[M3devel] zero sized structs?

[Antony Hosking]

On Oct 4, 2012, at 12:16 PM, Jay K <jay.krell at cornell.edu> wrote:

> Yes..the assertions all passes..but granted, maybe we don't have full coverage. I can look at the frontend, or just make it so in the backend.
> 
> 
> On further thought..by quick read..this M3x86.m3 code is questionable.
> While in_proc is usually false for declare_local, I think it really goes both ways.
> So it looks like size is sometimes rounded up to 4, sometimes left as zero.
> 
> 
> But what does it mean?
> Does a zero sized array get any storage?

No storage.

> In C and C++, two objects can't be at the same address, so, for example:
> struct foo { } bar; is not legal C. It is legal C++, however:

But you can have zero-length arrays in C.

> 
> 
> struct foo { } a,b;
> assert(&a != &b); will pass.
> 
> 
> and
> void* a = malloc(0);
> void* b = malloc(0);
> assert(a != b) will also pass in C and C++.
> 
> 
> That is, size 0 struct is invalid in C. In C++ it is rounded up to at least 1.
> malloc(0) is rounded up to at least 1.
> No two "objects" can have the same address.
> 
> 
> What is the meaning Modula-3?
> Rounding up to 1 or alignment seems easy/decent.
> Certainly these zero sized things are rare in our tree.
> 
> 
>  - Jay
> 
> 
> 
> CC: m3devel at elegosoft.com
> From: antony.hosking at gmail.com
> Subject: Re: [M3devel] zero sized structs?
> Date: Thu, 4 Oct 2012 09:21:28 -0400
> To: jay.krell at cornell.edu
> 
> Try to respect alignment. 
> 
> Sent from my iPad
> 
> On Oct 4, 2012, at 2:25 AM, Jay K <jay.krell at cornell.edu> wrote:
> 
> 
> 	declare_local	 noArgs 0 8 Struct -1522787086 T F 50 v.318
> 
> 
> and here is some relevant M3x86 code:
> 
>     IF u.in_proc THEN
>       v := get_temp_var (u, type, s, a, n);
>     ELSE
>       v := create_temp_var (u, type, s, a, n);
>     END;
> 
> 
> PROCEDURE get_temp_var (u: U; type: Type; s: ByteSize; a: Alignment;
>                         n: Name := M3ID.NoID): x86Var =
>   BEGIN
> 
>     (* round size and alignment up to 4 *)
> 
>     IF s < 4 THEN
>       s := 4;
>     END;
> 
>     IF a < 4 THEN
>       a := 4;
>     END;
> 
> 
> PROCEDURE create_temp_var (u: U; type: Type; s: ByteSize; a: Alignment;
>                            n: Name): x86Var =
>   VAR v := NewVar(u, type, 0, s, a, n);
>   BEGIN
>     v.loc := VLoc.temp;
>     v.parent := u.current_proc;
> 
>     u.current_proc.framesize := Word.And(u.current_proc.framesize + a - 1,
>                                          Alignmask[a]);
> 
>     INC(u.current_proc.framesize, s);
> 
>     v.offset := -u.current_proc.framesize;
> 
>     RETURN v;
>   END create_temp_var;
> 
> 
> which isn't clear by inspection...it actually looks like
> zero size is allowed through commonly...in_proc is rare,
> so create is common...assuming the frame is already aligned,
> the size remains unchanged...
> 
> 
> Relevant aside: I have been ignoring alignment.
> I'm inclined to go with something like:
> 
> <* ASSERT (size MOD alignment) = 0 *>
> 
> 
> If that ever fails, then I will change it to:
> 
> 
> FUNCTION RoundUp(a, b: INTEGER): INTEGER =
> BEGIN
>   WITH c = a MOD b DO
>     IF c # 0 THEN
>       RETURN a + b - c;
>     END;
>   END;
>   RETURN a;
> END;
> 
> size := RoundUp(size, alignment);
> 
> 
> and then, more to the point, I'll go with:
> 
> IF size = 0 THEN
>   size = 1;
> END;
> 
> 
> I haven't dug into what the gcc backend would do here, too much
> to dig through. :)
> 
> 
> Thoughts?
> 
> 
> I guess I should try both m3x86 and m3cc, and print the address
> of these things????
> 
> 
>  - Jay
> 
> 
> 
> From: jay.krell at cornell.edu
> To: m3devel at elegosoft.com
> Date: Thu, 4 Oct 2012 06:13:28 +0000
> Subject: [M3devel] zero sized structs?
> 
> Building obliqrt I get:
> 
> 
> ***
> *** runtime error:
> ***    <*ASSERT*> failed.
> ***    file "../src/M3C.m3", line 1768
> ***
> 
> 
> 
> PROCEDURE GetStructSizes_Declare(self: GetStructSizes_t; type: Type; byte_size: ByteSize): M3CG.Var =
> BEGIN
>     IF type = Type.Struct THEN
>         <* ASSERT byte_size > 0 *>
>         self.sizes[self.count] := byte_size;
>         INC(self.count);
>     END;
>     RETURN NIL;
> END GetStructSizes_Declare;
> 
> 
> due to presumably:
> 
> 
> PROCEDURE ApplyThreadClosure (self: ObliqThreadClosure): REFANY =
>   VAR noArgs: ARRAY [0 .. -1] OF ObValue.Val;
> 
> 
> ...
> 
> PROCEDURE HandleWork (self: ObliqWork) =
>   VAR noArgs: ARRAY [0 .. -1] OF ObValue.Val;
> 
> ..
> PROCEDURE EvalThread (                    self  : PackageThread;
> ...
> noArgs    : ARRAY [0 .. -1] OF ObValue.Val;
> 
> 
> What is the meaning of this?
> 
> 
>  - Jay

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