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cmd/compile/internal/noder: allow OCONVNOP for identical iface conversions

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In go.dev/cl/421821, I included a hack to force OCONVNOP back to
OCONVIFACE for conversions involving shape types and non-empty
interfaces. The comment correctly noted that this was only needed for
conversions between non-identical types, but the code was conservative
and applied to even conversions between identical types.

This CL adds an extra bool to record whether the conversion is between
identical types, so we can keep OCONVNOP instead of forcing back to
OCONVIFACE. This has a small improvement to generated code, because we
no longer need a convI2I call (as demonstrated by codegen/ifaces.go).

But more usefully, this is relevant to pruning unnecessary itab slots
in runtime dictionaries (next CL).

Change-Id: I94f89e961cd26629b925037fea58d283140766ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/427678
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
This commit is contained in:
Matthew Dempsky 2022-09-01 16:06:11 -07:00
parent 8af6c3348e
commit 34f0029a85
3 changed files with 27 additions and 2 deletions
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7
src/cmd/compile/internal/noder/reader.go
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@ -2432,6 +2432,7 @@ func (r *reader) expr() (res ir.Node) {
pos := r.pos()
typeWord, srcRType := r.convRTTI(pos)
dstTypeParam := r.Bool()
identical := r.Bool()
x := r.expr()
// TODO(mdempsky): Stop constructing expressions of untyped type.
@ -2460,8 +2461,10 @@ func (r *reader) expr() (res ir.Node) {
// Should this be moved down into typecheck.{Assign,Convert}op?
// This would be a non-issue if itabs were unique for each
// *underlying* interface type instead.
if n, ok := n.(*ir.ConvExpr); ok && n.Op() == ir.OCONVNOP && n.Type().IsInterface() && !n.Type().IsEmptyInterface() && (n.Type().HasShape() || n.X.Type().HasShape()) {
n.SetOp(ir.OCONVIFACE)
if !identical {
if n, ok := n.(*ir.ConvExpr); ok && n.Op() == ir.OCONVNOP && n.Type().IsInterface() && !n.Type().IsEmptyInterface() && (n.Type().HasShape() || n.X.Type().HasShape()) {
n.SetOp(ir.OCONVIFACE)
}
}
// spec: "If the type is a type parameter, the constant is converted