Add language to clarify shared/non-shared alias relationships

[GoogleCodeExporter]

This issue is responsive to the item 47 in the open issues list.

47. Aliasing guarantees?  Meaning of the restrict keyword when applied to a 
pointer-to-shared?

It is unclear from the brief problem statement above the detailed nature of the 
of the problem with respect to restrict qualified pointer target types. 
Therefore no particular proposal is expressed in the problem description.  
Further comments added to this issue are invited that amplify the problem 
description and that propose UPC specification changes.

Original issue reported on code.google.com by gary.funck on 22 May 2012 at 5:41

[GoogleCodeExporter]

As Gary indicated, there is not much "meat" to the problem statement.
So, I'll take a shot at throwing out my random thoughts in the hope of finding 
something we want/need to put in the spec:

If we consider the restrict keyword in C99, the guarantee is that given two or 
more pointers with the "restrict" qualifier they will each address (at least 
"dynamically") distinct (non-overlapping) objects and they cannot address an 
"extern" qualified object.  The gory details are found in the original C 
proposal here: http://www.lysator.liu.se/c/restrict.html

In the "linear" address space of C that all makes sense.
In UPC however, one can "reshape" a pointer by cast: for instance taking an 
indefinite pointer-to-shared (with it's "regular C" layout) and casting it to a 
cyclic layout.  So, does that change what "restrict" should mean?  I would hope 
it does not.

My experience with using "restrict" is limited to function parameters, though 
the C99 spec allows wider used.  Based on the "gut feeling" that comes along 
with "restrict" in this context I expect that as an implementer I can freely 
reorder accesses made via the two (or more) distinct pointers knowing that no 
changes to the program behavior can result.

So, do we want to clarify that "restrict" still means the same thing?

Does this interact or conflict with the "strict" modifier?
My feeling here (with only brief thought) is that the "strict" qualifier 
PROHIBITS exactly the re-ordering that "restrict" is meant to enable.  So, I 
would think these two qualifiers should NOT be permitted to be applied to the 
same pointer.

What about the "relaxed" qualifier?
The "restrict" qualifier applied to a relaxed pointer would, I believe, allow 
one to legally assume that the "same address" rule won't apply to shared 
accesses via two relaxed/restricted pointers.  So, there does seem to be some 
"performance utility" to "relaxed" in combination with "shared", by allowing 
optimization that we otherwise cannot perform even in the relaxed model.

Original comment by [email protected] on 1 Jun 2012 at 7:06

[GoogleCodeExporter]

Oops. The very last sentence of my previous comment read:

So, there does seem to be some "performance utility" to "relaxed" in 
combination with "shared", by allowing optimization that we otherwise cannot 
perform even in the relaxed model.

The first instance of "relaxed" should have said "restrict":

So, there does seem to be some "performance utility" to "restrict" in 
combination with "shared", by allowing optimization that we otherwise cannot 
perform even in the relaxed model.

Original comment by [email protected] on 1 Jun 2012 at 7:17

[GoogleCodeExporter]

The C99 semantics of restrict revolve around the objects accessed via 
dereference of the qualified pointer, so the fact that you traversed a wide 
pointer to reach those elements and the exact blocking factor in the type of 
that pointer should be completely irrelevant. In my opinion restrict already 
makes perfect sense on a pointer-to-shared and no further clarification is 
required.

I agree that the presence of strict on a restrict-qualified pointer probably 
negates any useful access optimizations, but the language concept being 
expressed by the annotation is still fully meaningful, so I see no reason to 
prohibit it.

I invite further examples where the combination is unclear, otherwise moving to 
close with WontFix.

Original comment by danbonachea on 15 Jun 2012 at 3:37

[GoogleCodeExporter]

I originally added this issue to the list.  I perhaps should not have mentioned 
restrict and taken us on that detour.  restrict is related to aliasing, but not 
to my main point.  Let me start with a few examples:

    shared int* p;
    shared int a[THREADS];

It is perfectly valid to point p at any part of array a.  A reference through 
p, therefore, may modify an element of array a.  So far not surprising.

Now consider:

    shared int* p;
    int b[100];

It is not legal in UPC to cast a pointer-to-local to a pointer-to-shared, so 
one cannot write p = (shared int*)&b[0], for example.  A compiler safely can 
assume that references through p cannot modify an element of array b.  I'm not 
sure how many compilers make use of this fact, but it exists.

