself
pure-but-unmemoisable.
This is incorrect.
Then I must have misunderstood your proposal. Either you did a poor job
of explaining it, or I did a poor job of trying to understand it, or
it's not as simple as you make it out to be.
Since I'm not really interested in discussing the details of floating
point rounding modes and memoisation, I'll just leave it at that.
Is there any real need to treat floating point state differently from
other global state in regard to pure?
Yes. Because it's all-pervasive.
It only affects floating point operations. Other pieces of global state
(e.g. the global locale) affect other operations. Once you make a
special case for floating point mode, it's easy to argue that the
special case should be extended to other pieces of global data. A
general solution generally beats special cases.
Or maybe I'm completely overthinking this.
Unless you treat it specially, you
cannot use floating point in any pure function. That creates a
restriction which is so severe that 'pure' becomes useless. Something
like the following template function could not be pure:
T add(T a, T b) { return a + b; }
In the presence of structs and class executing arbitrary code in opPlus,
you can't make this pure anyway. Unless you force opPlus itself to be
pure, which is almost certainly too restrictive for the current
definition of pure.
--
Rainer Deyke - rain...@eldwood.com
Re: Floating point rounding modes: we should restrict them slightly
Walter Bright wrote:
Don wrote:
PROPOSAL:
Change the spec by adding the line to float.html:
If the floating-point rounding mode is changed within a function, it
must be restored before the function exits. If this rule is violated
(for example, by the use of inline asm), the rounding mode used for
subsequent calculations is undefined.
But that doesn't allow for changing the rounding mode at a global level,
then rerunning one's computation to determine how rounding affects their
final result.
It doesn't prevent that at all. You just need to change the rounding
mode before running your computation (at the start of main(), for
example), and then reset it after performing the computation. What it
does do is prevent different terms within a single expression from
interacting which other.
eg
double foo() {
return x() + y();
}
x() and y() can use whichever rounding modes they like, and if they are
impure, they can affect one another, but under this proposal, they
cannot change the meaning of the addition inside foo().
Re: Floating point rounding modes: we should restrict them slightly
On 2009-09-13 06:14:02 -0400, Don nos...@nospam.com said:
double foo() {
return x() + y();
}
x() and y() can use whichever rounding modes they like, and if they are
impure, they can affect one another, but under this proposal, they
cannot change the meaning of the addition inside foo().
Problems still may occur if x and y do memoization however. You call
x() with one rounding mode, it memoize the results, then call x() with
another rounding mode and it reuses the previous results while it
shouldn't.
Basicaly, any memoization of x() should be discarded when you change
the rounding mode. For instance the compiler should not reuse the value
of x(), even if x is pure, in the following code:
auto a = x();
setRoundingMode(...);
auto b = x();
Basically, setRoundingMode should act as some sort of barrier for the
optimizer, but how?
--
Michel Fortin
michel.for...@michelf.com
http://michelf.com/
Re: Floating point rounding modes: we should restrict them slightly
Michel Fortin wrote:
On 2009-09-13 06:14:02 -0400, Don nos...@nospam.com said:
double foo() {
return x() + y();
}
x() and y() can use whichever rounding modes they like, and if they
are impure, they can affect one another, but under this proposal, they
cannot change the meaning of the addition inside foo().
Problems still may occur if x and y do memoization however.
Yes, but this is an orthogonal issue, which I have discussed previously.
Basicaly, any memoization of x() should be discarded when you change the
rounding mode.
Basically, setRoundingMode should act as some sort of barrier for the
optimizer, but how?
It's not a big deal, because rounding mode is changed so rarely, so you
don't need to exactly catch all the cases where memoisation can safely
be used; you only need to disallow all of the ones where it is unsafe.
I proposed two options: (1) by analogy to SafeD, allow rounding mode and
float exception flags to be respected only in specially marked modules
('module(advancedfloatingpoint)' or similar). Memoisation should be
disabled in such modules. (As a further optimisation, the compiler can
re-enable memoisation when such functions are called from
non-advancedfloatingpoint modules).
(2) provide a runtime call to disable memoisation. You'd be obliged to
call this every time you changed the rounding mode or exception status
from the default.
I favour (1) because it provides huge optimisation potential for modules
with default floating point.
The point of all my proposals on this topic, is that these features
cause a lot of trouble if they're completely unrestrained in the way
that C/C++ does it, but the semantics required to completely domesticate
them are not burdensome, and can be done without loss of useful
functionality.
Re: Floating point rounding modes: we should restrict them slightly
Walter Bright wrote: Don wrote: PROPOSAL: Change the spec by adding the line to float.html: If the floating-point rounding mode is changed within a function, it must be restored before the function exits. If this rule is violated (for example, by the use of inline asm), the rounding mode used for subsequent calculations is undefined. But that doesn't allow for changing the rounding mode at a global level, then rerunning one's computation to determine how rounding affects their final result. Ok, I understand now. This makes sense.
