cgen: fix x86 segment-override memory operands in basic inline asm

vlib/v/gen/c/cgen.v

@@ -6281,7 +6281,10 @@ fn (mut g Gen) asm_arg(arg ast.AsmArg, stmt ast.AsmStmt) {
 		}
 		ast.AsmAddressing {
 			if arg.segment != '' {
-				g.write('%%${arg.segment}:')
+				if !stmt.is_basic {
+					g.write('%') // escape percent with percent in extended assembly
+				}
+				g.write('%${arg.segment}:')
 			}
 			base := arg.base
 			index := arg.index

vlib/v/gen/c/testdata/asm_segment_override.c.must_have

@@ -0,0 +1,6 @@
+"mov %rsp, %gs:24\n\t"
+"mov %gs:16, %rsp\n\t"
+"mov %%fs:8, %%rax\n\t"
+"mov %%rax, %[a]\n\t"
+"mov %%gs:16, %%rax\n\t"
+"jne %l[over]\n\t"

vlib/v/gen/c/testdata/asm_segment_override.vv

@@ -0,0 +1,43 @@
+fn basic_asm() {
+	// basic assembly (no operand lists) is passed through verbatim by the C
+	// compiler, so segment overrides must use a single, unescaped `%`
+	asm volatile amd64 {
+		mov gs:[24], rsp
+		mov rsp, gs:[16]
+	}
+}
+
+fn extended_asm() u64 {
+	// extended assembly templates are processed by the C compiler, so
+	// segment overrides need the escaped `%%` form there
+	mut a := u64(0)
+	asm amd64 {
+		mov rax, fs:[8]
+		mov a, rax
+		; =r (a)
+		; ; rax
+	}
+	return a
+}
+
+fn goto_asm() {
+	// `asm goto` blocks are emitted as extended assembly even without any
+	// output/input/clobber operands, so they need the escaped `%%` form too
+	asm goto amd64 {
+		mov rax, gs:[16]
+		cmp rax, 0
+		jne over
+		; ; ; ; over
+	}
+	over:
+}
+
+fn main() {
+	// guard the calls behind a runtime condition, so that running the
+	// program does not actually dereference the segmented addresses
+	if arguments().len > 1000 {
+		basic_asm()
+		println(extended_asm())
+		goto_asm()
+	}
+}

vlib/v/parser/asm.v

@@ -289,15 +289,17 @@ fn (mut p Parser) asm_stmt(is_top_level bool) ast.AsmStmt {
 	scope.end_pos = p.prev_tok.pos
 
 	return ast.AsmStmt{
-		arch:          arch
-		is_goto:       is_goto
-		is_volatile:   is_volatile
-		templates:     templates
-		output:        output
-		input:         input
-		clobbered:     clobbered
-		pos:           pos.extend(p.prev_tok.pos())
-		is_basic:      is_top_level || output.len + input.len + clobbered.len == 0
+		arch:        arch
+		is_goto:     is_goto
+		is_volatile: is_volatile
+		templates:   templates
+		output:      output
+		input:       input
+		clobbered:   clobbered
+		pos:         pos.extend(p.prev_tok.pos())
+		// `asm goto` blocks are always emitted as extended assembly (they need the
+		// label section), even when they have no output/input/clobber operands
+		is_basic:      is_top_level || (!is_goto && output.len + input.len + clobbered.len == 0)
 		scope:         scope
 		global_labels: global_labels
 		local_labels:  local_labels