On Saturday, 27 July 2013 at 08:35:36 UTC, deadalnix wrote:
> On Saturday, 27 July 2013 at 08:08:01 UTC, monarch_dodra wrote:
>> Thank you for the answer. I expected D to do strict aliasing for the reasons you mentioned. This does come up with two follow up question though:
>>
>> 1. Does strict aliasing apply to slices?
>> 2. C++ uses 'char' as a 'neutral' type that can alias to anything. What about D? Does char fill that role? Does ubyte?
>
> We have void* and void[], I think they should have that role.

You can't read or write to void though (can you?), which is one of the main points in doing an un-strict alias: Raw binary storage.

On 7/27/2013 1:57 AM, David Nadlinger wrote:
> On Saturday, 27 July 2013 at 06:58:04 UTC, Walter Bright wrote:
>> Although it isn't in the spec, D should be "strict aliasing". This is because:
>>
>> 1. it enables better code generation
>>
>> 2. there are ways, such as unions, to get the other aliasing that doesn't
>> break strict aliasing
>
> We need to carefully formalize this then, and quickly. The problem GCC, Clang
> and others are facing is that (as you are probably aware) 2. isn't guaranteed to
> work for type-casting pointers either by the specs, but people want to be able
> to do this nonetheless.
>
> Thus, they both accept pointer aliasing through union types, trying to optimize
> as much as possible while avoiding to break people's expectations and existing
> code. This is a very unfortunate situation for both compiler developers and
> users; just search for something like "gcc strict aliasing" on StackOverflow for
> examples.
>
> There is already quite a lot of D code out there that violates the C-style
> strict aliasing rules.

I agree. Want to do an enhancement request on bugzilla for it?

On Saturday, 27 July 2013 at 08:58:22 UTC, Walter Bright wrote:
> On 7/27/2013 1:08 AM, monarch_dodra wrote:
>> 1. Does strict aliasing apply to slices?
>
> I don't know what you mean.

double d;
uint* p = cast(int*)&d; //unsafe aliasing

vs

double[] d = new double[](1);
uint[] p = cast(uint[])d; //unsafe aliasing ?

On Saturday, 27 July 2013 at 06:58:04 UTC, Walter Bright wrote:
> 2. there are ways, such as unions, to get the other aliasing that doesn't break strict aliasing

It would be great to have something like GCC's solution: warn when pointer casts may violate the strict aliasing rule, and provide a flag to disable it.

On Saturday, 27 July 2013 at 09:05:32 UTC, ponce wrote:
> It would be great to have something like GCC's solution: warn when pointer casts may violate the strict aliasing rule, and provide a flag to disable it.

BTW, C++ compilers usually have an effective way to disambiguate pointer aliasing so that loop code generation is better.

restrict may helps, strict aliasing may helps, but in my experience a direct annotation "I know what I'm doing, this loop does not alias" or "no loop dependency" is often more effective. This obviously require to trust the optimizing programmer a bit :)

Walter Bright:

> Although it isn't in the spec, D should be "strict aliasing". This is because:
>
> 1. it enables better code generation
>
> 2. there are ways, such as unions, to get the other aliasing that doesn't break strict aliasing

Is it good to add to Phobos a small template (named like "PointerCast" or something similar) that uses a union internally to perform pointer type conversions?

Is then the compiler going to warn the programmer when the pointer type aliasing rule is violated? I mean when the D code uses cast() between different pointer types (beside constness). An alternative design is to even deprecate (and later turn those into errors, where the error message suggests to use PointerCast).


Bye,
bearophile

On Saturday, 27 July 2013 at 09:03:57 UTC, monarch_dodra wrote:
> On Saturday, 27 July 2013 at 08:58:22 UTC, Walter Bright wrote:
>> On 7/27/2013 1:08 AM, monarch_dodra wrote:
>>> 1. Does strict aliasing apply to slices?
>>
>> I don't know what you mean.
>
> double d;
> uint* p = cast(int*)&d; //unsafe aliasing
>
> vs
>
> double[] d = new double[](1);
> uint[] p = cast(uint[])d; //unsafe aliasing ?

That is the same thing at the end.

27.07.2013 12:59, Walter Bright пишет:
> On 7/27/2013 1:57 AM, David Nadlinger wrote:
>> On Saturday, 27 July 2013 at 06:58:04 UTC, Walter Bright wrote:
>>> Although it isn't in the spec, D should be "strict aliasing". This is
>>> because:
>>>
>>> 1. it enables better code generation
>>>
>>> 2. there are ways, such as unions, to get the other aliasing that
>>> doesn't
>>> break strict aliasing
>>
>> We need to carefully formalize this then, and quickly. The problem
>> GCC, Clang
>> and others are facing is that (as you are probably aware) 2. isn't
>> guaranteed to
>> work for type-casting pointers either by the specs, but people want to
>> be able
>> to do this nonetheless.
>>
>> Thus, they both accept pointer aliasing through union types, trying to
>> optimize
>> as much as possible while avoiding to break people's expectations and
>> existing
>> code. This is a very unfortunate situation for both compiler
>> developers and
>> users; just search for something like "gcc strict aliasing" on
>> StackOverflow for
>> examples.
>>
>> There is already quite a lot of D code out there that violates the
>> C-style
>> strict aliasing rules.
>
> I agree. Want to do an enhancement request on bugzilla for it?
>

So is enhancement request filed?

-- 
Денис В. Шеломовский
Denis V. Shelomovskij

On Monday, 29 July 2013 at 05:05:54 UTC, Denis Shelomovskij wrote:
> So is enhancement request filed?

Now it is: http://d.puremagic.com/issues/show_bug.cgi?id=10750

Sorry, I'm not following the lists closely right now due to university work.

David

On Saturday, 3 August 2013 at 11:39:44 UTC, David Nadlinger wrote:
> On Monday, 29 July 2013 at 05:05:54 UTC, Denis Shelomovskij wrote:
>> So is enhancement request filed?
>
> Now it is: http://d.puremagic.com/issues/show_bug.cgi?id=10750
>
> Sorry, I'm not following the lists closely right now due to university work.
>
> David

C++ (and C, afaik), solved the problem by saying "char" can alias anything. I'm not sure that's a good idea to do the same in D, since a char actually represents something very specific, and comes with bagage.

We could have (u)byte do this, but at the same time, I see now reason why *they* should have to pay for lax aliasing.

A "simple" solutions I see:

Introduce the "raw" basic data type. It's basically a ubyte for all intents and purposes, but can alias to anything. I think this is a good idea, as the useage of the ward "raw" is immediately very explicit and self documenting about what is going on:

auto rawSlice = (cast(raw*)(&arbitraryData))[0 .. arbitraryData.sizeof];

The *cost* here, of course, is the introduction of a new *type*. The cost is very high, but at the same type, it deals with the problem (I believe), in the most elegant fashion possible.

Thoughts?