cmd/compile: make no-op rewrite funcs smaller

This doesn't change any behavior, but should help the compiler realise
that these funcs really do nothing at all.

Change-Id: Ib26c02ef264691acac983538ec300f91d6ff98db
Reviewed-on: https://go-review.googlesource.com/c/go/+/280314
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build gen
 // +build gen
 
 // This program generates Go code that applies rewrite rules to a Value.
@@ -194,7 +195,9 @@ func genRulesSuffix(arch arch, suff string) {
 		swc.add(stmtf("return rewriteValue%s%s_%s(v)", arch.name, suff, op))
 		sw.add(swc)
 	}
-	fn.add(sw)
+	if len(sw.List) > 0 { // skip if empty
+		fn.add(sw)
+	}
 	fn.add(stmtf("return false"))
 	genFile.add(fn)
 
@@ -264,7 +267,9 @@ func genRulesSuffix(arch arch, suff string) {
 		}
 		sw.add(swc)
 	}
-	fn.add(sw)
+	if len(sw.List) > 0 { // skip if empty
+		fn.add(sw)
+	}
 	fn.add(stmtf("return false"))
 	genFile.add(fn)
 

src/cmd/compile/internal/ssa/rewrite386splitload.go

@@ -156,7 +156,5 @@ func rewriteValue386splitload_Op386CMPWload(v *Value) bool {
 	}
 }
 func rewriteBlock386splitload(b *Block) bool {
-	switch b.Kind {
-	}
 	return false
 }

src/cmd/compile/internal/ssa/rewriteAMD64splitload.go

@@ -847,7 +847,5 @@ func rewriteValueAMD64splitload_OpAMD64CMPWloadidx2(v *Value) bool {
 	}
 }
 func rewriteBlockAMD64splitload(b *Block) bool {
-	switch b.Kind {
-	}
 	return false
 }

src/cmd/compile/internal/ssa/rewriteWasm.go

@@ -4902,7 +4902,5 @@ func rewriteValueWasm_OpZeroExt8to64(v *Value) bool {
 	}
 }
 func rewriteBlockWasm(b *Block) bool {
-	switch b.Kind {
-	}
 	return false
 }

src/cmd/compile/internal/ssa/rewritedec.go

@@ -409,7 +409,5 @@ func rewriteValuedec_OpStringPtr(v *Value) bool {
 	return false
 }
 func rewriteBlockdec(b *Block) bool {
-	switch b.Kind {
-	}
 	return false
 }

src/cmd/compile/internal/ssa/rewritedec64.go

@@ -2458,7 +2458,5 @@ func rewriteValuedec64_OpZeroExt8to64(v *Value) bool {
 	}
 }
 func rewriteBlockdec64(b *Block) bool {
-	switch b.Kind {
-	}
 	return false
 }