Experimental cmp+select instruction fusion #1491
Description
Currently Wasmi only fuses comparison (such as i32.eq, f64.lt) and conditional branch instructions (such as if and br_if) together.
This led to a huge speed-up in interpreter performance due to Wasmi having to execute fewer instructions and having to perform fewer stack accesses.
For Wasm's select instructions Wasmi currently already has quite a few instruction variants, mostly to deal with true_val and false_val immediate values.
This issue tracks progress and evaluates the idea to re-design Wasmi's select intruction to allow for fusing it with comparison instructions similar to the existing cmp+branch op-fusion.
This new design necessitates to no longer care about true_val and false_val immediates and simply allocate immediate values there as function local constants. So we end up with the following set of select instruction variants:
select_{i32,i64}_{eq,ne,and,or,xor,nand,nor,xnor}select_{i32,i64}_{lt,le}_{s,u}select_{f32,f64}_{eq,ne,lt,le,not_lt,not_le}