[llvm-commits] [PATCH] Allow SelectionDAGBuilder to reorder loads past stores

[Andrew Trick]

On Dec 19, 2011, at 9:42 AM, Hal Finkel wrote:

> The current SelectionDAGBuilder does not allow loads to be reordered
> past stores, and does not allow stores to be reordered. This is a side
> effect of the way the critical chain is constructed: there is a queue of
> pending loads that is flushed (in parallel) to the root of the chain
> upon encountering any store (and that store is also appended to the root
> of the chain). Among other things, loop unrolling is far less effective
> than it otherwise could be.
> 
> The attached patch allows SelectionDAGBuilder to use the available alias
> analysis to reorder independent loads and stores. It changes the queue
> of pending loads into a more general queue of pending memory operations,
> and flushes, in parallel, all potentially-conflicting loads and stores
> as necessary.

Hal,

I haven't tried out your patch, so correct me if I'm wrong. I don't
understand how this can work with a simple alias analysis interface
like we have in LLVM.

Root
ld A
st B
st C

mayAlias(A,B) = true
mayAlias(A,C) = true
mayAlias(B,C) = false

So we have the chain Root->A->B and Root->C which may result in the schedule:

st C
ld A
st B

It seems that we have to order all stores if we want them to "cover" preceding loads. What am I missing?

-Andy