Nondeterminism in WebAssembly

WebAssembly is a portable sandboxed platform with limited, local, nondeterminism.

  • Limited: nondeterministic execution can only occur in a small number of well-defined cases (described below) and, in those cases, the implementation may select from a limited set of possible behaviors.
  • Local: when nondeterministic execution occurs, the effect is local, there is no “spooky action at a distance”.

The rationale document details why WebAssembly is designed as detailed in this document.

The following is a list of the places where the WebAssembly specification currently admits nondeterminism:

  • New features will be added to WebAssembly, which means different implementations will have different support for each feature. This can be detected with has_feature, but is still a source of differences between executions.
  • When threads are added as a feature :unicorn:, even without shared memory, nondeterminism will be visible through the global sequence of API calls. With shared memory, the result of load operators is nondeterministic.
  • Except when otherwise specified, when an arithmetic operator returns NaN, there is nondeterminism in determining the specific bits of the NaN. However, wasm does still provide the guarantee that NaN values returned from an operation will not have 1 bits in their fraction field that aren’t set in any NaN values in the input operands, except for the most significant bit of the fraction field (which most operators set to 1).
  • Except when otherwise specified, when an arithmetic operator with a floating point result type receives no NaN input values and produces a NaN result value, the sign bit of the NaN result value is nondeterministic.
  • Fixed-width SIMD may want some flexibility :unicorn:
    • In SIMD.js, floating point values may or may not have subnormals flushed to zero.
    • In SIMD.js, operators ending in “Approximation” return approximations that may vary between platforms.
  • Environment-dependent resource limits may be exhausted. A few examples:
    • Memory allocation may fail.
    • The runtime can fail to allocate a physical page when a memory location is first accessed (e.g. through a load or store), even if that memory was virtually reserved by the maximum size property of the memory section.
    • Program stack may get exhausted (e.g., because function call depth is too big, or functions have too many locals, or infinite recursion). Note that this stack isn’t located in the program-accessible linear memory.
    • Resources such as handles may get exhausted.
    • Any other resource could get exhausted at any time. Caveat emptor.

Users of C, C++, and similar languages should be aware that operators which have defined or constrained behavior in WebAssembly itself may nonetheless still have undefined behavior at the source code level.