Stream: git-wasmtime

Topic: wasmtime / Issue #2611 Implement limiting WebAssembly exe...

view this post on Zulip Wasmtime GitHub notifications bot (Jan 27 2021 at 22:44):

cfallin commented on Issue #2611:

I'll take a look -- thanks for putting this together so quickly! A few quick notes on the isel thoughts:

For example one optimization might be to not have a fuel_var and instead periodically do addq $fuel_consumed, offset(%vminterrupts_ptr) which avoids consuming extra registers. Similarly cmpq $0, offset(%vminterrupts_ptr) could be generated as well

The former is an RMW compound op and could be done pretty reasonably with the current load-op merging framework; we haven't gotten around to it yet, but it's clear how to do so if we decide it's important. The latter is actually a bit more interesting: I recently fixed a bug (#2576) that arose because of certain combinations of load-and-compare that would sink a load beyond its first use by simply disallowing compare-from-memory. That too can be re-optimized but would take more machinery.

Anyway, all that to say that there are reasons you couldn't get either of those instructions to come out the backend :-)

view this post on Zulip Wasmtime GitHub notifications bot (Jan 27 2021 at 22:53):

alexcrichton commented on Issue #2611:

Oh also to clarify, I don't think it's just isel that can be improved, but rather I think the frontend codegen could also be improved if we decide the time should be invested. For example an empty function (func nop) generates this x86 code with the new backend (annotated by me): 

0000000000000000 _wasm_function_4:
       ;; prologue stack-check to ensure we're not out of stack space
       ;; note that this is only here because function calls are made internally
       0: 55                            pushq   %rbp
       1: 48 89 e5                      movq    %rsp, %rbp
       4: 4c 8b 17                      movq    (%rdi), %r10
       7: 4d 8b 12                      movq    (%r10), %r10
       a: 49 39 e2                      cmpq    %rsp, %r10
       d: 0f 86 02 00 00 00             jbe 2 <_wasm_function_4+0x15>
      13: 0f 0b                         ud2

      ;; allocating stack space, saving registers
      15: 48 83 ec 10                   subq    $16, %rsp
      19: 4c 89 24 24                   movq    %r12, (%rsp)

      ;; load vminterrupts_ptr into r12
      1d: 4c 8b 27                      movq    (%rdi), %r12

      ;; increment the fuel consumed by 1 where 8(%r12) is where the previous count is
      20: be 01 00 00 00                movl    $1, %esi
      25: 49 03 74 24 08                addq    8(%r12), %rsi

      ;; check if fuel is gerater than zero
      2a: 48 31 c0                      xorq    %rax, %rax
      2d: 48 39 c6                      cmpq    %rax, %rsi
      30: 0f 8c 13 00 00 00             jl  19 <_wasm_function_4+0x49>

      ;; save the fuel count, load the intrinsic address, call it
      36: 49 89 74 24 08                movq    %rsi, 8(%r12)
      3b: 48 8b b7 f0 06 00 00          movq    1776(%rdi), %rsi
      42: ff d6                         callq   *%rsi

      ;; restore the fuel_var for this function (unnecessary instruction, happens uncondititonally after call)
      44: 49 8b 74 24 08                movq    8(%r12), %rsi

      ;; save fuel_var at the end of the function (unnecessary instruction, happens unconditionally at function end)
      49: 49 89 74 24 08                movq    %rsi, 8(%r12)

      ;; function epilogue
      4e: 4c 8b 24 24                   movq    (%rsp), %r12
      52: 48 83 c4 10                   addq    $16, %rsp
      56: 48 89 ec                      movq    %rbp, %rsp
      59: 5d                            popq    %rbp
      5a: c3                            retq

Larger functions like https://github.com/bytecodealliance/sightglass/blob/faf101e50f7af6243fe3e910cf9455eecd9e1368/benchmarks-next/shootout-ackermann/benchmark.wat#L25-L49 end up having a lot more code generated with instrumentation enabled. I haven't yet gone through instruction-by-instruction yet, though, to see what could be improved.

view this post on Zulip Wasmtime GitHub notifications bot (Jan 27 2021 at 23:54):

github-actions[bot] commented on Issue #2611:

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view this post on Zulip Wasmtime GitHub notifications bot (Jan 28 2021 at 21:47):

alexcrichton commented on Issue #2611:

Ok this should be rebased and updated to have add_fuel

Last updated: Dec 23 2024 at 12:05 UTC