Fix #14773: Reuse the param slots for the tailrec local mutable vars. #14865
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…vars.
The tailrec phase generates code that looks like
class Bar {
def foo(x1: Int, x2: Int): Int = {
var this$: Bar = this;
var taillocal$x1: Int = x1;
var taillocal$x2: Int = x2;
...
// body where `this`, `x1` and `x2` are replaced by
// `this$`, `taillocal$x1` and `taillocal$x2`
}
}
This generates bytecode where the `this` value and the parameters
never actually change, and are never used. Instead, the synthetic
mutable variables are used instead.
As described in the linked issue, this confuses debuggers, which
only display the never-changing original `this` value and
parameters.
In this commit, we intercept this shape of code in the back-end.
We reliable identify tailrec-generated `ValDef`s from their
semantic names, with an additional safety check that they are
`Synthetic | Mutable`. When we find this shape, we do not allocate
local slots for the `var`s, and instead reuse the slots for `this`
and the parameters. We skip past the `ValDef`s so that code
generation does not re-emit useless assignments.
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The tailrec phase generates code that looks like
This generates bytecode where the
thisvalue and the parameters never actually change, and are never used. Instead, the synthetic mutable variables are used instead.As described in the linked issue, this confuses debuggers, which only display the never-changing original
thisvalue and parameters.In this commit, we intercept this shape of code in the back-end. We reliable identify tailrec-generated
ValDefs from their semantic names, with an additional safety check that they areSynthetic | Mutable. When we find this shape, we do not allocate local slots for thevars, and instead reuse the slots forthisand the parameters. We skip past theValDefs so that code generation does not re-emit useless assignments.