[PATCH] This patch introduces MemorySSA, a virtual SSA form for memory.Details on what it looks like are in MemorySSA.h

[Daniel Berlin]

On Wed, Feb 25, 2015 at 11:05 AM, James Molloy <james.molloy at arm.com> wrote:

> Hi Daniel,
>
> One inline comment about the design. Sorry for the naiveity.
>
> Cheers,
>
> James
>
>
> ================
> Comment at: include/llvm/Transforms/Utils/MemorySSA.h:150
> @@ +149,3 @@
> +// All defs also have a use
> +class MemoryDef : public MemoryUse {
> +public:
> ----------------
> I don't understand this: Firstly, why must all defs have a use?


So let me first give the theoretical answer:

MemorySSA is, abstractly about heap versions.
Every store generates a new heap version.
Every use is associated with a give heap version
In order to be able to know what the heap version was prior to a given
store, the def is also a use.

It is intended to perfectly represent the semantics of the actual memory
operations, but give a way to have factored use-def chains over memory
operations that you can disambiguate without needing to walk back through
all instructions in the CFG to find clobbers.


Practically:

Because you need some way to link def-def, or else you have nowhere to walk
if the current store is a "false def" for a given load.

IE given

store @a, %c
store @b, %d
load @a

MemorySSA for this will be:

; 1 = MemoryDef(0)
store @a, %c
;2 = MemoryDef(1)
store @b, %d
;MemoryUse(2)
%e = load @a

When you walk from the load at %e, you  get to the store at  2=
MemoryDef(1), say "does this really alias my load", get "no", and want to
keep going.  You can do this, because there is a def-def chain, so you go
from 2 = MemoryDef(1) to 1 = MemoryDef(0), ask the same question, get
"yes", and stop.

If def's did not have uses, it would look like this instead:

; 1 = MemoryDef()
store @a, %c
;2 = MemoryDef()
store @b, %d
;MemoryUse(2)
%e = load @a


Now you do the same walk.
You get to the store at 2 = MemoryDef(), query whether it is a real alias,
get "no", and you are stuck, because you have no chain that leads back to
the next store :)




We can store without loading the resulting value back.


> Secondly, you've used inheritance which is an "is-a" relationship. Is
> there something fundamental I'm missing here?
>


All memorydefs are in fact, memoryuses as well.
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