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Sign-extension operators proposal for WebAssembly

This page describes a proposal for the post-MVP sign-extension operator feature.

This proposal adds five new integer instructions for sign-extending 8-bit, 16-bit, and 32-bit values.

New Sign-extending Operators

To support sign-extending, five new sign-extension operators are added:

  • i32.extend8_s: extend a signed 8-bit integer to a 32-bit integer
  • i32.extend16_s: extend a signed 16-bit integer to a 32-bit integer
  • i64.extend8_s: extend a signed 8-bit integer to a 64-bit integer
  • i64.extend16_s: extend a signed 16-bit integer to a 64-bit integer
  • i64.extend32_s: extend a signed 32-bit integer to a 64-bit integer

Note that i64.extend32_s was not originally included when this proposal was discussed in the May 2017 CG meeting. The reason given was that the behavior matches i64.extend_s/i32. It was later discovered that this is not correct, as i64.extend_s/i32 sign-extends an i32 value to i64, whereas i64.extend32_s sign-extends an i64 value to i64. The behavior of i64.extend32_s can be emulated with i32.wrap/i64 followed by i64.extend_s/i32, but the same can be said of the sign-extending load operations. Therefore, i64.extend32_s has been added for consistency.

Spec Changes

The instruction syntax is modified as follows:

instr ::= ... |
          inn.extend8_s | inn.extend16_s | i64.extend32_s

The instruction binary format is modified as follows:

instr ::= ...
        | 0xC0                  =>  i32.extend8_s
        | 0xC1                  =>  i32.extend16_s
        | 0xC2                  =>  i64.extend8_s
        | 0xC3                  =>  i64.extend16_s
        | 0xC4                  =>  i64.extend32_s