Why does a 'new' statement produce so many temporary variables

Andrew Haley aph@redhat.com
Mon Aug 15 09:40:00 GMT 2011


On 08/09/2011 08:18 AM, Li junsong wrote:
> Hi,
> 
> I am writing a plugin to add a pass in gcc 4.6.1. the pass will do
> some optimizations before high gimple lowering,
> moving some gimple nodes from one place to another. In cc1, the gimple
> code is quite
> understandable.  But I'm confused by the gimple that Java front end converts to.
> 
> I dump the file, here is a piece of code doing "new".
> 
> #ref#2#4 = _Jv_AllocObjectNoFinalizer (&Number.class$$);
> #ref#3#6 = #ref#2#4;
> #ref#2#4 = #ref#3#6;
> D.459 = #slot#1#1;
> #slot#4#7 = D.459;
> #ref#3#6.1 = #ref#3#6;
> (#ref#3#6.1, #slot#4#7);
> 
> the corresponding Java source code is
> 
>         int sum = 4;
>         Number number;
>         if ( sum < 100 )
>             number = new Number(sum);   <=here
> 
> the Number class is a simple one:
> 
> class Number
> {
>     int i;
>     Number(int inputi){
>         i = inputi;
>     }
> }
> 
> Here comes my question:
>  1. I am wondering why we don't use 'NEW_EXPR' to represent a "new" statement?
>      The 'NEW_EXPR' has already been defined in the cp-tree.def.( As I know, the
>      "new" statement in cp front end is also represented as that in
> Java front end,
>      which is "gimple_call" in gimple code, "ADDR_EXPR" in tree code,
> is it? why?)

Why would we want to use 'NEW_EXPR' ?  We simply call the allocator
which creates the object.

>  2. How can I figure out which pieces of code are corresponding
>      to 'NEW' statement? ( Can I always first find the
> "_Jv_AllocObjectNoFinalize" or "_Jv_AllocObject"
>      gimple_call and then "<init>" gimple_call to locate the code?)

I think so.

>  3. I find that there is no document to describe the implementation of
> Java front end.
>      I don't understand why we need so many temporary variables:
> 
>      #ref#2#4 = _Jv_AllocObjectNoFinalizer (&Number.class$$);
>      #ref#3#6 = #ref#2#4;
>      #ref#2#4 = #ref#3#6;
> 
>      Here, the #ref#3#6 and #ref#2#4 is a kind waste. But each "new" statement
>      does the same thing.

It's not really a waste: we assume that the optimizer will delete
useless temporaries, and as far as I know it does.

Andrew.



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