[LLVMdev] LLVM IR in DAG form

[Jeehoon Kang]

Thank you David and Jeremy!

I am quite convinced that LLVM IR in SSA form already expresses data
dependence quite well, as said David and Jeremy. Expressing IR in DAG may
enable more optimizations on memory operations, but the benefit seems to be
not so much.

Furthermore, I strongly agree with Jeremy in that instruction orders should
be preserved for -O1 for debugging purposes.

Thank you,
Jeehoon


2015-02-21 21:41 GMT+09:00 Jeremy Lakeman <Jeremy.Lakeman at gmail.com>:

>
>
> On Sat, Feb 21, 2015 at 6:38 PM, David Chisnall <
> David.Chisnall at cl.cam.ac.uk> wrote:
>
>>
>> > On 21 Feb 2015, at 05:59, Jeehoon Kang <jeehoon.kang at sf.snu.ac.kr>
>> wrote:
>> >
>> > this is Jeehoon Kang, a CS PhD student and a newbie to LLVM.
>> >
>> > I am wondering why LLVM IR's basic block consists of a list of
>> instructions,
>> > rather than a DAG of instruction as in the low level (ISelectionDAG).
>>
>> SSA form is implicitly a DAG, defined by the uses relation (registers in
>> LLVM can be thought of as edges between instructions).  It is not *solely*
>> a DAG, however.  For example, in some places the order of two loads matters
>> - particularly when they are atomic operations - it's only side-effect-free
>> operations that can be represented entirely as a DAG.  In general,
>> optimisations that work best with a DAG representation deal with use-def
>> chains and are not explicitly aware of the sequence of instructions in the
>> basic blocks unless they need to be.
>>
>> The order of loads is still essentially a directed graph. Currently that
> information is implicit in the basic block order, and optimisations need to
> know if it is safe to screw around with them. Perhaps these relationships
> would be better represented explicitly instead, in which case the order of
> instructions in a block would be less relevant.
>
> Though of course machine instructions need to be ordered, -O0 shouldn't
> mess with the order of operations for debugging purposes, and you do need
> some deterministic way to iterate over instructions. So I'm not certain
> there'd be much benefit in trying to remove the current ordering of
> instructions. If you want to walk the instructions as a DAG you can, if you
> want to walk them in execution order you can do that too.
>
>
>> David
>>
>>
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>
>


-- 
Jeehoon Kang (Ph.D. student) <http://sf.snu.ac.kr/jeehoon.kang>
Software Foundations Laboratory <http://sf.snu.ac.kr>
Seoul National University <http://www.snu.ac.kr>
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