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RTS: fix pages calculation in grow_memory #2633

Merged
merged 3 commits into from
Jun 30, 2021
Merged

RTS: fix pages calculation in grow_memory #2633

merged 3 commits into from
Jun 30, 2021

Commits on Jun 30, 2021

  1. RTS: fix pages calculation in grow_memory 

    Originally discussed here: #2573 (comment)

Previously grow_memory would allocate one more page than necessary when
the argument is a multiple of Wasm page size. New calculation fixes
that. The code is basically

    (ptr + WASM_PAGE_SIZE - 1) / WASM_PAGE_SIZE

I.e. divide argument by WASM_PAGE_SIZE, round up. However we do this in
u64 to avoid overflows in `ptr` is larger than `u32::MAX -
WASM_PAGE_SIZE + 1`

Diff of mo-rts.wasm before and after:

    --- mo-rts.wat  2021-06-30 08:50:10.319312072 +0300
    +++ ../../motoko_2/rts/mo-rts.wat       2021-06-30 08:50:08.507322577 +0300
    @@ -18760,12 +18760,14 @@
       (func $motoko_rts::alloc::alloc_impl::grow_memory::hd1a4f99ad46d13e9 (type 7) (param i32)
         block  ;; label = @1
           local.get 0
    -      i32.const 16
    -      i32.shr_u
    +      i64.extend_i32_u
    +      i64.const 65535
    +      i64.add
    +      i64.const 16
    +      i64.shr_u
    +      i32.wrap_i64
           memory.size
           i32.sub
    -      i32.const 1
    -      i32.add
           local.tee 0
           i32.const 1
           i32.lt_s

So two extra instructions.

CanCan backend benchmark reports 0.004% increase in cycles (42,055,656
to 42,057,358, +1,702).
    @osa1
    osa1 committed Jun 30, 2021
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  2. RTS: Slightly improve grow_memory performance 

    Improves cycles in a benchmark from 42,057,358 to 42,056,507 (-0.002%),
moving `total_pages - current_pages` to the slow path.
    @osa1
    osa1 committed Jun 30, 2021
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  3. Update rts/motoko-rts/src/alloc/gc.rs 

    Co-authored-by: Claudio Russo <claudio@dfinity.org>
    @osa1 @crusso
    osa1 and crusso authored Jun 30, 2021
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