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WASM to WAVM

Not all WASM instructions are 1:1 with WAVM opcodes. This document lists those which are not, and explains how they're expressed in WAVM. Many of the WAVM representations use opcodes not in WASM, which are documented in wavm-custom-opcodes.md.

block and loop​

In WASM, a block contains instructions. Branch instructions exit a fixed number of blocks, jumping to their destination. A normal block's destination is the end of the block, whereas a loop's destination is the start of the loop.

In WAVM, instructions are flat. At transpilation time, any branch instructions are replaced with jumps to the corresponding block's destination. This means that WAVM interpreters don't need to track blocks, and thus block instructions are unnecessary.

if and else​

These are translated to a block with an ArbitraryJumpIf as follows:

begin block with endpoint end
  conditional jump to else
  [instructions inside if statement]
  branch
  else: [instructions inside else statement]
end

br and br_if​

br and br_if are translated into ArbitraryJump and ArbitraryJumpIf respectively. The jump locations can be known at transpilation time, making blocks obsolete.

br_table​

br_table is translated to a check for each possible branch in the table, and then if none of the checks hit, a branch of the default level.

Each of the non-default branches has a conditional jump to a section afterwards, containing a drop for the selector, and then a jump to the target branch.

local.tee​

local.tee is translated to a WAVM Dup and then a LocalSet.

return​

To translate a return, the number of return values must be known from the function signature. A WAVM MoveFromStackToInternal is added for each return value. Then, a loop checks IsStackBoundary (which implicitly pops a value) until it's true and the stack boundary has been popped. Next, a MoveFromInternalToStack is added for each return value to put the return values back on the stack. Finally, a WAVM Return is added, returning control flow to the caller.

Floating point instructions​

A floating point library module must be present to translate floating point instructions. They are translated by bitcasting f32 and f64 arguments to i32s and i64s, then a cross module call to the floating point library, and finally bitcasts of any return values from i32s and i64s to f32s and f64s.