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

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.