[m-dev.] for review: a replacement of code_exprn.m

[Fergus Henderson]

On 06-Aug-2000, Zoltan Somogyi <zs at cs.mu.OZ.AU> wrote:
> compiler/pragma_c_gen.m:
> 	When using var_locn, ensure that none of the input arguments is
> 	assigned to r1, in order to avoid forcing the C compiler to generate
> 	code to shuffle it out of the way.

I don't understand that.
Why would the C compiler generate code to shuffle r1?

> compiler/mercury_compile.m:
> compiler/code_gen.m:
> 	Turn off tracing in any predicate that originates in a builtin module
> 	for backend_by_preds and backend_by_phases respectively.
>
> 	Look up options in the globals structure in the module_info, not in the
> 	globals structure in the I/O state, since this is where we turn off
> 	tracing. (We should later make sure that other parts of the compiler
> 	are also consistent on this issue.)

I think it would be much better to just set the globals in both the io__state
and the module_info.  Trying to ensure that other parts of the
compiler only access the globals via the module_info rather than via
the io__state is not going to work, because there are lots of places
in the compiler that only get passed the io__state, not the module_info.
Passing down the module_info to all of those places would be a nightmare...

> +++ call_gen.m	2000/08/03 08:53:28
> @@ -97,19 +98,14 @@
>  	code_info__get_instmap(InstMap),
>  	{ goal_info_get_instmap_delta(GoalInfo, InstMapDelta) },
>  	{ instmap__apply_instmap_delta(InstMap, InstMapDelta, ReturnInstMap) },
> -	{ call_gen__output_arg_locs(ArgsInfos, OutputArgLocs) },
> -		% We must update the code generator state to reflect
> -		% the situation after the call before building
> -		% the return liveness info. No later code in this
> -		% predicate depends on the old state.
> -	call_gen__rebuild_registers(ArgsInfos),
> -	code_info__generate_return_live_lvalues(OutputArgLocs, ReturnInstMap,
> -		ReturnLiveLvalues),
>  
> +		% Update the code generator state to reflect the situation
> +		% after the call.
> +	call_gen__handle_return(ArgsInfos, ReturnInstMap, ReturnLiveLvalues),
> +
>  		% Make the call.
>  	code_info__get_module_info(ModuleInfo),
	...

With this change, the code & comments are somewhat mystifying: why
does it "update the code generator state to reflect the sitatuation
after the call" *before* (generating code to) make the call?

Hence I prefer the old comment.

[... to be continued ...]

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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