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>