cmd/compile: fix cmpstring rewrite rule

We need to ensure that the Select0 lives in the same block as
its argument. Divide up the rule into 2 so that we can put the
parts in the right places.

(This would be simpler if we could use @block syntax mid-rule, but
that feature currently only works at the top level.)

This fixes the ssacheck builder after CL 578835

Change-Id: Id26a01d9fac0684e0b732d35d0f7999f6de07825
Reviewed-on: https://go-review.googlesource.com/c/go/+/580815
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

2 files changed, 56 insertions, 32 deletions

diff --git a/src/cmd/compile/internal/ssa/_gen/generic.rules b/src/cmd/compile/internal/ssa/_gen/generic.rules
index 398601e81b..70bac217fa 100644
--- a/src/cmd/compile/internal/ssa/_gen/generic.rules
+++ b/src/cmd/compile/internal/ssa/_gen/generic.rules
@@ -2799,7 +2799,11 @@
 // same memory state can reuse the results of the first call.
 // See issue 61725.
 // Note that this could pretty easily generalize to any pure function.
-(StaticLECall {f} x y m:(SelectN [1] c:(StaticLECall {g} x y mem)))
+(SelectN [0] (StaticLECall {f} x y (SelectN [1] c:(StaticLECall {g} x y mem))))
   && isSameCall(f, "runtime.cmpstring")
   && isSameCall(g, "runtime.cmpstring")
-=> (MakeResult (SelectN [0] <typ.Int> c) m)
+=> @c.Block (SelectN [0] <typ.Int> c)
+
+// If we don't use the result of cmpstring, might as well not call it.
+// Note that this could pretty easily generalize to any pure function.
+(SelectN [1] c:(StaticLECall {f} _ _ mem)) && c.Uses == 1 && isSameCall(f, "runtime.cmpstring") && clobber(c) => mem
diff --git a/src/cmd/compile/internal/ssa/rewritegeneric.go b/src/cmd/compile/internal/ssa/rewritegeneric.go
index 98c94bc1ba..cfed828a82 100644
--- a/src/cmd/compile/internal/ssa/rewritegeneric.go
+++ b/src/cmd/compile/internal/ssa/rewritegeneric.go
@@ -28081,6 +28081,7 @@ func rewriteValuegeneric_OpSelectN(v *Value) bool {
 	v_0 := v.Args[0]
 	b := v.Block
 	config := b.Func.Config
+	typ := &b.Func.Config.Types
 	// match: (SelectN [0] (MakeResult x ___))
 	// result: x
 	for {
@@ -28439,6 +28440,55 @@ func rewriteValuegeneric_OpSelectN(v *Value) bool {
 		v.copyOf(newLen)
 		return true
 	}
+	// match: (SelectN [0] (StaticLECall {f} x y (SelectN [1] c:(StaticLECall {g} x y mem))))
+	// cond: isSameCall(f, "runtime.cmpstring") && isSameCall(g, "runtime.cmpstring")
+	// result: @c.Block (SelectN [0] <typ.Int> c)
+	for {
+		if auxIntToInt64(v.AuxInt) != 0 || v_0.Op != OpStaticLECall || len(v_0.Args) != 3 {
+			break
+		}
+		f := auxToCall(v_0.Aux)
+		_ = v_0.Args[2]
+		x := v_0.Args[0]
+		y := v_0.Args[1]
+		v_0_2 := v_0.Args[2]
+		if v_0_2.Op != OpSelectN || auxIntToInt64(v_0_2.AuxInt) != 1 {
+			break
+		}
+		c := v_0_2.Args[0]
+		if c.Op != OpStaticLECall || len(c.Args) != 3 {
+			break
+		}
+		g := auxToCall(c.Aux)
+		if x != c.Args[0] || y != c.Args[1] || !(isSameCall(f, "runtime.cmpstring") && isSameCall(g, "runtime.cmpstring")) {
+			break
+		}
+		b = c.Block
+		v0 := b.NewValue0(v.Pos, OpSelectN, typ.Int)
+		v.copyOf(v0)
+		v0.AuxInt = int64ToAuxInt(0)
+		v0.AddArg(c)
+		return true
+	}
+	// match: (SelectN [1] c:(StaticLECall {f} _ _ mem))
+	// cond: c.Uses == 1 && isSameCall(f, "runtime.cmpstring") && clobber(c)
+	// result: mem
+	for {
+		if auxIntToInt64(v.AuxInt) != 1 {
+			break
+		}
+		c := v_0
+		if c.Op != OpStaticLECall || len(c.Args) != 3 {
+			break
+		}
+		f := auxToCall(c.Aux)
+		mem := c.Args[2]
+		if !(c.Uses == 1 && isSameCall(f, "runtime.cmpstring") && clobber(c)) {
+			break
+		}
+		v.copyOf(mem)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpSignExt16to32(v *Value) bool {
@@ -29311,36 +29361,6 @@ func rewriteValuegeneric_OpStaticLECall(v *Value) bool {
 		v.AddArg2(v0, mem)
 		return true
 	}
-	// match: (StaticLECall {f} x y m:(SelectN [1] c:(StaticLECall {g} x y mem)))
-	// cond: isSameCall(f, "runtime.cmpstring") && isSameCall(g, "runtime.cmpstring")
-	// result: (MakeResult (SelectN [0] <typ.Int> c) m)
-	for {
-		if len(v.Args) != 3 {
-			break
-		}
-		f := auxToCall(v.Aux)
-		_ = v.Args[2]
-		x := v.Args[0]
-		y := v.Args[1]
-		m := v.Args[2]
-		if m.Op != OpSelectN || auxIntToInt64(m.AuxInt) != 1 {
-			break
-		}
-		c := m.Args[0]
-		if c.Op != OpStaticLECall || len(c.Args) != 3 {
-			break
-		}
-		g := auxToCall(c.Aux)
-		if x != c.Args[0] || y != c.Args[1] || !(isSameCall(f, "runtime.cmpstring") && isSameCall(g, "runtime.cmpstring")) {
-			break
-		}
-		v.reset(OpMakeResult)
-		v0 := b.NewValue0(v.Pos, OpSelectN, typ.Int)
-		v0.AuxInt = int64ToAuxInt(0)
-		v0.AddArg(c)
-		v.AddArg2(v0, m)
-		return true
-	}
 	return false
 }
 func rewriteValuegeneric_OpStore(v *Value) bool {