cmd/compile: rework/reduce partially lived argument spilling

In CL 307909 we generate code that spills pointer-typed argument
registers if it is part of an SSA-able aggregate. The current
code spill the register unconditionally. Sometimes it is
unnecessary, because it is already spilled, or it is never live.

This CL reworks the spill generation. We move it to the end of
compilation, after liveness analysis, so we have information about
if a spill is necessary, and only generate spills for the
necessary ones.

Change-Id: I8d60be9b2c47651aeda14f5e2d1bbd207c134b26
Reviewed-on: https://go-review.googlesource.com/c/go/+/309331
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 17 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/liveness/plive.go b/src/cmd/compile/internal/liveness/plive.go
index 4395aaeeb6..53feb6cc32 100644
--- a/src/cmd/compile/internal/liveness/plive.go
+++ b/src/cmd/compile/internal/liveness/plive.go
@@ -141,6 +141,11 @@ type liveness struct {
 
 	cache progeffectscache
 
+	// partLiveArgs includes input arguments (PPARAM) that may
+	// be partially live. That is, it is considered live because
+	// a part of it is used, but we may not initialize all parts.
+	partLiveArgs map[*ir.Name]bool
+
 	doClobber bool // Whether to clobber dead stack slots in this function.
 }
 
@@ -268,6 +273,12 @@ func (lv *liveness) valueEffects(v *ssa.Value) (int32, liveEffect) {
 		}
 	}
 
+	if n.Class == ir.PPARAM && !n.Addrtaken() && n.Type().Width > int64(types.PtrSize) {
+		// Only aggregate-typed arguments that are not address-taken can be
+		// partially live.
+		lv.partLiveArgs[n] = true
+	}
+
 	var effect liveEffect
 	// Read is a read, obviously.
 	//
@@ -394,6 +405,8 @@ func newliveness(fn *ir.Func, f *ssa.Func, vars []*ir.Name, idx map[*ir.Name]int
 
 	lv.markUnsafePoints()
 
+	lv.partLiveArgs = make(map[*ir.Name]bool)
+
 	lv.enableClobber()
 
 	return lv
@@ -1310,8 +1323,9 @@ func (lv *liveness) emit() (argsSym, liveSym *obj.LSym) {
 // Entry pointer for Compute analysis. Solves for the Compute of
 // pointer variables in the function and emits a runtime data
 // structure read by the garbage collector.
-// Returns a map from GC safe points to their corresponding stack map index.
-func Compute(curfn *ir.Func, f *ssa.Func, stkptrsize int64, pp *objw.Progs) Map {
+// Returns a map from GC safe points to their corresponding stack map index,
+// and a map that contains all input parameters that may be partially live.
+func Compute(curfn *ir.Func, f *ssa.Func, stkptrsize int64, pp *objw.Progs) (Map, map[*ir.Name]bool) {
 	// Construct the global liveness state.
 	vars, idx := getvariables(curfn)
 	lv := newliveness(curfn, f, vars, idx, stkptrsize)
@@ -1373,7 +1387,7 @@ func Compute(curfn *ir.Func, f *ssa.Func, stkptrsize int64, pp *objw.Progs) Map
 		p.To.Sym = x
 	}
 
-	return lv.livenessMap
+	return lv.livenessMap, lv.partLiveArgs
 }
 
 func (lv *liveness) emitStackObjects() *obj.LSym {