Now here's where it gets messy...

Arguments of this function may alias since q1 could equal (int*)p:

    void foo1( shared int* p, int* q1 );

Arguments of this function may not alias(!)

    void foo2( shared int* p, int q2[] );

because C99 draws a distinction between int* and int[] in 6.7.5.2.8 and, as 
established above, UPC makes it illegal to point a pointer-to-shared at 
non-shared-qualified data.  So in foo2, p can never point into the q2 array.  
Thus, an implementation can assume that p and q2 do not alias...or can it?

You will find that C compilers are perfectly okay with the following call:

    void test( int q[] );
    int* x;
    ...
    test( x );

Therefore, it is possible to cast a pointer-to-shared to a pointer-to-local, 
pass that pointer-to-local to a function with an array parameter, and have a 
UPC compiler incorrectly assume that the non-shared array parameter cannot 
alias with any shared data.  You might argue that one needs this declaration 
instead:

    void foo2( shared int* restrict p, int q2[restrict] );

But I'm arguing that code explicitly states a no-aliasing relationship that 
appears to be already implied by C99 and UPC 1.2, when in fact it is not true.

I propose that we add some language to UPC 1.2 6.4.3.1 to say that it does not 
imply this particular no-aliasing relationship.  Or perhaps near UPC 1.2 
6.5.1.1 near footnote 14 (which also talks about aliasing),

Original comment by [email protected] on 15 Jun 2012 at 7:43

• Added labels: Milestone-Spec-1.3

[GoogleCodeExporter]

Update title to better reflect the issue that I wanted to raise.

Original comment by [email protected] on 15 Jun 2012 at 7:57

• Changed title: Add language to clarify shared/non-shared alias relationships

[GoogleCodeExporter]

Re-assign Troy as owner.

Original comment by gary.funck on 29 Jun 2012 at 7:28

[GoogleCodeExporter]

Original comment by gary.funck on 3 Jul 2012 at 3:10

• Added labels: Type-Clarification
• Removed labels: Type-Defect

[GoogleCodeExporter]

Set default Consensus to "Low".

Original comment by gary.funck on 19 Aug 2012 at 11:26

• Added labels: Consensus-Low

[GoogleCodeExporter]

For completeness, here is the text Troy quoted from 6.7.5.2-8:

"EXAMPLE 2 Note the distinction between the declarations
   extern int *x;
   extern int y[];
The first declares x to be a pointer to int; the second declares y to be an 
array of int of unspecified size
(an incomplete type), the storage for which is defined elsewhere."

Note this designates the type (int y[]) as an INCOMPLETE type. 

However, it appears there is some subtle "adjustment" which automatically takes 
place when one of these incomplete array types appears as a formal parameter to 
a function.

6.5.2.2 constraint and semantics on function calling (emphasis added):

"Each argument shall have a type such that its value may be assigned to an 
object with the unqualified version of the type of its corresponding parameter. 
...
In preparing for the call to a function, the arguments are evaluated, and each 
parameter is assigned the value of the corresponding argument[78]
[78]...A parameter declared to have ARRAY or function type is ADJUSTED TO HAVE 
A POINTER TYPE as described in 6.9.1."

6.9.1-7: Function definitions:

"the type of EACH PARAMETER IS ADJUSTED as described in 6.7.5.3 for a parameter 
type list; the resulting type shall be an OBJECT type."

6.7.5.3: Function declarators:

"After adjustment, the parameters in a parameter type list in a function 
declarator that is part of a definition of that function SHALL NOT HAVE 
INCOMPLETE TYPE."

My interpretation of these C99 passages is that in a declaration of the form:

   void foo2( shared int* p, int q2[] );

the second parameter is automatically "adjusted" to instead be:

   void foo2( shared int* p, int *q2 );

Troy's basis for comment 4 seems to be that the first form is somehow different 
from the second and implies some sort of freedom from aliasing, but the C99 
verbiage quoted above seems to indicate they are in fact equivalent, and 
moreover the first is automatically "adjusted" (aka rewritten) to be the 
second. Furthermore, in the absence of restrict there is nothing to prohibit 
aliasing of these incomplete array formal arguments in pure C, eg:

void foo(int x[], int y[], int *q) { }
int main() {
  int a[7];
  foo(a,a,a); /* permitted */
  return 0;
}

This code is fully legal (or at least `gcc -pedantic` accepts it and I can't 
find anything in C99 to prohibit it) and all three arguments of foo are aliased.

