On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:
> On 19 April 2014 14:33, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>> On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:
>>> On 19 April 2014 13:02, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>>> On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:
>>>>> On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:
>>>>>> I'm currently testing out a GCC optimisation that allows you to set call argument flags.  The current assumptions being:
>>>>>>
>>>>>> in parameters  =>  Assume no escape, no clobber (read-only). ref parameters, classes and pointers  =>  Assume worst case. default  =>  Assume no escape.
>>>>>
>>>>> That should read:
>>>>>
>>>>> ref parameters, inout parameters, classes and pointers.
>>>>>
>>>>> The default of assuming no escape is an experiment - I may limit this to only scalar types, and parameters marked as 'scope'  (So long as no one plans on deprecating it soon :)
>>>>
>>>> What does "assume no escape" actually mean?
>>>> [The above list doesn't really make sense. W/o context, it's
>>>> hard to even tell why, hence the question.]
>>>>
>>>
>>> Actually, I might change the default to assume worst case.  I've just tried this out, which is still valid.
>>>
>>> class C {
>>>    int * p;
>>>    this(int x) {
>>>      p = &x; // escapes the address of the parameter.
>>>    }
>>> }
>>
>> This might be currently accepted, but it is clearly invalid
>> (escapes local; the only way to make it work safely would
>> be to silently copy 'x' to the GC-managed heap, which would be
>> way too costly).
>>
>>
>>    A f(A a) { g(&a); return a; } // likewise with ref instead of pointer.
>>
>> This is OK (even if ideally 'g' should be forbidden from escaping 'a').
>>
>> Similarly:
>>
>>    A f(A a) {
>>       auto o = register(&a); // can modify 'a'
>>       o.blah();              // ditto
>>       doneWith(o);           // ditto
>>       return a;
>>    }
>>
>>
>> What I was wondering was things like whether that "assume no escape" property was transitive; if /locally/ escaping was disallowed, and to what extent. What does "assume no escape" mean at all? In your examples you're mentioning refs together with pointers, that would only make sense if no-escape were transitive -- but then treating all args as no-escape would be very wrong.
>>
>>
>>> Worse, scope doesn't error on the general case either.
>>>
>>> class D {
>>>    int * p;
>>>    this(scope int x) {
>>>      p = &x; // escapes the address of the scope parameter.
>>>    }
>>> }
>>
>> D's "scope" isn't enforced in any way right now, which means that code could exist that is invalid, but currently works. It would break silently(!) when compiled with a decent compiler, which still doesn't enforce scope.
>>
> 
> People should get bug fixing soon then.  =)

Until some kind of diagnostics appear, most of those bugs won't even be found. It's too easy to write "auto f (in A a)" and then forget about the implicit 'scope' when modifying the function body.


>>> Do these examples give you a good example?
>>
>> I'm worried about a) invalid assumptions making it into GDC;
>> b) certain valid assumptions making into GDC. The latter because
>> it could mean that code that's incorrect, but still accepted by
>> the other compilers could silently break when compiled with GDC.
>>
> 
> Invalid assumptions rarely make it into GDC.  The testsuite is a good

AFAICT what you're proposing *is* invalid. I can't be sure because
it's not clear what that "no-escape" property means; that's why I
asked about it twice already...
Clearly, escaping objects reachable indirectly via function arguments
is perfectly fine (eg string slicing), yet you wanted to treat args as
no-escape by default.

Also, treating /some/ types specially wouldn't be ideal;

   struct A { int a; /* no pointers or classes */ }
   struct B { int* b; /*...*/ }

   f(A); // should be treated similarly to 'f(int)'
   f(B); // should be treated similarly to 'f(int*)'

Yes, not doing it is "just" a missed optimization, but in practice it means that wrapping types becomes even more expensive in D (it's already almost prohibitively so - eg returning small one-element structs from functions needlessly happens by ref).

> bench for this, as well as several projects (now I've got dub set-up)
> to test it in the wild.

These problems will result in invalid optimizations, so can be hard to trigger and may come and go away randomly.


