TJump Optimization

Since trampoline-jump-if and trampoline functions are performance-critical, we are trying to optimize them as much as possible. The standard form of trampoline-jump-if looks like

if ctx, skip := otel_trampoline_onenter(&arg); skip {
    otel_trampoline_onexit(ctx, &retval)
    return ...
} else {
    defer otel_trampoline_onexit(ctx, &retval)
    ...
}

The obvious optimization opportunities are cases when onEnter or onExit hooks are not present. For the latter case, we can replace the defer statement to empty statement, you might argue that we can remove the whole else block, but there might be more than one trampoline-jump-if in the same function, they are nested in the else block, i.e.

if ctx, skip := otel_trampoline_onenter(&arg); skip {
    otel_trampoline_onexit(ctx, &retval)
    return ...
} else {
    ;
    ...
}

For the former case, it's a bit more complicated. We need to manually construct CallContext on the fly and pass it to onExit trampoline defer call and rewrite the whole condition to always false. The corresponding code snippet is

if false {
    ;
} else {
    defer otel_trampoline_onexit(&CallContext{...}, &retval)
    ...
}

The if skeleton should be kept as is, otherwise inlining of trampoline-jump-if will not work. During compiling, the dce and sccp passes will remove the whole then block. That's not the whole story. We can further optimize the tjump iff the onEnter hook does not use SkipCall. In this case, we can rewrite condition of trampoline-jump-if to always false, remove return statement in then block, they are memory-aware and may generate memory SSA values during compilation.

if ctx,_ := otel_trampoline_onenter(&arg); false {
    ;
} else {
    defer otel_trampoline_onexit(ctx, &retval)
    ...
}

The compiler responsible for hoisting the initialization statement out of the if skeleton, and the dce and sccp passes will remove the whole then block. All these trampoline functions looks as if they are executed sequentially, i.e.

ctx,_ := otel_trampoline_onenter(&arg);
defer otel_trampoline_onexit(ctx, &retval)

Note that this optimization pass is fraigle as it really heavily depends on the structure of trampoline-jump-if and trampoline functions. Any change in tjump should be carefully examined.

TJump Optimization ​

TJump Optimization