cmd/compile: move []byte->string map key optimization to ssa

If we call slicebytetostring immediately (with no intervening writes)
before calling map access or delete functions with the resulting
string as the key, then we can just use the ptr/len of the
slicebytetostring argument as the key. This avoids an allocation.

Fixes #44898
Update #71132

There's old code in cmd/compile/internal/walk/order.go that handles
some of these cases.

1. m[string(b)]
2. s := string(b); m[s]
3. m[[2]string{string(b1),string(b2)}]

The old code handled cases 1&3. The new code handles cases 1&2.
We'll leave the old code around to keep 3 working, although it seems
not terribly common.

Case 2 happens particularly after inlining, so it is pretty common.

Change-Id: I8913226ca79d2c65f4e2bd69a38ac8c976a57e43
Reviewed-on: https://go-review.googlesource.com/c/go/+/640656
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/walk/order.go b/src/cmd/compile/internal/walk/order.go
index 858fc706ab..8967b7dbba 100644
--- a/src/cmd/compile/internal/walk/order.go
+++ b/src/cmd/compile/internal/walk/order.go
@@ -334,6 +334,14 @@ func (o *orderState) mapKeyTemp(outerPos src.XPos, t *types.Type, n ir.Node) ir.
 // It would be nice to handle these generally, but because
 // []byte keys are not allowed in maps, the use of string(k)
 // comes up in important cases in practice. See issue 3512.
+//
+// Note that this code does not handle the case:
+//
+//      s := string(k)
+//      x = m[s]
+//
+// Cases like this are handled during SSA, search for slicebytetostring
+// in ../ssa/_gen/generic.rules.
 func mapKeyReplaceStrConv(n ir.Node) bool {
 	var replaced bool
 	switch n.Op() {