So I think this means that as a general rule incomplete array type formal 
arguments are equivalent to a pointer-to-local, and (in the absence of 
restrict) can always be aliased with other pointer arguments. This is an 
(unstated) aliasing property of pure C99 which exists completely independently 
of any UPC extensions. C99 does not list all the ways in which pointers can be 
aliased, so it would seem odd for us to start doing that. Perhaps this is 
something we can add to the Rationale doc under "Advice to Implementers"?

I still move to close with NoChange.

Original comment by danbonachea on 26 Sep 2012 at 11:15

[GoogleCodeExporter]

We discussed this issue at the 1/17 telecon and did not reach a consensus.

We agreed to table the issue for now, and perhaps reconsider it at a later date 
with a concrete proposal from Troy.

Original comment by danbonachea on 17 Jan 2013 at 7:49

• Added labels: Milestone-Spec-1.4
• Removed labels: Milestone-Spec-1.3

[GoogleCodeExporter]

Proposed language to add to UPC Section 6.4.3 "Cast and assignment expressions" 
as new paragraphs 8 and 9:

8   A pointer-to-shared cannot alias with an array that is not shared 
qualified; however a pointer-to-shared can alias with a pointer-to-local, even 
if that pointer-to-local arises via type adjustment of an array parameter that 
is not shared qualified.

9   EXAMPLE 2

    /* ISO/IEC00 Section 6.7.5.3 Paragraph 7 adjusts q to int*  */
    void foo( shared int* p, int q[] ); /* p and q may alias */

    static shared int x[10*THREADS];
    int* y = (int*)&x[MYTHREAD];

    foo( &x[MYTHREAD], y );

Original comment by [email protected] on 22 Jan 2013 at 9:26

[GoogleCodeExporter]

I think Troy's text would be a great addition to the "Advice to Implementers" 
document: http://code.google.com/p/upc-specification/wiki/UPCSpecCompanion

However I'm still opposed to this going into the language spec for several 
reasons:

1. It addresses a purely implementation issue - an aliasing property which is 
already derivable from a close reading of C99/UPC semantics, and is completely 
irrelevant to users. Realistically the only people who may ever need this 
information are probably all on this mailing list.

2. There is nothing remotely like this in C99. It defines how objects may 
legally be accessed (6.5-7 and 6.7.3.1), and leaves it up to implementers to 
infer what optimizations are possible that won't affect the observable behavior 
of the program in a way that differs from execution on the abstract machine. In 
fact C99 5.1.2.3 even states: "The semantic descriptions in this International 
Standard describe the behavior of an abstract machine in which issues of 
optimization are irrelevant."

3. It begs the question of what are the other aliasing properties of the 
language that an implementer needs to write a correct alias analysis.

4. The proposed language makes an overly broad statement: "a pointer-to-shared 
can alias with a pointer-to-local". This could be misread to mean that any two 
such pointers may be aliased. This property is true in the very narrow case 
shown in proposed example, but there are in fact many more examples where two 
randomly-chosen pointers in the program may NOT be aliased. The most obvious 
case is simply adding restrict to the arguments in the example. Other common 
cases include when the referent types are incompatible, or when analysis of 
surrounding code can prove the pointers reference two objects known to be 
distinct.

Original comment by danbonachea on 22 Jan 2013 at 11:26

[GoogleCodeExporter]

We need more people to choose a side on this issue...

1) The added language is relevant and helpful to programmers working within a 
C-based language targeting HPC because it helps to know when they need to worry 
about assisting the compiler by adding "restrict."  These paragraphs explicitly 
call out a situation where it may appear that aliasing is prohibited but is in 
fact allowed.  "restrict" is necessary (for greatest performance) if the 
programmer knows that they aren't generating aliases.

2) There is no constraint on the UPC spec that it must include only statements 
that sound like things in C99.  We can say whatever we want (and could even 
contradict C99), it's just that the spec falls back on C99 for anything that we 
don't mention.

3) I'm not suggesting that we include an in depth discussion of all possible 
C99 aliases among pointers-to-local or UPC aliases among pointers-to-shared.  
This section in the UPC spec discusses casts between pointers-to-shared and 
pointers-to-local, and because UPC permits that (in one direction), it opens up 
the possibility of aliases between those different kinds of pointers.  This 
section is the place to clarify any unusual side effect of that consequence.