Re: Floating point rounding modes: we should restrict them slightly
Don wrote: PROPOSAL: Change the spec by adding the line to float.html: If the floating-point rounding mode is changed within a function, it must be restored before the function exits. If this rule is violated (for example, by the use of inline asm), the rounding mode used for subsequent calculations is undefined. I added it into float.html for D2.
Re: Floating point rounding modes: we should restrict them slightly
Don wrote: snip Floating point settings are just another case of the same thing, except that currently violations in relation to the former are allowed. There's a fundamental difference between them: the floating point settings are a hardware feature and it is IMPOSSIBLE to avoid them. Actually, you _can_ avoid changing the floating point settings in the course of an average program. But still You can choose not to use locale settings. Or, you can pass them as a parameter, which doesn't work for floating point settings. As long as it's the built-in arithmetic operators you're concerned about. But that said, wrapping every arithmetic operation in a function to purify it of dependence on floating point settings wouldn't be ideal. Please do not get sidetracked on pure functions, it is orthogonal to this issue. I'm not sure about this. ISTM arithmetic operators are essentially pure functions, and so the compiler may treat them as such. Stewart.
Re: Floating point rounding modes: we should restrict them slightly
Walter Bright: Don: PROPOSAL: Change the spec by adding the line to float.html: If the floating-point rounding mode is changed within a function, it must be restored before the function exits. If this rule is violated (for example, by the use of inline asm), the rounding mode used for subsequent calculations is undefined. I added it into float.html for D2. An important purpose of a not bug-prone language is remove as many undefined situations as possible. If you add that to D2 specs, can the compiler catch at compile time all cases of violations to that rule? D has something for the actions that must be done before the function exits, the scope(exit). Can something like that be used to automatically avoid undefined rounding situations? Bye, bearophile
Re: Floating point rounding modes: we should restrict them slightly
bearophile wrote: An important purpose of a not bug-prone language is remove as many undefined situations as possible. That's right. If you add that to D2 specs, can the compiler catch at compile time all cases of violations to that rule? Unlikely. But that has to be weighed against the former behavior being defined in a useless way. D has something for the actions that must be done before the function exits, the scope(exit). Can something like that be used to automatically avoid undefined rounding situations? Using an RAII object to do it might be better.
Re: Floating point rounding modes: we should restrict them slightly
Don wrote: snip LISTING ONE: real x = foo(1) + foo(2) + foo(3); LISTING TWO: real x1 = foo(1); real x2 = foo(2); real x3 = foo(3); real x = x1 + x2 + x3; In C and C++ (and currently in D), they are NOT equivalent! foo() is allowed to change the floating-point rounding mode, so it could change the meaning of '+'. snip This is just one aspect of it. You could also call a pure function, change the rounding mode, and then call the pure function again on the same arguments. The value returned would change, thereby violating the purity. There has been some talk about pure functions in relation to locales: http://d.puremagic.com/issues/show_bug.cgi?id=3057 http://www.digitalmars.com/d/archives/digitalmars/D/std.locale_85081.html Floating point settings are just another case of the same thing, except that currently violations in relation to the former are allowed. Maybe the right solution is some kind of hidden parameter mechanism Stewart.
Re: Floating point rounding modes: we should restrict them slightly
Don wrote: PROPOSAL: Change the spec by adding the line to float.html: If the floating-point rounding mode is changed within a function, it must be restored before the function exits. If this rule is violated (for example, by the use of inline asm), the rounding mode used for subsequent calculations is undefined. But that doesn't allow for changing the rounding mode at a global level, then rerunning one's computation to determine how rounding affects their final result.
Floating point rounding modes: we should restrict them slightly
A quiz:
Conside a function
real foo() {}
This is not pure, so that it may side-effects, but even so, are listings
one and two equivalent?
LISTING ONE:
real x = foo(1) + foo(2) + foo(3);
LISTING TWO:
real x1 = foo(1);
real x2 = foo(2);
real x3 = foo(3);
real x = x1 + x2 + x3;
In C and C++ (and currently in D), they are NOT equivalent!
foo() is allowed to change the floating-point rounding mode, so it could
change the meaning of '+'.
This is quite ridiculous. This is a 'freedom' that comes with a heavy
price tag.
Actually doing this would make code nearly impossible to debug, and I
don't think anyone is stupid enough to actually do this
...but as long as it's theoretically possible, it prevents the compiler
from doing useful optimisations.
Disallowing this kind of obscure, dangerous nonsense is, I think, part
of the core mission of D.
PROPOSAL:
Change the spec by adding the line to float.html:
If the floating-point rounding mode is changed within a function, it
must be restored before the function exits. If this rule is violated
(for example, by the use of inline asm), the rounding mode used for
subsequent calculations is undefined.
This slight tightening of semantics can have a significant benefit for
all floating-point code.
