From 499d0e3d3146283feb7a20a2394ca34d0d116ffc Mon Sep 17 00:00:00 2001
From: Felix Ehlers <fleximus@users.noreply.github.com>
Date: Tue, 14 Jul 2026 14:07:20 +0200
Subject: [PATCH 1/2] cgen: fix x86 segment-override memory operands in basic
 inline asm (fix #27777)

---
 vlib/v/gen/c/cgen.v                           |  5 +++-
 .../testdata/asm_segment_override.c.must_have |  4 +++
 vlib/v/gen/c/testdata/asm_segment_override.vv | 30 +++++++++++++++++++
 3 files changed, 38 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/gen/c/testdata/asm_segment_override.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/asm_segment_override.vv

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 7d67009e360977..afb88903b60a25 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/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
diff --git a/vlib/v/gen/c/testdata/asm_segment_override.c.must_have b/vlib/v/gen/c/testdata/asm_segment_override.c.must_have
new file mode 100644
index 00000000000000..cbffd58437d40e
--- /dev/null
+++ b/vlib/v/gen/c/testdata/asm_segment_override.c.must_have
@@ -0,0 +1,4 @@
+"mov %rsp, %gs:24\n\t"
+"mov %gs:16, %rsp\n\t"
+"mov %%fs:8, %%rax\n\t"
+"mov %%rax, %[a]\n\t"
diff --git a/vlib/v/gen/c/testdata/asm_segment_override.vv b/vlib/v/gen/c/testdata/asm_segment_override.vv
new file mode 100644
index 00000000000000..5f3495888f099a
--- /dev/null
+++ b/vlib/v/gen/c/testdata/asm_segment_override.vv
@@ -0,0 +1,30 @@
+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 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())
+	}
+}

From 9ece160d2e1885a59d613252e323a79be1a55250 Mon Sep 17 00:00:00 2001
From: Felix Ehlers <fleximus@users.noreply.github.com>
Date: Fri, 17 Jul 2026 06:12:40 +0200
Subject: [PATCH 2/2] parser: treat asm goto blocks as extended assembly, so
 template operands are escaped correctly

---
 .../testdata/asm_segment_override.c.must_have |  2 ++
 vlib/v/gen/c/testdata/asm_segment_override.vv | 13 ++++++++++++
 vlib/v/parser/asm.v                           | 20 ++++++++++---------
 3 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/vlib/v/gen/c/testdata/asm_segment_override.c.must_have b/vlib/v/gen/c/testdata/asm_segment_override.c.must_have
index cbffd58437d40e..60b793af609f0a 100644
--- a/vlib/v/gen/c/testdata/asm_segment_override.c.must_have
+++ b/vlib/v/gen/c/testdata/asm_segment_override.c.must_have
@@ -2,3 +2,5 @@
 "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"
diff --git a/vlib/v/gen/c/testdata/asm_segment_override.vv b/vlib/v/gen/c/testdata/asm_segment_override.vv
index 5f3495888f099a..c63624c05ec373 100644
--- a/vlib/v/gen/c/testdata/asm_segment_override.vv
+++ b/vlib/v/gen/c/testdata/asm_segment_override.vv
@@ -20,11 +20,24 @@ fn extended_asm() u64 {
 	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()
 	}
 }
diff --git a/vlib/v/parser/asm.v b/vlib/v/parser/asm.v
index 91f3ec8871ed9e..6b1b30dadd29ec 100644
--- a/vlib/v/parser/asm.v
+++ b/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