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What are the preconditions of newArray#

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Johan Tibell

19 Aug 2011 19 Aug '11

11:04 a.m.

Hi, I'm seeing a segfault which I suspect is due to allocating Array#s of size 0, using newArray#. Are zero length arrays allowed? What are the preconditions of newArray#? It'd be great if they were documented. -- Johan

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Edward Z. Yang

21 Aug 21 Aug

11:55 p.m.

stg_newArrayzh in rts/PrimOps.cmm doesn't appear to give any indication, so this might be a good patch to add. But I'm curious: what would allocating Array#s of size 0 do? Null pointers? That sounds dangerous... Edward Excerpts from Johan Tibell's message of Fri Aug 19 11:04:48 -0400 2011:

Hi,

I'm seeing a segfault which I suspect is due to allocating Array#s of size 0, using newArray#. Are zero length arrays allowed? What are the preconditions of newArray#? It'd be great if they were documented.

-- Johan

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Johan Tibell

22 Aug 22 Aug

3:43 a.m.

On Mon, Aug 22, 2011 at 5:55 AM, Edward Z. Yang wrote:

stg_newArrayzh in rts/PrimOps.cmm doesn't appear to give any indication, so this might be a good patch to add. But I'm curious: what would allocating Array#s of size 0 do? Null pointers? That sounds dangerous...

I would imagine that a zero sized array would be a StgArrPtrs header with its size field set to 0. It's not a very useful thing to have, I admit. If someone (Simon?) can confirm that we don't intend to support zero-length array I'll push a patch that adds a comment. Johan

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Edward Kmett

11:54 a.m.

It would still be nice to have a consistent base case. On Mon, Aug 22, 2011 at 3:43 AM, Johan Tibell wrote:

On Mon, Aug 22, 2011 at 5:55 AM, Edward Z. Yang wrote:

...
stg_newArrayzh in rts/PrimOps.cmm doesn't appear to give any indication, so this might be a good patch to add. But I'm curious: what would allocating Array#s of size 0 do? Null pointers? That sounds dangerous...

I would imagine that a zero sized array would be a StgArrPtrs header with its size field set to 0. It's not a very useful thing to have, I admit. If someone (Simon?) can confirm that we don't intend to support zero-length array I'll push a patch that adds a comment.

Johan

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Johan Tibell

12:08 p.m.

I agree (unless it has a performance cost). I had to fix a couple of bugs in my code associated with generating zero-length arrays. On Mon, Aug 22, 2011 at 5:54 PM, Edward Kmett wrote:

It would still be nice to have a consistent base case.

On Mon, Aug 22, 2011 at 3:43 AM, Johan Tibell wrote:

...
On Mon, Aug 22, 2011 at 5:55 AM, Edward Z. Yang wrote:

...
stg_newArrayzh in rts/PrimOps.cmm doesn't appear to give any indication, so this might be a good patch to add. But I'm curious: what would allocating Array#s of size 0 do? Null pointers? That sounds dangerous...

I would imagine that a zero sized array would be a StgArrPtrs header with its size field set to 0. It's not a very useful thing to have, I admit. If someone (Simon?) can confirm that we don't intend to support zero-length array I'll push a patch that adds a comment.

Johan

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Simon Marlow

23 Aug 23 Aug

3:47 p.m.

An Array# of size zero is a perfectly reasonable thing. If it doesn't work, it should (and I vaguely recall making it work at some point in the past, but perhaps I failed to add a test and as a result it has rotted...) Cheers, Simon On 22/08/11 17:08, Johan Tibell wrote:

I agree (unless it has a performance cost). I had to fix a couple of bugs in my code associated with generating zero-length arrays.

On Mon, Aug 22, 2011 at 5:54 PM, Edward Kmett wrote:

...
It would still be nice to have a consistent base case.

On Mon, Aug 22, 2011 at 3:43 AM, Johan Tibell wrote:

...
On Mon, Aug 22, 2011 at 5:55 AM, Edward Z. Yang wrote:

...
stg_newArrayzh in rts/PrimOps.cmm doesn't appear to give any indication, so this might be a good patch to add. But I'm curious: what would allocating Array#s of size 0 do? Null pointers? That sounds dangerous...

I would imagine that a zero sized array would be a StgArrPtrs header with its size field set to 0. It's not a very useful thing to have, I admit. If someone (Simon?) can confirm that we don't intend to support zero-length array I'll push a patch that adds a comment.

Johan

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Johan Tibell

4:04 p.m.

It could well be that it's some later primop that's failing due to the empty size, like my new copyArray# primop. If that's the case I could fix it but I would probably would have to add a branch to the copyArray# primop, which I'm reluctant to do. On Tue, Aug 23, 2011 at 9:47 PM, Simon Marlow wrote:

An Array# of size zero is a perfectly reasonable thing. If it doesn't work, it should (and I vaguely recall making it work at some point in the past, but perhaps I failed to add a test and as a result it has rotted...)