4) I chose "can" instead of "may" to avoid saying "may alias" which has a 
specific pointer-aliasing definition already.  We can work on the wording.  The 
point is that without this clarification, a programmer may think that the 
compiler can assume that no alias exists and that they don't need to bother 
with "restrict," but that's not true.  "restrict" may be beneficial here.

Original comment by [email protected] on 23 Jan 2013 at 12:12

[GoogleCodeExporter]

> We need more people to choose a side on this issue...

I agree it may be helpful for more people to state their opinion on this one.

> it helps to know when they need to worry about assisting the compiler by 
adding "restrict."  
> These paragraphs explicitly call out a situation where it may appear that 
aliasing is prohibited 
> but is in fact allowed.  "restrict" is necessary (for greatest performance) 
if the programmer 
> knows that they aren't generating aliases.
> ... they don't need to bother with "restrict," but that's not true.  
"restrict" may be beneficial here.

I don't buy this argument. You're basically saying the average UPC programmer 
believes he can replicate the compiler's alias analysis in his head and wants 
to infer the minimal set of restrict keywords he needs to get the optimizations 
he wants to be applied. Excluding programmers who helped write the compiler, or 
some very detailed user feedback from the optimizer, that doesn't seem like a 
realistic way to optimize. A more principled and portable approach for a 
performance-conscious author to optimize their UPC program is to insert a 
restrict annotation at *every* function declaration used by inner loops where 
he knows the pointer arguments will never be aliased. Ie express the 
non-aliased property wherever the program design ensures it, and let the 
optimizer sort it out. If the analysis in one compiler is already smart enough 
to prove lack of aliasing in some of those cases, the restrict keyword makes no 
difference but hurts nothing. A compiler with a less aggressive analysis may 
benefit more from the programmer's annotation.

> There is no constraint on the UPC spec that it must include only statements 
that sound like things in C99.

Agreed, but C99 is also an excellent model for us, since it's very closely 
related and has endured decades of scrutiny and tweaking from many more users, 
implementers and language writers than our project. Also, if we're still 
considering the possibility of the UPC spec somehow/someday being promoted to a 
more formal part of the C language, we should try not to gratuitously diverge 
from the form and conventions of the C spec (not to be confused with language 
features).

Also, as I argued in comment 9, the aliasing scenario you propose to specify 
already exists in pure C99:

   void foo(int x[], int y[], int *q) { } 

All three arguments to this pure C99 function may legally be aliased, although 
a casual C programmer might not immediately realize that. Regardless, the C 
spec doesn't explicitly state that or any other aliasing property - it's just 
an inferred property of the language semantics.

Original comment by danbonachea on 23 Jan 2013 at 3:18

[GoogleCodeExporter]

I think Troy has valid points that would be valuable to express *somewhere* 
writing.
However, I agree with Dan that given C99 as our "basis" these clarifications 
are not appropriate for inclusion in the normative portion of the UPC spec.

Original comment by [email protected] on 23 Jan 2013 at 6:57

[GoogleCodeExporter]

I understand that this information could go into the UPCSpecCompanion document 
that Dan linked in Comment #12, but the proposed text in Comment #11 is brief, 
fits with an existing section of the spec, and is similar to an existing 
aliasing clarification in Footnote 14.  Is there some distinction between the 
proposed text and Footnote 14 that would make you want to keep Footnote 14 in 
the spec but relegate the proposed text to the UPCSpecCompanion, or would you 
also want to remove Footnote 14 from the spec and put it into the 
UPCSpecCompanion?

Original comment by [email protected] on 23 Jan 2013 at 4:50

[GoogleCodeExporter]

Troy,

I assume you are talking about the following from the 1.2 spec:

14  This is a powerful statement which allows, for example, that in an 
implementation
sizeof(shared int *) may differ from sizeof (shared [10] int *) and if T and S
are pointer-to-shared types with different block sizes, then T* and S* cannot 
be aliases.

If so, then I would answer "YES" to your question "would you also want to 
remove Footnote 14 from the spec and put it into the UPCSpecCompanion?".

