cmd/compile: remove self copies in tail-call wrappers

The previous CL re-enables tail calls for method wrappers. But
with the changed IR and SSA representation, for stack arguments
it generates self copies. This CL makes the compiler detect the
self copies and remove them.

Change-Id: I7252572a1a47834f28b6706e45906e2356408e02
Reviewed-on: https://go-review.googlesource.com/c/go/+/350349
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/expand_calls.go b/src/cmd/compile/internal/ssa/expand_calls.go
index 79434f33d3..a0f0e653aa 100644
--- a/src/cmd/compile/internal/ssa/expand_calls.go
+++ b/src/cmd/compile/internal/ssa/expand_calls.go
@@ -1098,6 +1098,17 @@ func (x *expandState) rewriteArgs(v *Value, firstArg int) {
 			if a.MemoryArg() != m0 {
 				x.f.Fatalf("Op...LECall and OpDereference have mismatched mem, %s and %s", v.LongString(), a.LongString())
 			}
+			if v.Op == OpTailLECall {
+				// It's common for a tail call passing the same arguments (e.g. method wrapper),
+				// so this would be a self copy. Detect this and optimize it out.
+				a0 := a.Args[0]
+				if a0.Op == OpLocalAddr {
+					n := a0.Aux.(*ir.Name)
+					if n.Class == ir.PPARAM && n.FrameOffset()+x.f.Config.ctxt.FixedFrameSize() == aOffset {
+						continue
+					}
+				}
+			}
 			// "Dereference" of addressed (probably not-SSA-eligible) value becomes Move
 			// TODO(register args) this will be more complicated with registers in the picture.
 			mem = x.rewriteDereference(v.Block, sp, a, mem, aOffset, aux.SizeOfArg(auxI), aType, a.Pos)
@@ -1110,6 +1121,14 @@ func (x *expandState) rewriteArgs(v *Value, firstArg int) {
 			} else {
 				aOffset = aux.OffsetOfArg(auxI)
 			}
+			if v.Op == OpTailLECall && a.Op == OpArg && a.AuxInt == 0 {
+				// It's common for a tail call passing the same arguments (e.g. method wrapper),
+				// so this would be a self copy. Detect this and optimize it out.
+				n := a.Aux.(*ir.Name)
+				if n.Class == ir.PPARAM && n.FrameOffset()+x.f.Config.ctxt.FixedFrameSize() == aOffset {
+					continue
+				}
+			}
 			if x.debug > 1 {
 				x.Printf("...storeArg %s, %v, %d\n", a.LongString(), aType, aOffset)
 			}