go/analysis/passes/asmdecl: permit return jump without writing to results

RET f(SB) is a tail call. It is okay to not write the results,
as it has not returned to the caller at this point.

Change-Id: I670486d02285c3a346cbc93e91be3b9e61ab77bd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/264319
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>

go/analysis/passes/asmdecl/asmdecl.go

@@ -308,7 +308,8 @@ Files:
 				continue
 			}
 
-			if strings.Contains(line, "RET") {
+			if strings.Contains(line, "RET") && !strings.Contains(line, "(SB)") {
+				// RET f(SB) is a tail call. It is okay to not write the results.
 				retLine = append(retLine, lineno)
 			}
 

go/analysis/passes/asmdecl/testdata/src/a/asm.go

@@ -51,3 +51,5 @@ func fvariadic(int, ...int)
 func pickStableABI(x int)
 func pickInternalABI(x int)
 func pickFutureABI(x int)
+
+func retjmp() int

go/analysis/passes/asmdecl/testdata/src/a/asm1.s

@@ -345,3 +345,7 @@ TEXT ·pickInternalABI<ABIInternal>(SB), NOSPLIT, $32
 TEXT ·pickFutureABI<ABISomethingNotyetInvented>(SB), NOSPLIT, $32
 	MOVQ	x+0(FP), AX
 	RET
+
+// return jump
+TEXT ·retjmp(SB), NOSPLIT, $0-8
+	RET	retjmp1(SB) // It's okay to not write results if there's a tail call.