Re: [omr-dev] ILValue load() method

["Mark Stoodley" <mstoodle@xxxxxxxxxx>]

It is a rule, not an optimization: nodes
can only be referenced directly within a single (extended) block. To communicate
a value from one block to another requires storing to a temp and then loading
in the other (extended) block.

A block is the standard basic block
of compiler textbooks: a single entry and single normal exit flow (exception
flows don't count).

An extended block is a linear sequence
of blocks with no entry points other than the first one in the sequence.
There can be "side" exits (at the end of any block in the sequence).

At the beginning of a compilation, there
are only basic blocks. An optimization pass called basic block extension
(done as part of block ordering) identifies blocks that can be combined
into extended basic blocks and adds a flag to the second and subsequent
blocks that you can see in JIT logs, e.g.:
        n17n
     BBStart <block_4>(freq 302) (extension of previous block) (in loop 3)

The idea is that if you're evaluating
a node X in an extended basic block, you can be sure that you have already
evaluated every tree top from the beginning of the block until the tree
that anchors X .

Mark Stoodley		8200 Warden Avenue
Senior Software Developer		Markham, L6G 1C7
IBM Runtime Technologies		Canada
Phone:	+1-905-413-5831
e-mail:	mstoodle@xxxxxxxxxx
We cannot solve our problems with the same thinking we used when we created them - Albert Einstein

From:      
 Dibyendu Majumdar <mobile@xxxxxxxxxxxxxxx>
To:      
 omr developer discussions
<omr-dev@xxxxxxxxxxx>
Date:      
 2018/06/12 07:21 PM
Subject:    
   [omr-dev] ILValue
load() method
Sent by:    
   omr-dev-bounces@xxxxxxxxxxx

---

Hi,

I notice that ILValue::load() method creates a temporary and stores
the value in the Block that computed the value, if the value is being
loaded in another block. I would like to understand why this is
necessary. Is it a rule that a value computed in one block cannot be
accessed directly in another block unless it is stored in the original
block? Is this an optimization of some sort?

Regards
Dibyendu
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