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Date: Fri, 23 May 2008 20:27:37 +0200
From: Fabrice Bellard <fabrice@bellard.org>
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Subject: Re: [Qemu-devel] [4544] added tcg_temp_free() and improved the
	handling of constants
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	<200805231903.35923.paul@codesourcery.com>
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To: qemu-devel@nongnu.org
Cc: Paul Brook <paul@codesourcery.com>

I will update the TCG documentation soon to explain it and other 
modifications I am about to do.

In short, tcg_const() is now exactly equivalent to a temporary, hence it 
should be freed. Freeing temporaries is useful to reduce translation 
time and memory usage. However it has little consequences on the quality 
of the generated code.

Fabrice.

Paul Brook wrote:
>> added tcg_temp_free() and improved the handling of constants
> 
> Should the return value of tcg_const_i32 et. al. be considered immutable, or 
> is it ok to use it as a temporary?
> 
> e.g.:
> 
> static TCGv do_frob(TCGv base, int addend)
> {
>   TCGv tmp = tcg_const_i32(addend);
>   tcg_gen_helper_frob(tmp, tmp, base)
>   return tmp;
> }
> 
> or should this be written as:
> 
> static TCGv do_frob(TCGv base, int addend)
> {
>   TCGv tmp = tcg_temp_new(TCG_TYPE_I32);
>   tcg_gen_movi_i32(tmp, addend);
>   tcg_gen_helper_frob(tmp, tmp, base)
>   return tmp;
> }
> 
> Currently it's fairly hard to enforce this restriction automatically, but we 
> need to decide whether it's a feature or a bug.
> 
> Paul
>