1986850 - wasm-gc: Improve precision of alias-checking for structure loads/stores

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, enhancement, P3)

Description

[Julian Seward [:jseward]]

At the moment, our alias analysis for struct loads/stores only differentiates
between the in-line data area, the out-line data area and the place where the
IL->OOL pointer is stored. That means any structure store invalidates
potential CSEs from other structure loads and severely limits the extent to
which structure loads can participate in GVN and LICM.

For example (at the MIR level),

    x1 = struct.get ptr1 field#0
    struct.set ptr2 field#42 <value>
    x2 = struct.get ptr1 field#0
    x3 = x1 + x2

the two struct.gets won't get commoned up even though the intervening
struct.set obviously can't alias them.

Additionally, quite a few MIR nodes have the default aliases-everything
AliasSet, even when this is overkill. In particular MWasmParameter is like
that, which presumably means that any use of MWasmParameter invalidates
potential CSEs from all preceding MIR nodes that read memory. We should fix
that too.

Comment 1

[Julian Seward [:jseward]]

(In reply to Julian Seward [:jseward] from comment #0)

AliasSet, even when this is overkill. In particular MWasmParameter is like
that, which presumably means that any use of MWasmParameter invalidates
potential CSEs from all preceding MIR nodes that read memory.

This claim is incorrect: MWasmParameter nodes are never moved away from
the start of entry block. So aliasing issues between them and other nodes
are a non-problem.

Comment 2

[Julian Seward [:jseward]]

Attachment: bug1986850-wasm-gc-tbaa.diff-1

WIP. First version that doesn't cause stuff (JS3/*-wasm) to crash.

Comment 3

[Julian Seward [:jseward]]

Attachment: Bug 1986850 - wasm-gc: Improve precision of alias-checking for structure loads/stores. r=bvisness,rhunt.

This improves alias checking for wasm wasm struct loads/stores, and has some
ridealong cleanups for other wasm alias-set markings.

• new method MWasmLoadField::mightAlias, TypesMightBeRelatedByInheritance,
  which add some better checks for struct aliasing

• ridealongs:

  • MWasmLoadField: set movable if there's no trap-site info
  • MWasmLoadField::congruentTo: tidy up (no functional change)

• ridealongs: MWasmNeg, MWasmStackArg WasmRegisterResult
  WasmFloatRegisterResult WasmBuiltinFloatRegisterResult WasmRegister64Result
  MWasmRefAsNonNull WasmRefCastAbstract WasmRefCastConcrete:
  add missing empty-alias-set annotations, as detected during testing

• ridealong: MIROps.yaml: WasmBoundsCheckRange32: add missing alias_set of
  none and missing guard marking (the latter an outright bug)

Comment 4

[Julian Seward [:jseward]]

Perf effects for the patch in comment 3 (Intel Tiger Lake, best of 10 runs):

                        before                after
                      cycles:u             cycles:u    improvement

j2cl-box2d-w     4,783,244,656        4,702,389,288       +1.7%
Df-complex-w    75,419,255,753       75,351,208,857       +0.1%
Df-todomvc-w    24,013,916,189       23,959,147,626       +0.2%
Ko-compose-w    80,062,108,322       80,146,126,481       -0.1%

Comment 5

[Julian Seward [:jseward]]

Remeasured. Is worth around 2% for j2cl-box2d and is a wash for
the other three. Best of 10 runs:

Total Score:
               basis    after
j2cl-box2d    157.27   160.51
Df-todomvc     23.35    23.47
Ko-compose      3.97     3.99
Df-complex      4.29     4.30


cycles:u, millions
               basis    after
j2cl-box2d     4,751    4,660 
Df-todomvc    23,962   23,939
Ko-compose    79,127   79,049
Df-complex    74,799   74,934


instructions:u, millions
               basis    after
j2cl-box2d    11,620   11,452 
Df-todomvc    29,171   29,151
Ko-compose   115,081  114,969
Df-complex   107,311  107,400


L1-dcache-load-misses:u, millions
               basis    after
j2cl-box2d     196        195
Df-todomvc     592        594
Ko-compose     1,859    1,863
Df-complex     1,655    1,651

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/ebe50f018f9b
https://hg.mozilla.org/integration/autoland/rev/fe37936b961b
wasm-gc: Improve precision of alias-checking for structure loads/stores.  r=bvisness,rhunt.

Comment 7

[Atila Butkovits]

https://hg.mozilla.org/mozilla-central/rev/fe37936b961b