> Saying that, we have had to revert some optimisation cases as D's schizophrenic nature of enforcing attributes and behaviours is becoming increasingly dismal.
> 
> eg:
> - nothrow has *no* guarantee, period, because it still allows
> unrecoverable errors being thrown, and allows people to catch said
> unrecoverable errors.
> - pure is a tough nut to crack also.  The theory should allow you to
> be able to cache return values, but in practise...

D's "pure" doesn't have much in common with the "normal" pure concept;
exposing gcc's pure via an attribute and completely ignoring D's
version is probably the only practical solution, anything else would
be too costly, result in too small gains, and be too hard to get right.

> - The nature of debug statements breaks guarantees of both nothrow and
> pure, possibly many more.
> - Defining reliable strict aliasing rules, it turns out, is not that
> simple (this is something that Walter has mentioned about D should
> have good guarantees for, ie: D arrays).

Short term, disabling strict aliasing is the only option. I was scared
of the impact it had before you even started to add support for it a
few years ago (the codegen was already different w/ -fno-strict-aliasing
before, which meant that I immediately had to disable it everywhere...)
There probably does not exist a single D program that respects strict
aliasing rules, other than by chance. The perf gains are minimal globally,
but the potential for silent data corruption is huge.

> I'm just in investigating all avenues, as I usually do.  There is no reason why 'in' shouldn't have more powerful guarantees IMO, and what

Of course "scope" (which is part of "in) should be taken advantage of. I'm concerned about args not marked as "scope" being treated wrongly.

[The scope-related bugs in D programs is a /language/ or frontend problem,
 not a reason to avoid doing the right thing in GDC. It's just that in
 practice what can happen is that GDC will be seen as miscompiling
 "working" D programs...]

artur

On 19 April 2014 17:10, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
> On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:
>> On 19 April 2014 14:33, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>> On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:
>>>> On 19 April 2014 13:02, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>>>> On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:
>>>>>> On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:
>>>>>>> I'm currently testing out a GCC optimisation that allows you to set call argument flags.  The current assumptions being:
>>>>>>>
>>>>>>> in parameters  =>  Assume no escape, no clobber (read-only). ref parameters, classes and pointers  =>  Assume worst case. default  =>  Assume no escape.
>>>>>>
>>>>>> That should read:
>>>>>>
>>>>>> ref parameters, inout parameters, classes and pointers.
>>>>>>
>>>>>> The default of assuming no escape is an experiment - I may limit this to only scalar types, and parameters marked as 'scope'  (So long as no one plans on deprecating it soon :)
>>>>>
>>>>> What does "assume no escape" actually mean?
>>>>> [The above list doesn't really make sense. W/o context, it's
>>>>> hard to even tell why, hence the question.]
>>>>>
>>>>
>>>> Actually, I might change the default to assume worst case.  I've just tried this out, which is still valid.
>>>>
>>>> class C {
>>>>    int * p;
>>>>    this(int x) {
>>>>      p = &x; // escapes the address of the parameter.
>>>>    }
>>>> }
>>>
>>> This might be currently accepted, but it is clearly invalid
>>> (escapes local; the only way to make it work safely would
>>> be to silently copy 'x' to the GC-managed heap, which would be
>>> way too costly).
>>>
>>>
>>>    A f(A a) { g(&a); return a; } // likewise with ref instead of pointer.
>>>
>>> This is OK (even if ideally 'g' should be forbidden from escaping 'a').
>>>
>>> Similarly:
>>>
>>>    A f(A a) {
>>>       auto o = register(&a); // can modify 'a'
>>>       o.blah();              // ditto
>>>       doneWith(o);           // ditto
>>>       return a;
>>>    }
>>>
>>>
>>> What I was wondering was things like whether that "assume no escape" property was transitive; if /locally/ escaping was disallowed, and to what extent. What does "assume no escape" mean at all? In your examples you're mentioning refs together with pointers, that would only make sense if no-escape were transitive -- but then treating all args as no-escape would be very wrong.
>>>
>>>
>>>> Worse, scope doesn't error on the general case either.
>>>>
>>>> class D {
>>>>    int * p;
>>>>    this(scope int x) {
>>>>      p = &x; // escapes the address of the scope parameter.
>>>>    }
>>>> }
>>>
>>> D's "scope" isn't enforced in any way right now, which means that code could exist that is invalid, but currently works. It would break silently(!) when compiled with a decent compiler, which still doesn't enforce scope.
>>>
>>
>> People should get bug fixing soon then.  =)
>
> Until some kind of diagnostics appear, most of those bugs won't even be found. It's too easy to write "auto f (in A a)" and then forget about the implicit 'scope' when modifying the function body.
>
>
>>>> Do these examples give you a good example?
>>>
>>> I'm worried about a) invalid assumptions making it into GDC;
>>> b) certain valid assumptions making into GDC. The latter because
>>> it could mean that code that's incorrect, but still accepted by
>>> the other compilers could silently break when compiled with GDC.
>>>
>>
>> Invalid assumptions rarely make it into GDC.  The testsuite is a good
>
> AFAICT what you're proposing *is* invalid. I can't be sure because
> it's not clear what that "no-escape" property means; that's why I
> asked about it twice already...
> Clearly, escaping objects reachable indirectly via function arguments
> is perfectly fine (eg string slicing), yet you wanted to treat args as
> no-escape by default.
>

