[M3devel] a trouble with passing records by value..

[Tony Hosking]

On 31 Aug 2010, at 19:13, Jay K wrote:

> Another change we need is to check that the backend and frontend agree on the sizes and alignments of types.
> gcc does have a "packed" notion. It might be useful, like if it lets us set size/alignment and impedes repeat computation.

Why would it not?  We specify alignment...

> It might also be good to have the backend feed its computed sizes/alignments back to the front end.
> It'd be via more files.

YUCK!!!!!!!!!  Not a good plan...

> 
>  - Jay
> 
> ----------------------------------------
>> From: jay.krell at cornell.edu
>> To: hosking at cs.purdue.edu
>> Date: Tue, 31 Aug 2010 23:09:56 +0000
>> CC: m3devel at elegosoft.com
>> Subject: Re: [M3devel] a trouble with passing records by value..
>> 
>> 
>> I'm possibly going to try changing the target-specific code in gcc to never pass structs in registers.
>> Yucky.
>> 
>> I'm also going to try giving temporaries types.
>> Another m3cg change like pop_struct.
>> Given the latest internal error I saw.
>> Maybe review m3cg for more missing type information.
>> 
>> - Jay
>> 
>> ----------------------------------------
>>> From: jay.krell at cornell.edu
>>> To: hosking at cs.purdue.edu
>>> CC: m3devel at elegosoft.com
>>> Subject: RE: [M3devel] a trouble with passing records by value..
>>> Date: Tue, 31 Aug 2010 23:05:08 +0000
>>> 
>>> 
>>> t1 must still be passed in registers. The ABI can't be changed by that.
>>> 
>>> - Jay
>>> 
>>> ----------------------------------------
>>>> From: hosking at cs.purdue.edu
>>>> Date: Tue, 31 Aug 2010 09:15:32 -0400
>>>> To: jay.krell at cornell.edu
>>>> CC: m3devel at elegosoft.com
>>>> Subject: Re: [M3devel] a trouble with passing records by value..
>>>> 
>>>> What happens if you take the address of t inside ActionLookup?
>>>> What happens if you take the address of t1 inside main?
>>>> 
>>>> On 31 Aug 2010, at 07:25, Jay K wrote:
>>>> 
>>>>> 
>>>>> Given something like this:
>>>>> 
>>>>> jbook2:p247 jay$ more 1.c
>>>>> #include
>>>>> 
>>>>> typedef struct { long code; long value; } T1;
>>>>> 
>>>>> void ActionLookup(T1 t, long code, long value);
>>>>> 
>>>>> void ActionLookup(T1 t, long code, long value)
>>>>> {
>>>>> assert(t.code == code);
>>>>> assert(t.value == value);
>>>>> }
>>>>> 
>>>>> int main()
>>>>> {
>>>>> T1 t1 = {2,2};
>>>>> ActionLookup(t1, 2, 2);
>>>>> return 0;
>>>>> }
>>>>> j
>>>>> 
>>>>> 
>>>>> on some platforms, such as AMD64_DARWIN, T1 is passed in registers. Good.
>>>>> 
>>>>> 
>>>>> However, one of the unfortunate aspects of our Modula-3 system is that when you reference e.g. t1.value,
>>>>> the backend isn't told you are accessing the "value" "field" of "t1", and it figures out where that is,
>>>>> but rather 64bits at offset 64bits into t1. Therefore t1 must have an address. Therefore it can't be in registers.
>>>>> Darn.
>>>>> 
>>>>> 
>>>>> If m3cg were higher level this could be better.
>>>>> 
>>>>> 
>>>>> There should be a viable compromise where the parameter is passed in registers, and only "homed"
>>>>> to some stack location if its address is used -- e.g. to pass unused parameters in registers.
>>>>> But given the inefficiency of field accesses, I'm not sure it is worth trying?
>>>>> 
>>>>> 
>>>>> Maybe we should have M3CG include field references?
>>>>> 
>>>>> 
>>>>> There is this basic problem that the interface between m3front and m3cc isn't really at the
>>>>> right level for m3cc. But it is probably for m3front. m3front wants a lower level code generator.
>>>>> 
>>>>> 
>>>>> - Jay
>>>>> 
>>>> 
>>> 
>> 
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