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

[Tony Hosking]

On 31 Aug 2010, at 19:09, Jay K wrote:

> 
> I'm possibly going to try changing the target-specific code in gcc to never pass structs in registers.
> Yucky.

I strongly advise against that hack.

> 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.

This would be better...

> 
>  - 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
>>>> 
>>> 
>> 
>