Not wanted - experimented.  I could not think at the time a use case where a parameter could escape.  And *I* have shown a very valid use case to *disprove myself*.  I'm not sure of your continual persistence of bringing this up as this is no longer relevant to me.  Other than possibly the silent breaking of 'scope' or 'in' parameters that are misused (accepts invalid bugs).


>> bench for this, as well as several projects (now I've got dub set-up)
>> to test it in the wild.
>
> These problems will result in invalid optimizations, so can be hard to trigger and may come and go away randomly.
>
>
>> Saying that, we have had to revert some optimisation cases as D's schizophrenic nature of enforcing attributes and behaviours is becoming increasingly dismal.
>>
>> eg:
>> - nothrow has *no* guarantee, period, because it still allows
>> unrecoverable errors being thrown, and allows people to catch said
>> unrecoverable errors.
>> - pure is a tough nut to crack also.  The theory should allow you to
>> be able to cache return values, but in practise...
>
> D's "pure" doesn't have much in common with the "normal" pure concept; exposing gcc's pure via an attribute and completely ignoring D's version is probably the only practical solution, anything else would be too costly, result in too small gains, and be too hard to get right.
>

Is what we do.  I think the current conditions for a pure in the C sense function in D is more or less impossible to achieve.  The only guarantee that we can cling onto is that the transitive nature of pure means that functions can be const folded as deep as the pure rabbit hole goes.

>> - The nature of debug statements breaks guarantees of both nothrow and
>> pure, possibly many more.
>> - Defining reliable strict aliasing rules, it turns out, is not that
>> simple (this is something that Walter has mentioned about D should
>> have good guarantees for, ie: D arrays).
>
> Short term, disabling strict aliasing is the only option. I was scared
> of the impact it had before you even started to add support for it a
> few years ago (the codegen was already different w/ -fno-strict-aliasing
> before, which meant that I immediately had to disable it everywhere...)
> There probably does not exist a single D program that respects strict
> aliasing rules, other than by chance. The perf gains are minimal globally,
> but the potential for silent data corruption is huge.
>

I think the performance gains are worthy of note, most benchmarks tend to suggest that code is at least 5% slower with strict aliasing optimisations turned off.


> I'm just in investigating all avenues, as I usually do.  There is no
>> reason why 'in' shouldn't have more powerful guarantees IMO, and what
>
> Of course "scope" (which is part of "in) should be taken advantage of. I'm concerned about args not marked as "scope" being treated wrongly.
>
> [The scope-related bugs in D programs is a /language/ or frontend problem,
>  not a reason to avoid doing the right thing in GDC. It's just that in
>  practice what can happen is that GDC will be seen as miscompiling
>  "working" D programs...]
>

Indeed.  However my approach may be the reverse here as my attention span is short lived when it comes to working/moving around the various areas of gdc (Implement the optimisation, then fix the accepts invalid upstream), otherwise it may not be for another 2 years till I look at this again.