If there were no Companion or other document in which to put this kind of 
elaboration, then I'd reluctantly accept them in the Spec itself.  I look at 
the Spec as providing the equivalent of "Axioms" and both Footnote 14 and your 
proposed addition are more like "Theorems" that one can derive from the axioms. 
 The analogy is far from perfect, but felt like a good way to express my view 
compactly.

-Paul

Original comment by [email protected] on 23 Jan 2013 at 9:35

[GoogleCodeExporter]

Thanks Paul.  As long as both that footnote and my proposed language are in the 
same document, I can accept putting them both into either the spec or the 
companion document.  It would not be helpful to have aliasing clarifications 
split across two different documents.

To use your analogy, I like having both the axioms and the theorems (especially 
the non-obvious ones) in a single document, but I can understand your point of 
view too.

Original comment by [email protected] on 23 Jan 2013 at 9:51

[GoogleCodeExporter]

I strongly disagree about removing footnote 14, because I feel it has a much 
broader audience than the clarification proposed in this issue. I've argued in 
comments 12 and 14 that the clarification proposed in this issue is only 
relevant to optimizer writers. Troy has argued that it may rarely also be of 
interest to programmers who want to micromanage their restrict keywords. 

Whichever view you accept, I hope we can agree that the current issue applies 
to a property that is strictly "beneath" the level of the abstract machine (the 
central goal of the semantic specification). Aliasing properties that an 
optimizer can infer from a function declaration has no semantic impact on the 
abstract machine; by definition any correct optimizations on a legal program 
must preserve the external semantic behavior of the unoptimized abstract 
machine. Stated another way, the topic of this issue never has an impact on the 
*correctness* of a user program, merely the compiler analysis used to perform 
optimizations.

In contrast, the primary purpose of footnote 14 clarifies a non-obvious 
restriction on the correctness of user programs, and thus has a much wider 
audience. The footnote is annotating semantic para 13:

     The block size is a part of the type compatibility(14)

Most users lacking a deep understanding of C99 would not understand the full 
correctness implications of this powerful statement without a footnote. 
Specifically, the footnote clarifies that a UPC program which does something 
like the following is invalid and leads to undefined behavior:

shared int *p1 = upc_alloc(1);
shared int **pp1 = &p1;
shared [10] int **pp2 = (void *)pp1;
**pp2 = 5;

Note this program is fully type-correct and should not be rejected by the type 
checker, but it is erroneous because it has aliased two pointers (pp1 and pp2) 
with incompatible referent types (and those types are incompatible solely 
because of the difference in block size and para 13). This program is likely to 
demonstrate incorrect behavior at runtime in any UPC compiler that optimizes 
phaseless PTS using a dedicated representation. In fact, the motivation for 
this semantic restriction on the user was precisely to allow implementations to 
perform that powerful optimization.

Footnote 14 clarifies a semantic restriction that affects the correctness of 
programs and that restriction DOES apply at the level of the abstract machine. 
All users need to understand this property in order to write correct UPC 
programs with aggressive use of pointer-to-shared. Despite the fact the word 
"alias" appears in the text of the footnote, this text is mostly targeted at 
end-users and not optimizer writers. I agree with Paul's analogy about 
providing semantic axioms and letting compiler writers derive the theorems they 
need to write a correct optimization, but I don't think we should expect the 
same level of sophistication from casual UPC programmers. Important, 
non-obvious restrictions on program correctness should be spelled out 
explicitly for users in the interests of clarity, and that's what footnote 14 
does.

Original comment by danbonachea on 24 Jan 2013 at 7:12

[GoogleCodeExporter]

As an aside, I think the importance of this issue pales in comparison to issues 
3 and 85, which are directly blocking progress on the ratification of 1.3. My 
impression from the call is we'd agreed to table this for now in the interests 
of focusing on the high-priority issues?

Original comment by danbonachea on 24 Jan 2013 at 7:37

• Added labels: Priority-Low
• Removed labels: Priority-Medium

[GoogleCodeExporter]

RE importance, note the owner field for this issue versus issues 3 and 85.  I'm 
dealing with this issue and Steve is handling issues 3 and 85, so this 
discussion isn't impeding his progress on 3 or 85.