Cheers, Simon

On 22/08/11 17:08, Johan Tibell wrote:

...
I agree (unless it has a performance cost). I had to fix a couple of bugs in my code associated with generating zero-length arrays.

On Mon, Aug 22, 2011 at 5:54 PM, Edward Kmett wrote:

...
It would still be nice to have a consistent base case.

On Mon, Aug 22, 2011 at 3:43 AM, Johan Tibell wrote:

...
On Mon, Aug 22, 2011 at 5:55 AM, Edward Z. Yang wrote:

...
stg_newArrayzh in rts/PrimOps.cmm doesn't appear to give any indication, so this might be a good patch to add. But I'm curious: what would allocating Array#s of size 0 do? Null pointers? That sounds dangerous...

I would imagine that a zero sized array would be a StgArrPtrs header with its size field set to 0. It's not a very useful thing to have, I admit. If someone (Simon?) can confirm that we don't intend to support zero-length array I'll push a patch that adds a comment.

Johan

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Simon Marlow

4:31 p.m.

You could make it a precondition of copyArray#, although that's slightly less pleasant from a user's perspective. Cheers, Simon On 23/08/11 21:04, Johan Tibell wrote:

It could well be that it's some later primop that's failing due to the empty size, like my new copyArray# primop. If that's the case I could fix it but I would probably would have to add a branch to the copyArray# primop, which I'm reluctant to do.

On Tue, Aug 23, 2011 at 9:47 PM, Simon Marlow wrote:

...
An Array# of size zero is a perfectly reasonable thing. If it doesn't work, it should (and I vaguely recall making it work at some point in the past, but perhaps I failed to add a test and as a result it has rotted...)

Cheers, Simon

On 22/08/11 17:08, Johan Tibell wrote:

...
I agree (unless it has a performance cost). I had to fix a couple of bugs in my code associated with generating zero-length arrays.

On Mon, Aug 22, 2011 at 5:54 PM, Edward Kmett wrote:

...
It would still be nice to have a consistent base case.

On Mon, Aug 22, 2011 at 3:43 AM, Johan Tibell wrote:

...
On Mon, Aug 22, 2011 at 5:55 AM, Edward Z. Yang wrote:

...
stg_newArrayzh in rts/PrimOps.cmm doesn't appear to give any indication, so this might be a good patch to add. But I'm curious: what would allocating Array#s of size 0 do? Null pointers? That sounds dangerous...

I would imagine that a zero sized array would be a StgArrPtrs header with its size field set to 0. It's not a very useful thing to have, I admit. If someone (Simon?) can confirm that we don't intend to support zero-length array I'll push a patch that adds a comment.

Johan

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Johan Tibell

4:38 p.m.

Right. That's an option. On Tue, Aug 23, 2011 at 10:31 PM, Simon Marlow wrote:

You could make it a precondition of copyArray#, although that's slightly less pleasant from a user's perspective.

Cheers, Simon

On 23/08/11 21:04, Johan Tibell wrote:

...
It could well be that it's some later primop that's failing due to the empty size, like my new copyArray# primop. If that's the case I could fix it but I would probably would have to add a branch to the copyArray# primop, which I'm reluctant to do.

On Tue, Aug 23, 2011 at 9:47 PM, Simon Marlow wrote:

...
An Array# of size zero is a perfectly reasonable thing. If it doesn't work, it should (and I vaguely recall making it work at some point in the past, but perhaps I failed to add a test and as a result it has rotted...)

Cheers, Simon

On 22/08/11 17:08, Johan Tibell wrote:

...
I agree (unless it has a performance cost). I had to fix a couple of bugs in my code associated with generating zero-length arrays.

On Mon, Aug 22, 2011 at 5:54 PM, Edward Kmett wrote:

...
It would still be nice to have a consistent base case.

On Mon, Aug 22, 2011 at 3:43 AM, Johan Tibell wrote:

...
On Mon, Aug 22, 2011 at 5:55 AM, Edward Z. Yang wrote: > > stg_newArrayzh in rts/PrimOps.cmm doesn't appear to give any > indication, > so this might be a good patch to add. But I'm curious: what would > allocating Array#s of size 0 do? Null pointers? That sounds > dangerous...

I would imagine that a zero sized array would be a StgArrPtrs header with its size field set to 0. It's not a very useful thing to have, I admit. If someone (Simon?) can confirm that we don't intend to support zero-length array I'll push a patch that adds a comment.

Johan

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Edward Kmett
Edward Z. Yang
Johan Tibell
Simon Marlow