On 04/20/14 03:00, Iain Buclaw via Digitalmars-d wrote:
> On 19 April 2014 17:10, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>> On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:
>>> On 19 April 2014 14:33, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>>> On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:
>>>>> On 19 April 2014 13:02, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>>>>> On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:
>>>>>>> On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:
>>>>>>>> I'm currently testing out a GCC optimisation that allows you to set call argument flags.  The current assumptions being:
>>>>>>>>
>>>>>>>> in parameters  =>  Assume no escape, no clobber (read-only). ref parameters, classes and pointers  =>  Assume worst case. default  =>  Assume no escape.
>>>>>>>
>>>>>>> That should read:
>>>>>>>
>>>>>>> ref parameters, inout parameters, classes and pointers.
>>>>>>>
>>>>>>> The default of assuming no escape is an experiment - I may limit this to only scalar types, and parameters marked as 'scope'  (So long as no one plans on deprecating it soon :)
>>>>>>
>>>>>> What does "assume no escape" actually mean?
>>>>>> [The above list doesn't really make sense. W/o context, it's
>>>>>> hard to even tell why, hence the question.]

>>>> What I was wondering was things like whether that "assume no escape" property was transitive; if /locally/ escaping was disallowed, and to what extent. What does "assume no escape" mean at all? In your examples you're mentioning refs together with pointers, that would only make sense if no-escape were transitive -- but then treating all args as no-escape would be very wrong.

>>>> I'm worried about a) invalid assumptions making it into GDC;
>>>> b) certain valid assumptions making into GDC. The latter because
>>>> it could mean that code that's incorrect, but still accepted by
>>>> the other compilers could silently break when compiled with GDC.
>>>
>>> Invalid assumptions rarely make it into GDC.  The testsuite is a good
>>
>> AFAICT what you're proposing *is* invalid. I can't be sure because
>> it's not clear what that "no-escape" property means; that's why I
>> asked about it twice already...
>> Clearly, escaping objects reachable indirectly via function arguments
>> is perfectly fine (eg string slicing), yet you wanted to treat args as
>> no-escape by default.
> 
> Not wanted - experimented.  I could not think at the time a use case where a parameter could escape.  And *I* have shown a very valid use case to *disprove myself*.  I'm not sure of your continual persistence of bringing this up as this is no longer relevant to me.  Other than

Several reasons. First, I did not realize that you had changed your mind.
Second, I'm just trying to figure out what that 'assume-no-escape'
property implies. From a C based compiler I would have expected a feature
that disallowed address-of on objects, but did not affect any other
object referenced from that one (for handling C's 'register' storage class).
Third, I'm paranoid when it comes to GDC regressions. Once upon the time
I had a working D compiler, one that was able to deliver maybe ~75% of
GCC functionality, and where the main issues were with the language and
frontend, not with the tool. This was a few years ago, after you'd fixed
the many small problems I had run into. The type of those issues made it
very clear that I was the only one that was trying to really use GDC.
Unfortunately what happened then was that things started to get worse, and
by worse I mean "unusable"; GDC ceased to be able to deliver functionality
and performance that was in the same league as GCC C/C++. LTO stopped
working (which is not only crucial for a language w/o macros and low-level
control, but was useful as a work-around for tool issues, like the lack
of cross module inlining). The accepted language changed in a way that
was completely backward incompatible and would have required an extra
preprocessing step (pragma-attributes were removed and became hard
errors). GCC attributes became inaccessible; things like @pure, @malloc,
@ifunc, @align, @cold, @regparm all used to work, but now didn't. @inline,
@noinline and @flatten were added back as /true/ D-style attributes later,
and seem to work great. Thanks for exposing those, but my concern is that
dealing with them one-by-one does not scale - that's why I haven't filed
any bug reports wrt the other ones. Your work on getting D support merged
into GCC is critically important for the survival of the language; things
that are merely optimizations can happen later, or will be mostly irrelevant
(if the merge never happens).
But the situation right now is that the latest GDC version that works is
the gcc4.6 one (IIRC 2.057 based) - I have tried to migrate to a newer one
maybe 5 or 6 times over the last 1.5 year, and always run into some kind of
problem immediately after trying a new build. Like the last time, with the
failure of inlining of certain trivial functions. So when I then read about
a proposed new optimization, which sounds dubious to me, I ask about
more details, as I'm horrified that I could encounter yet another new GDC
specific problem during the next round of the let's-upgrade-gdc experience...


