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 i32
s and i64
s,
then a cross module call to the floating point library,
and finally bitcasts of any return values from i32
s and i64
s to f32
s and f64
s.