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Re: Padding between char array/VLA in struct?

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On 8/3/2021 11:10 AM, Andrey Tarasevich wrote:
> On 8/3/2021 8:40 AM, BGB wrote:
>>
>> FWIW: My compiler (BGBCC) also supports VLAs, though only for local 
>> variables and similar. In these contexts, they effectively decay into 
>> pointers, and are transformed into an "alloca()" call.
>> ...
>> But, as can be noted, my compiler doesn't do anything too fancy to 
>> support "alloca()"; in effect this gets turned by the compiler into a 
>> special runtime call for "call malloc() and then add the returned 
>> pointer to a linked list", and then in the epilog a function call is 
>> added to free anything that was added to the list.
>>
>> I also make no claim that multidimensional VLAs "actually work"...
>> ...
> 
> It appears that the amount of effort you spent implementing your 
> pseudo-VLA would be quite sufficient to implement the standard-compliant 
> VLA. You just needed to channel that effort slightly differently. Which 
> makes one wonder why you decided to eschew the standard semantics...
> 

Partly it was because this was what seemed easier, and also allowed 
doing pretty much all of the VLA stuff to be handled in the front-end 
(things like lambdas are also handled in the frontend, as are many 
operations involving "variant" types, *1, ...).

With the ABI design, variable-sized stack frames would have added more 
complexity relative to fixed-size stack frames (there is no frame 
pointer or similar in this case, and everything on-stack is referenced 
using fixed displacements relative to the stack pointer, and 
prolog/epilog adjustments use fixed offsets, ...).


Note that this is generally for a target where:
   I am using 128K as the default stack size;
     (Except for interrupt handlers, which use an 8K stack).
   The target is (or assumes) No-MMU operation;
   The RAM space is typically measured in MB.

Using malloc ends up preferable for larger memory allocations (VLAs or 
large stack arrays), since it isn't bound by the stack-size limit, and 
doesn't assume spending inordinate amounts of RAM on the program stacks.

Though, a few cases exist where the total RAM is measured in KB (and the 
heap is just sorta wedged between ".bss" and the stack).


*1: The variant types are a non-standard extension which add dynamic 
type-checking via tagged pointers; and turns pretty much every operation 
on these types into a function call into the runtime.

...

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Padding between char array/VLA in struct? Ian Pilcher <arequipeno@gmail.com> - 2021-08-02 12:48 -0500
  Re: Padding between char array/VLA in struct? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-02 14:07 -0400
    Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-02 11:25 -0700
      Re: Padding between char array/VLA in struct? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-02 19:21 -0400
  Re: Padding between char array/VLA in struct? Bart <bc@freeuk.com> - 2021-08-02 19:11 +0100
    Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-02 11:27 -0700
      Re: Padding between char array/VLA in struct? Bart <bc@freeuk.com> - 2021-08-02 20:43 +0100
        Re: Padding between char array/VLA in struct? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-08-02 13:11 -0700
          Re: Padding between char array/VLA in struct? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-08-02 13:18 -0700
          Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-02 13:31 -0700
            Re: Padding between char array/VLA in struct? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-08-02 19:50 -0700
              Re: Padding between char array/VLA in struct? David Brown <david.brown@hesbynett.no> - 2021-08-04 14:01 +0200
        Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-02 13:19 -0700
        Re: Padding between char array/VLA in struct? BGB <cr88192@gmail.com> - 2021-08-03 10:40 -0500
          Re: Padding between char array/VLA in struct? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-08-03 09:10 -0700
            Re: Padding between char array/VLA in struct? BGB <cr88192@gmail.com> - 2021-08-03 12:52 -0500
              Re: Padding between char array/VLA in struct? BGB <cr88192@gmail.com> - 2021-08-04 12:05 -0500
  Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-02 11:12 -0700
    Re: Padding between char array/VLA in struct? Bart <bc@freeuk.com> - 2021-08-02 19:23 +0100
      Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-02 13:15 -0700
  Re: Padding between char array/VLA in struct? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-08-02 11:32 -0700
  Re: Padding between char array/VLA in struct? Ian Pilcher <arequipeno@gmail.com> - 2021-08-02 13:59 -0500
    Re: Padding between char array/VLA in struct? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-02 12:47 -0700
      Re: Padding between char array/VLA in struct? Ian Pilcher <arequipeno@gmail.com> - 2021-08-02 15:27 -0500
        Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-02 13:36 -0700
        Re: Padding between char array/VLA in struct? antispam@math.uni.wroc.pl - 2021-08-02 21:52 +0000
    Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-02 13:22 -0700
      Re: Padding between char array/VLA in struct? Manfred <noname@add.invalid> - 2021-08-04 17:01 +0200
        Re: Padding between char array/VLA in struct? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-04 10:11 -0700

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