>>> - Defining reliable strict aliasing rules, it turns out, is not that simple (this is something that Walter has mentioned about D should have good guarantees for, ie: D arrays).
>>
>> Short term, disabling strict aliasing is the only option. I was scared
>> of the impact it had before you even started to add support for it a
>> few years ago (the codegen was already different w/ -fno-strict-aliasing
>> before, which meant that I immediately had to disable it everywhere...)
>> There probably does not exist a single D program that respects strict
>> aliasing rules, other than by chance. The perf gains are minimal globally,
>> but the potential for silent data corruption is huge.
> 
> I think the performance gains are worthy of note, most benchmarks tend to suggest that code is at least 5% slower with strict aliasing optimisations turned off.

I'd like to avoid something like the GCC C strict-aliasing disaster.
String-aliasing helps some code, but breaks some other (legacy) code.
Turning on strict-aliasing by default and recompiling the whole userspace
(hundreds of packages, thousands of sources) is not going to make any
noticeable difference for a desktop user, yet it is likely to trigger
data-corrupting bugs /somewhere/. So the result is practically no perf
gain, but a non-zero chance of corrupting user data, often by programs
that are not perf sensitive at all - that's not a good trade-off.
D doesn't have the MLOCs of legacy code, but I doubt that the D code
that exists was written with strict aliasing in mind. Especially since
the rules are not exactly well defined.

artur

On 20 April 2014 13:19, Artur Skawina via Digitalmars-d < digitalmars-d@puremagic.com> wrote:
> On 04/20/14 03:00, Iain Buclaw via Digitalmars-d wrote:
>> On 19 April 2014 17:10, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>> On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:
>>>> On 19 April 2014 14:33, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>>>> On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:
>>>>>> On 19 April 2014 13:02, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>>>>>> On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:
>>>>>>>> On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:
>>>>>>>>> I'm currently testing out a GCC optimisation that allows you to
set call argument flags. The current assumptions being:
>>>>>>>>>
>>>>>>>>> in parameters => Assume no escape, no clobber (read-only). ref parameters, classes and pointers => Assume worst case. default => Assume no escape.
>>>>>>>>
>>>>>>>> That should read:
>>>>>>>>
>>>>>>>> ref parameters, inout parameters, classes and pointers.
>>>>>>>>
>>>>>>>> The default of assuming no escape is an experiment - I may limit
this to only scalar types, and parameters marked as 'scope' (So long as no one plans on deprecating it soon :)
>>>>>>>
>>>>>>> What does "assume no escape" actually mean?
>>>>>>> [The above list doesn't really make sense. W/o context, it's
>>>>>>> hard to even tell why, hence the question.]
>
>>>>> What I was wondering was things like whether that "assume no escape" property was transitive; if /locally/ escaping was disallowed, and to what extent. What does "assume no escape" mean at all? In your examples you're mentioning refs together with pointers, that would only make sense if no-escape were transitive -- but then treating all args as no-escape would be very wrong.
>
>>>>> I'm worried about a) invalid assumptions making it into GDC;
>>>>> b) certain valid assumptions making into GDC. The latter because
>>>>> it could mean that code that's incorrect, but still accepted by
>>>>> the other compilers could silently break when compiled with GDC.
>>>>
>>>> Invalid assumptions rarely make it into GDC. The testsuite is a good
>>>
>>> AFAICT what you're proposing *is* invalid. I can't be sure because
>>> it's not clear what that "no-escape" property means; that's why I
>>> asked about it twice already...
>>> Clearly, escaping objects reachable indirectly via function arguments
>>> is perfectly fine (eg string slicing), yet you wanted to treat args as
>>> no-escape by default.
>>
>> Not wanted - experimented. I could not think at the time a use case where a parameter could escape. And *I* have shown a very valid use case to *disprove myself*. I'm not sure of your continual persistence of bringing this up as this is no longer relevant to me. Other than
>
> Several reasons. First, I did not realize that you had changed your mind. Second, I'm just trying to figure out what that 'assume-no-escape' property implies. From a C based compiler I would have expected a feature that disallowed address-of on objects, but did not affect any other object referenced from that one (for handling C's 'register' storage
class).
> Third, I'm paranoid when it comes to GDC regressions. Once upon the time I had a working D compiler, one that was able to deliver maybe ~75% of GCC functionality, and where the main issues were with the language and frontend, not with the tool. This was a few years ago, after you'd fixed the many small problems I had run into. The type of those issues made it very clear that I was the only one that was trying to really use GDC. Unfortunately what happened then was that things started to get worse, and by worse I mean "unusable"; GDC ceased to be able to deliver functionality and performance that was in the same league as GCC C/C++. LTO stopped working (which is not only crucial for a language w/o macros and low-level control, but was useful as a work-around for tool issues, like the lack of cross module inlining).

