Initialize tags back to 0 before using it.

Initialize tags back to 0 before using it.
Committed: https://code.google.com/p/dart/source/detail?r=6711

[cshapiro, 2012-04-18 20:52:00]

lgtm

https://chromiumcodereview.appspot.com/10021046/diff/3/vm/object.cc
File vm/object.cc (right):

https://chromiumcodereview.appspot.com/10021046/diff/3/vm/object.cc#newcode8539
vm/object.cc:8539: tags = 0;
This seems like a strange idiom but I may not understand what is actually going
on.  Rather than doing this

  tag = 0
  tag = DoSomething(tag)
  raw->ptr()->tags_ = tags

Why not just do

  uword tag = DoSomething(0)
  raw->ptr()->tags_ = tag

It was unclear to me what the effect of the tags = 0 assignment was when I
looked at this code.  Keeping the def and use as close together as possible
would be great.

[siva, 2012-04-18 21:21:17]

https://chromiumcodereview.appspot.com/10021046/diff/3/vm/object.cc
File vm/object.cc (right):

https://chromiumcodereview.appspot.com/10021046/diff/3/vm/object.cc#newcode8539
vm/object.cc:8539: tags = 0;
This is in preparation for merging the tags fields and the header field into one
word.

The pattern at that point will look as:

header_bits = 0;
header_bits = UpdateTag info...
header_bits = UpdateSize...
header_bits = UpdateClassId ...
raw->ptr()->header_ = header_bits;


On 2012/04/18 20:52:00, cshapiro wrote:
> This seems like a strange idiom but I may not understand what is actually
going
> on.  Rather than doing this
> 
>   tag = 0
>   tag = DoSomething(tag)
>   raw->ptr()->tags_ = tags
> 
> Why not just do
> 
>   uword tag = DoSomething(0)
>   raw->ptr()->tags_ = tag
> 
> It was unclear to me what the effect of the tags = 0 assignment was when I
> looked at this code.  Keeping the def and use as close together as possible
> would be great.