2 files changed, 47 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/_gen/generic.rules b/src/cmd/compile/internal/ssa/_gen/generic.rules
index ccdb1d8a77..0fc0f54433 100644
--- a/src/cmd/compile/internal/ssa/_gen/generic.rules
+++ b/src/cmd/compile/internal/ssa/_gen/generic.rules
@@ -2539,6 +2539,13 @@
 (SelectN [0] call:(StaticLECall {sym} a x)) && needRaceCleanup(sym, call) && clobber(call) => x
 (SelectN [0] call:(StaticLECall {sym} x)) && needRaceCleanup(sym, call) && clobber(call) => x
 
+// When rewriting append to growslice, we use as the the new length the result of
+// growslice so that we don't have to spill/restore the new length around the growslice call.
+// The exception here is that if the new length is a constant, avoiding spilling it
+// is pointless and its constantness is sometimes useful for subsequent optimizations.
+// See issue 56440.
+(SliceLen (SelectN [0] (StaticLECall {sym} _ newLen:(Const(64|32)) _ _ _ _))) && isSameCall(sym, "runtime.growslice") => newLen
+
 // Collapse moving A -> B -> C into just A -> C.
 // Later passes (deadstore, elim unread auto) will remove the A -> B move, if possible.
 // This happens most commonly when B is an autotmp inserted earlier
diff --git a/src/cmd/compile/internal/ssa/rewritegeneric.go b/src/cmd/compile/internal/ssa/rewritegeneric.go
index de7f3e8bc7..6598c0e483 100644
--- a/src/cmd/compile/internal/ssa/rewritegeneric.go
+++ b/src/cmd/compile/internal/ssa/rewritegeneric.go
@@ -26977,6 +26977,46 @@ func rewriteValuegeneric_OpSliceLen(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
+	// match: (SliceLen (SelectN [0] (StaticLECall {sym} _ newLen:(Const64) _ _ _ _)))
+	// cond: isSameCall(sym, "runtime.growslice")
+	// result: newLen
+	for {
+		if v_0.Op != OpSelectN || auxIntToInt64(v_0.AuxInt) != 0 {
+			break
+		}
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpStaticLECall || len(v_0_0.Args) != 6 {
+			break
+		}
+		sym := auxToCall(v_0_0.Aux)
+		_ = v_0_0.Args[1]
+		newLen := v_0_0.Args[1]
+		if newLen.Op != OpConst64 || !(isSameCall(sym, "runtime.growslice")) {
+			break
+		}
+		v.copyOf(newLen)
+		return true
+	}
+	// match: (SliceLen (SelectN [0] (StaticLECall {sym} _ newLen:(Const32) _ _ _ _)))
+	// cond: isSameCall(sym, "runtime.growslice")
+	// result: newLen
+	for {
+		if v_0.Op != OpSelectN || auxIntToInt64(v_0.AuxInt) != 0 {
+			break
+		}
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpStaticLECall || len(v_0_0.Args) != 6 {
+			break
+		}
+		sym := auxToCall(v_0_0.Aux)
+		_ = v_0_0.Args[1]
+		newLen := v_0_0.Args[1]
+		if newLen.Op != OpConst32 || !(isSameCall(sym, "runtime.growslice")) {
+			break
+		}
+		v.copyOf(newLen)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpSlicePtr(v *Value) bool {