I have not seen this, infact the opposite.

As for speed of runtime.  Many of the optimisations that were removed in the last year have been found when porting to architectures that are less forgiving than x86.

I still don't what's up with LTO. It's not easy to debug, and fails in a way that doesn't seem to suggest a front-end problem (it chokes when reading LTO information written by itself).

> The accepted language changed in a way that
> was completely backward incompatible and would have required an extra
> preprocessing step (pragma-attributes were removed and became hard
> errors). GCC attributes became inaccessible; things like @pure, @malloc,
> @ifunc, @align, @cold, @regparm all used to work, but now didn't. @inline,
> @noinline and @flatten were added back as /true/ D-style attributes later,
> and seem to work great.

Many of these attributes to which need a good reason to be hashed out. Originally my intention was to only expose those that are impossible to infer otherwise in D through existing language features.

> Thanks for exposing those, but my concern is that
> dealing with them one-by-one does not scale - that's why I haven't filed
> any bug reports wrt the other ones.

They were exposed one-by-one before too. The entire source of routines was pretty much a copy-paste job from c-common.c. It has been suggested by the gcc maintainers to put those attributes into a more common area that can be shared between C/C++/ObjC and D - but there were too many subtle differences to make that work at all.  So instead I lopped of the head of the problem. The same as D iasm support, which was similarly dropped completely.

> Your work on getting D support merged
> into GCC is critically important for the survival of the language; things
> that are merely optimizations can happen later, or will be mostly
irrelevant
> (if the merge never happens).
> But the situation right now is that the latest GDC version that works is
> the gcc4.6 one (IIRC 2.057 based) - I have tried to migrate to a newer one
> maybe 5 or 6 times over the last 1.5 year, and always run into some kind
of
> problem immediately after trying a new build.

What projects are you having problems with. My experience says that there are more frontend implementation problems than gdc glue when moving across several major versions.

> Like the last time, with the
> failure of inlining of certain trivial functions. So when I then read
about
> a proposed new optimization, which sounds dubious to me, I ask about more details, as I'm horrified that I could encounter yet another new GDC specific problem during the next round of the let's-upgrade-gdc
experience...
>

The failure of inlining is not a blocker IMO - relying on 'hidden' features, NRVO being another example, in user code is a bug. Regardless of the validity.  This particular problem has been noted and it is being dealt with by improving the relationship between differing variants of const and mutable types.