RE footnote 14, yes, it is a more important clarification than the one that I 
am proposing, so could we keep footnote 14 in the spec but repeat it (quote it 
verbatim) in the companion document near the new text?  I guess I just want 
one-stop shopping for UPC aliasing information because there was a time that I 
went searching for guidance on UPC aliasing rules and the only thing I could 
find anywhere was footnote 14 and it was insufficient for my needs.  Just 
trying to make it easier for others despite whatever size audience that ends up 
being.

Side note:

RE phaseless PTS and "the motivation for this semantic restriction on the user 
was precisely to allow implementations to perform that powerful optimization" 
-- Cray intentionally got rid of phaseless PTS years ago.  It was very awkward 
to deal with PTS's of two different sizes in all phases of our compiler and 
runtime implementation, plus gave the generated code no measurable performance 
benefit despite its theoretical prospects.  It actually used to give multiple 
customers headaches when they would change the block size of a PTS inside a 
struct only to discover that doing so changed their struct size, struct 
packing, field alignments, etc. which they had carefully designed to fit in a 
cache line or to make some other manual optimization work correctly.  I like 
the no-alias implications of footnote 14 because it assists with compiler alias 
analysis, but don't find phaseless PTS to be important at all -- it's just 
confusing.

Original comment by [email protected] on 24 Jan 2013 at 6:55

[GoogleCodeExporter]

First, I'm not sure about the implications of the following proposed paragraph:
"8   A pointer-to-shared cannot alias with an array that is not shared 
qualified;"

I just made up a contrived aliasing example and could run it successfully with 
BUPC (with --enable-sptr-struct).

int main(int argc, char **argv)
{
  shared [0] char *p = 0; 
  char a[1] = {'U'};
  p += (size_t)&a[0];
  printf("*p is %c\n", *p);
  return 0;
}

Second, I also feel that Footnote 14 is too much a burden to the user because 
it requires the user to guarantee that there is always no aliasing between two 
pointers to pointer-to-shared.

To conclude, I agree with Paul's comment #17 that both Troy's proposed language 
and Footnote 14 should go to the companion document rather than the spec.

Original comment by [email protected] on 24 Jan 2013 at 8:09

[GoogleCodeExporter]

That example has undefined behavior (see ISO/IEC 9899 6.5.6 paragraph 8), so as 
far as the spec is concerned, there is no aliasing.  The fact that aliasing 
occurs with a particular implementation is irrelevant.

Original comment by [email protected] on 24 Jan 2013 at 8:18

[GoogleCodeExporter]

RE comment 23:

I also ran the example with Cray UPC :-).

But my main point is actually that it would be very "Risky" for a compiler to 
assume that a local array would not be modified through a pointer-to-shared, 
whether it's an accident or a buffer-overrun attack.  If I understood 
correctly, this assumption was the main motivation of this issue brought up in 
comment 4.

Original comment by [email protected] on 24 Jan 2013 at 8:46

[GoogleCodeExporter]

I agree with Steve that Yili's example in comment #22 is an erroneous program, 
and thus behavior is undefined and it doesn't matter what the optimizer does 
with it - garbage in, garbage out. Troy's text "A pointer-to-shared cannot 
alias with an array that is not shared qualified" is valid for all correct UPC 
programs, and thus a safe assumption for the alias analysis to make. There are 
more subtle ways to write an incorrect C99 program  (analogous to comment #19) 
which introduce aliasing which is equally erroneous with undefined effects - 
this is one of the fundamental downsides of a non-typesafe language and not 
something we can "fix".

" I also feel that Footnote 14 is too much a burden to the user"
"I like the no-alias implications of footnote 14 because it assists with 
compiler alias analysis, but don't find phaseless PTS to be important at all"

Footnote 14 and the para 13 it clarifies date back to UPC 1.1 (ie over 10 
years) and IMHO are not open to semantic change. Relaxing the para 13 
restriction would break many compilers, including breaking BUPC in fundamental 
ways. 

Our experiments over the years have turned up many examples where the phaseless 
PTS optimization makes a *huge* difference in the performance of 
pointer-intensive codes. It's possible that much of this benefit arises because 
BUPC is source-to-source, so the module generating assembly code lacks 
knowledge of PTS semantics and arithmetic. In any case, it's a very important 
optimization for us. It also has the nice benefit of recovering any performance 
cost that might be associated with fully-general blocked PTS for any codes that 
don't use that feature (global performance impact is one of the main arguments 
to remove fully-general array blocking from the language).

Original comment by danbonachea on 25 Jan 2013 at 12:12