cmd/compile: schedule in-register OpArg first

OpArgXXXReg values must be scheduled at the very top, as their
registers need to be live at the beginning before any other use
of the register.

Change-Id: Ic76768bb74da402adbe61db3b2d174ecd3f9fffc
Reviewed-on: https://go-review.googlesource.com/c/go/+/306329
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/ssa/schedule.go b/src/cmd/compile/internal/ssa/schedule.go
index c987647131..4e3e5e75e3 100644
--- a/src/cmd/compile/internal/ssa/schedule.go
+++ b/src/cmd/compile/internal/ssa/schedule.go
@@ -137,7 +137,14 @@ func schedule(f *Func) {
 			case v.Op == OpVarDef:
 				// We want all the vardefs next.
 				score[v.ID] = ScoreVarDef
-			case v.Op == OpArg || v.Op == OpArgIntReg || v.Op == OpArgFloatReg:
+			case v.Op == OpArgIntReg || v.Op == OpArgFloatReg:
+				// In-register args must be scheduled as early as possible to ensure that the
+				// context register is not stomped. They should only appear in the entry block.
+				if b != f.Entry {
+					f.Fatalf("%s appeared outside of entry block, b=%s", v.Op, b.String())
+				}
+				score[v.ID] = ScorePhi
+			case v.Op == OpArg:
 				// We want all the args as early as possible, for better debugging.
 				score[v.ID] = ScoreArg
 			case v.Type.IsMemory():