cmd/asm: accept TEXT f+0(SB) in -gensymabis mode

f+0(SB) is a non-standard but acceptable alias for f(SB).

Fixes #30968.

Change-Id: I499ccee4d3ff3ab4e47f75d99407aace858e59aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/174537Reviewed-by: Austin Clements <austin@google.com>

src/cmd/asm/internal/asm/operand_test.go

@@ -145,17 +145,18 @@ func TestFuncAddress(t *testing.T) {
 
 				isFuncSym := strings.HasSuffix(test.input, "(SB)") &&
 					// Ignore static symbols.
-					!strings.Contains(test.input, "<>") &&
-					// Ignore symbols with offsets.
-					!strings.Contains(test.input, "+")
+					!strings.Contains(test.input, "<>")
 
 				wantName := ""
 				if isFuncSym {
-					// Strip $|* and (SB).
+					// Strip $|* and (SB) and +Int.
 					wantName = test.output[:len(test.output)-4]
 					if strings.HasPrefix(wantName, "$") || strings.HasPrefix(wantName, "*") {
 						wantName = wantName[1:]
 					}
+					if i := strings.Index(wantName, "+"); i >= 0 {
+						wantName = wantName[:i]
+					}
 				}
 				if ok != isFuncSym || name != wantName {
 					t.Errorf("fail at %s as function address: got %s, %v; expected %s, %v", test.input, name, ok, wantName, isFuncSym)

src/cmd/asm/internal/asm/parse.go

@@ -800,9 +800,9 @@ func (p *Parser) setPseudoRegister(addr *obj.Addr, reg string, isStatic bool, pr
 
 // funcAddress parses an external function address. This is a
 // constrained form of the operand syntax that's always SB-based,
-// non-static, and has no additional offsets:
+// non-static, and has at most a simple integer offset:
 //
-//    [$|*]sym(SB)
+//    [$|*]sym[+Int](SB)
 func (p *Parser) funcAddress() (string, bool) {
 	switch p.peek() {
 	case '$', '*':
@@ -815,7 +815,14 @@ func (p *Parser) funcAddress() (string, bool) {
 	if tok.ScanToken != scanner.Ident || p.atStartOfRegister(name) {
 		return "", false
 	}
-	if p.next().ScanToken != '(' {
+	tok = p.next()
+	if tok.ScanToken == '+' {
+		if p.next().ScanToken != scanner.Int {
+			return "", false
+		}
+		tok = p.next()
+	}
+	if tok.ScanToken != '(' {
 		return "", false
 	}
 	if reg := p.next(); reg.ScanToken != scanner.Ident || reg.String() != "SB" {