Re: Floating point rounding modes: we should restrict them slightly
Rainer Deyke wrote: Don wrote: Change the spec by adding the line to float.html: If the floating-point rounding mode is changed within a function, it must be restored before the function exits. Even if that function is called 'fesetround'? Obviously not. In fact, I'd hope to even disallow fesetround() since it's too easy to abuse, and instead have an RAII struct to restore the mode automatically. But that's just syntax sugar.
Re: Floating point rounding modes: we should restrict them slightly
#ponce wrote: Why not imposing a rounding mode (ex : toward minus infinity) ? And then disallowing to change it in assembly. You can't impose a rounding mode in a system language: rounding mode is a crucial feature in a few rare but very important cases. The trick is to allow those cases without polluting everything else. This tiny proposal gets rid of most the damage. However, my previous proposal for fixing the interaction of 'pure' with rounding modes and exception handling, would allow the rounding mode to be round-to-nearest almost everywhere, and changeable only in impure functions inside modules marked module(advancedfloatingpoint). This could give performance benefits everywhere outside those modules.
Re: Floating point rounding modes: we should restrict them slightly
#ponce alil...@gmail.com wrote in message news:h8adr2$23p...@digitalmars.com... Why not imposing a rounding mode (ex : toward minus infinity) ? And then disallowing to change it in assembly. I mean, you can still do : round(x) as floor(x + 0.5L) ceil(x) as floor(x + 1.0L) Iit would cost some precision with large floating-point number, but not much since rounding such large numbers has few meaning. It would clear up the rounding-mode mess a bit. There's more to rounding modes than just converting / rounding to integer. Rounding affects all arithmetic operations, and whether an expresion evaluates to 0.97999 or .97998 can be important in some cases.
Re: Floating point rounding modes: we should restrict them slightly
On 2009-09-10 04:24:38 -0400, Don nos...@nospam.com said:
PROPOSAL:
Change the spec by adding the line to float.html:
If the floating-point rounding mode is changed within a function, it
must be restored before the function exits. If this rule is violated
(for example, by the use of inline asm), the rounding mode used for
subsequent calculations is undefined.
This slight tightening of semantics can have a significant benefit for
all floating-point code.
I perfectly agree. I'm just wondering how to enforce it in the language.
Later in the thread you say:
... have an RAII struct to restore the mode automatically. But that's
just syntax sugar.
But how do you prevent someone from writing:
auto s = new roundingMode(...);
The only way you can really make sure it's scoped to a specific
function is to call it from another function:
R performWithRoundingMode(R)(flag roundingMode, lazy R result)
{
flag oldRoundingMode = getRoundingMode();
setRoundingMode(roundingMode);
try
return result;
finally
setRoundingMode(oldRoundingMode);
}
auto i = performWithRoundingMode(TOWARDS_ZERO, 12 * 12);
Here you can't escape the scoping requirements.
--
Michel Fortin
michel.for...@michelf.com
http://michelf.com/
Re: Floating point rounding modes: we should restrict them slightly
Michel Fortin wrote:
On 2009-09-10 04:24:38 -0400, Don nos...@nospam.com said:
PROPOSAL:
Change the spec by adding the line to float.html:
If the floating-point rounding mode is changed within a function, it
must be restored before the function exits. If this rule is violated
(for example, by the use of inline asm), the rounding mode used for
subsequent calculations is undefined.
This slight tightening of semantics can have a significant benefit for
all floating-point code.
I perfectly agree. I'm just wondering how to enforce it in the language.
Yes, it's not so obvious how to enforce it. However, in reality it's a
rarely-used, system level feature, so relying on convention probably
isn't too terrible in the short term.
Later in the thread you say:
... have an RAII struct to restore the mode automatically. But that's
just syntax sugar.
But how do you prevent someone from writing:
auto s = new roundingMode(...);
scope _should_ take care of that. I'm not sure that it does. Still, we
just need to make it easy to do the right thing, and difficult to do the
wrong thing.
The only way you can really make sure it's scoped to a specific function
is to call it from another function:
R performWithRoundingMode(R)(flag roundingMode, lazy R result)
{
flag oldRoundingMode = getRoundingMode();
setRoundingMode(roundingMode);
try
return result;
finally
setRoundingMode(oldRoundingMode);
}
auto i = performWithRoundingMode(TOWARDS_ZERO, 12 * 12);
Here you can't escape the scoping requirements.
Hmmm. Not ideal, but could be worse.
But anyway, the important thing is to codify in the spec that the
floating point state must not change randomly.
For discussing syntax, I think we really need an interval arithmetic
module, so that we can ensure it all works well.
Re: Floating point rounding modes: we should restrict them slightly
#ponce wrote: Why not imposing a rounding mode (ex : toward minus infinity) ? And then disallowing to change it in assembly. I mean, you can still do : round(x) as floor(x + 0.5L) ceil(x) as floor(x + 1.0L) Iit would cost some precision with large floating-point number, but not much since rounding such large numbers has few meaning. It would clear up the rounding-mode mess a bit. Shouldn't ceil(1.0) return 1.0, and wouldn't floor(1.0 + 1.0L) return 2.0? L.
