cmd/compile: don't merge load+op if other op arg is still live

We want to merge a load and op into a single instruction

    l = LOAD ptr mem
    y = OP x l

into

    y = OPload x ptr mem

However, all of our OPload instructions require that y uses
the same register as x. If x is needed past this instruction, then
we must copy x somewhere else, losing the whole benefit of merging
the instructions in the first place.

Disable this optimization if x is live past the OP.

Also disable this optimization if the OP is in a deeper loop than the load.

Update #19595

Change-Id: I87f596aad7e91c9127bfb4705cbae47106e1e77a
Reviewed-on: https://go-review.googlesource.com/38337
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/likelyadjust.go b/src/cmd/compile/internal/ssa/likelyadjust.go
index 8a2fe1bbd2..72f0ae9c48 100644
--- a/src/cmd/compile/internal/ssa/likelyadjust.go
+++ b/src/cmd/compile/internal/ssa/likelyadjust.go
@@ -443,6 +443,14 @@ func (ln *loopnest) findExits() {
 	ln.initializedExits = true
 }
 
+// depth returns the loop nesting level of block b.
+func (ln *loopnest) depth(b ID) int16 {
+	if l := ln.b2l[b]; l != nil {
+		return l.depth
+	}
+	return 0
+}
+
 // recordIfExit checks sl (the loop containing b) to see if it
 // is outside of loop l, and if so, records b as an exit block
 // from l and returns true.