FYI: proposed changes to atomic load/store in SelectionDAG

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I have a set of changes out for review which are possibly note worthy, and backend contributors may wish to be aware of.

TLDR: atomic loads as normal LoadSDNodes w/an “isAtomic” flag.

Background

At the moment, we lower all atomic loads and stores as instances of AtomicSDNode (along with cmpxchg, and atomicrmw). This requires us to duplicate any isel rules we wish to apply for atomic loads or stores, but does have the nice property that it’s harder to introduce a silent miscompile by adding an transform which forgets about atomicity.

Proposed End Result

Represent atomic loads and stores as normal LoadSDNode or StoreSDNodes. Analogously to volatility, provide a flag on the node (stored in the MMO) which indicates whether the operation is atomic. All transforms updated to check isAtomic if needed.

The advantages of this representations are:

  1. Once the audit has been done, it makes it easier to keep atomic and non-atomic rules in sync.
  2. It makes GlobalISEL easier (by eliminating the need for the special case).
  3. Unify patterns flowing through other backend passes (i.e. unordered atomics and non-atomics shouldn’t generate radically different MI structures)

One open question is whether we do this just for unordered atomics, or for all atomics. I’d be open to either, but would start with just unordered to start with either way.

Migration Plan

This would be done on a per-backend basis, and to start with, I’m only proposing to port X86.

The basic strategy I plan on taking is:

  1. introduce infrastructure and a flag for testing ()
  2. Audit uses of isVolatile, and apply isAtomic conservatively*
  3. piecemeal conservative* update generic code and x86 backedge code in individual reviews w/tests for cases which didn’t check volatile, but can be found with inspection
  4. flip the (x86) flag at the end (with minimal diffs)
  5. Work through todo list identified in (2) and (3) exposing performance ops

(*) The “conservative” bit here is aimed at minimizing the number of diffs involved in (4). Ideally, there’d be none. In practice, getting it down to something reviewable by a human is the actual goal. Note that there are (currently) no paths which produce LoadSDNode or StoreSDNode with atomic MMOs, so we don’t need to worry about preserving any behavior there.

We’ve taken a very similar strategy twice before with success - once at IR level, and once at the MI level (post ISEL). I’ll probably need some help with some of the ISEL patterns since that’s the part I’m not familiar with.

Thoughts?

Philip

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I think you nailed the major concern, which is that compilers tend to have bugs where they accidentally fold atomic or volatile loads when they shouldn’t.

It sounds like you plan to do the audit and phase things in with a flag. The flag gives me some confidence that downstream consumers will be able to test this before it’s turned on by default. So, if you’re motivated enough to do the audit, go for it. :slight_smile:

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All of the changes in this sequence have landed, and the default for X86 has been flipped in git 246098.

There will be some cleanup work and further optimization work in a couple of weeks, but I’m going to give this a bit of burn-in period before doing anything else to give problems time to be reported.

Philip