On 20 Apr 2014 13:19, "Artur Skawina via Digitalmars-d" < digitalmars-d@puremagic.com> wrote:
>
> On 04/20/14 03:00, Iain Buclaw via Digitalmars-d wrote:
> > On 19 April 2014 17:10, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
> >> On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:
> >>> On 19 April 2014 14:33, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
> >>>> On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:
> >>>>> On 19 April 2014 13:02, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
> >>>>>> On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:
> >>>>>>> On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:
> >>>>>>>> I'm currently testing out a GCC optimisation that allows you to
set call argument flags.  The current assumptions being:
> >>>>>>>>
> >>>>>>>> in parameters  =>  Assume no escape, no clobber (read-only). ref parameters, classes and pointers  =>  Assume worst case. default  =>  Assume no escape.
> >>>>>>>
> >>>>>>> That should read:
> >>>>>>>
> >>>>>>> ref parameters, inout parameters, classes and pointers.
> >>>>>>>
> >>>>>>> The default of assuming no escape is an experiment - I may limit
this to only scalar types, and parameters marked as 'scope'  (So long as no one plans on deprecating it soon :)
> >>>>>>
> >>>>>> What does "assume no escape" actually mean?
> >>>>>> [The above list doesn't really make sense. W/o context, it's
> >>>>>> hard to even tell why, hence the question.]
>
> >>>> What I was wondering was things like whether that "assume no escape" property was transitive; if /locally/ escaping was disallowed, and to what extent. What does "assume no escape" mean at all? In your examples you're mentioning refs together with pointers, that would only make sense if no-escape were transitive -- but then treating all args as no-escape would be very wrong.
>
> >>>> I'm worried about a) invalid assumptions making it into GDC;
> >>>> b) certain valid assumptions making into GDC. The latter because
> >>>> it could mean that code that's incorrect, but still accepted by
> >>>> the other compilers could silently break when compiled with GDC.
> >>>
> >>> Invalid assumptions rarely make it into GDC.  The testsuite is a good
> >>
> >> AFAICT what you're proposing *is* invalid. I can't be sure because
> >> it's not clear what that "no-escape" property means; that's why I
> >> asked about it twice already...
> >> Clearly, escaping objects reachable indirectly via function arguments
> >> is perfectly fine (eg string slicing), yet you wanted to treat args as
> >> no-escape by default.
> >
> > Not wanted - experimented.  I could not think at the time a use case where a parameter could escape.  And *I* have shown a very valid use case to *disprove myself*.  I'm not sure of your continual persistence of bringing this up as this is no longer relevant to me.  Other than
>
> Several reasons. First, I did not realize that you had changed your mind.

It wouldn't be an experiment if I didn't change my mind 30 times. :)

> Second, I'm just trying to figure out what that 'assume-no-escape' property implies. From a C based compiler I would have expected a feature that disallowed address-of on objects, but did not affect any other object referenced from that one (for handling C's 'register' storage
class).

This is an optimisation not built with a C compiler in mind, nor is it leveraged by C/C++, so I don't think you should think of it in this way.

I was actually made aware of this particular attribute when someone spoke to a GNU/Fortran maintainer and thought that INTENT(IN) - or at least the no clobber aspect - could be used to improve optimisations around 'immutable' parameters. Something that gives us the one up that C-family const could never guarantee (currently there is no difference between const and immutable).  That is still on the table to do using this attribute pending investigation.

The no escape aspect leverages on what 'scope' and 'in' should be enforcing.

> Third, I'm paranoid when it comes to GDC regressions. Once upon the time I had a working D compiler, one that was able to deliver maybe ~75% of GCC functionality, and where the main issues were with the language and frontend, not with the tool.

GCC functionality is not to be confused with C/C++ functionality.  And most of the attributes exposed to C-family languages were nothing more an excess baggage, ie: @artificial makes no sense when there is no inline keyword.

Anything that needs raising would ideally be in a bug report, as I can't keep track of issues raised in threads.

Regards
Iain

On 21 Apr 2014 09:56, "Iain Buclaw" <ibuclaw@gdcproject.org> wrote:
>
> On 20 Apr 2014 13:19, "Artur Skawina via Digitalmars-d" <
digitalmars-d@puremagic.com> wrote:
> >
> > On 04/20/14 03:00, Iain Buclaw via Digitalmars-d wrote:
> > > On 19 April 2014 17:10, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
> > >> On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:
> > >>> On 19 April 2014 14:33, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
> > >>>> On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:
> > >>>>> On 19 April 2014 13:02, Artur Skawina via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
> > >>>>>> On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:
> > >>>>>>> On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:
> > >>>>>>>> I'm currently testing out a GCC optimisation that allows you
to set call argument flags.  The current assumptions being:
> > >>>>>>>>
> > >>>>>>>> in parameters  =>  Assume no escape, no clobber (read-only). ref parameters, classes and pointers  =>  Assume worst case. default  =>  Assume no escape.
> > >>>>>>>
> > >>>>>>> That should read:
> > >>>>>>>
> > >>>>>>> ref parameters, inout parameters, classes and pointers.
> > >>>>>>>
> > >>>>>>> The default of assuming no escape is an experiment - I may
limit this to only scalar types, and parameters marked as 'scope'  (So long as no one plans on deprecating it soon :)
> > >>>>>>
> > >>>>>> What does "assume no escape" actually mean?
> > >>>>>> [The above list doesn't really make sense. W/o context, it's
> > >>>>>> hard to even tell why, hence the question.]
> >
> > >>>> What I was wondering was things like whether that "assume no
escape"
> > >>>> property was transitive; if /locally/ escaping was disallowed, and to what extent. What does "assume no escape" mean at all? In your examples you're mentioning refs together with pointers, that would only make sense if no-escape were transitive -- but then treating
all
> > >>>> args as no-escape would be very wrong.
> >
> > >>>> I'm worried about a) invalid assumptions making it into GDC;
> > >>>> b) certain valid assumptions making into GDC. The latter because
> > >>>> it could mean that code that's incorrect, but still accepted by
> > >>>> the other compilers could silently break when compiled with GDC.
> > >>>
> > >>> Invalid assumptions rarely make it into GDC.  The testsuite is a
good
> > >>
> > >> AFAICT what you're proposing *is* invalid. I can't be sure because
> > >> it's not clear what that "no-escape" property means; that's why I
> > >> asked about it twice already...
> > >> Clearly, escaping objects reachable indirectly via function arguments
> > >> is perfectly fine (eg string slicing), yet you wanted to treat args
as
> > >> no-escape by default.
> > >
> > > Not wanted - experimented.  I could not think at the time a use case where a parameter could escape.  And *I* have shown a very valid use case to *disprove myself*.  I'm not sure of your continual persistence of bringing this up as this is no longer relevant to me.  Other than
> >
> > Several reasons. First, I did not realize that you had changed your
mind.
>
> It wouldn't be an experiment if I didn't change my mind 30 times. :)
>
> > Second, I'm just trying to figure out what that 'assume-no-escape' property implies. From a C based compiler I would have expected a
feature
> > that disallowed address-of on objects, but did not affect any other object referenced from that one (for handling C's 'register' storage
class).
>
> This is an optimisation not built with a C compiler in mind, nor is it
leveraged by C/C++, so I don't think you should think of it in this way.
>
> I was actually made aware of this particular attribute when someone spoke
to a GNU/Fortran maintainer and thought that INTENT(IN) - or at least the no clobber aspect - could be used to improve optimisations around 'immutable' parameters. Something that gives us the one up that C-family const could never guarantee (currently there is no difference between const and immutable).  That is still on the table to do using this attribute pending investigation.
>
> The no escape aspect leverages on what 'scope' and 'in' should be
enforcing.
>
> > Third, I'm paranoid when it comes to GDC regressions. Once upon the time I had a working D compiler, one that was able to deliver maybe ~75% of GCC functionality, and where the main issues were with the language and frontend, not with the tool.
>
> GCC functionality is not to be confused with C/C++ functionality.  And
most of the attributes exposed to C-family languages were nothing more an excess baggage, ie: @artificial makes no sense when there is no inline keyword.
>
> Anything that needs raising would ideally be in a bug report, as I can't
keep track of issues raised in threads.
>

As for the original topic.

STCin is an attribute ignored by the compiler (only talking about gdc) - which is a shame because marking parameters as 'in' I see is still encouraged.  And I would assume the same is true also for dmd and ldc, otherwise someone would have noticed this bug by now.

On 04/20/14 22:11, Iain Buclaw via Digitalmars-d wrote:
> The failure of inlining is not a blocker IMO

It is one in practice. A language with a compiler that can not even inline this trivial function:

   test   %rdi,%rdi
   sete   %al
   retq

is not a viable alternative to C.

Not everything can or should be an enum, and D doesn't have macros -- there is no workaround.

[That's "array.empty". More context, for people not reading the GDC list:
 http://forum.dlang.org/thread/mailman.75.1396605155.19942.d.gnu@puremagic.com ]

The bug may be hard to fix, and I really appreciate your effort, but this definitely *is* a major problem.


> relying on 'hidden' features, NRVO being another example, in user code is a bug.

Not if RVO is mandated by a spec. Yes, I had trouble with this in the past too. :) IIRC an auto-return-type related problem was fixed by Walter some time ago; but I still have to maintain the workaround, even after the bug has been fixed upstream...

artur