From 0c6c80c92cf02525e375e2f6a973dcf2fdec6e92 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 16:54:36 +0300
Subject: [PATCH] cgen: fix parallel compilation failing on windows (fixes
 #26877)

---
 vlib/v/builder/parallel_cc_failure_test.v  |  7 +++-
 vlib/v/gen/c/cgen.v                        |  3 ++
 vlib/v/gen/c/cheaders.v                    |  6 +++-
 vlib/v/gen/c/link_generated_c_files_test.v | 37 ++++++++++++++++++++++
 4 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/vlib/v/builder/parallel_cc_failure_test.v b/vlib/v/builder/parallel_cc_failure_test.v
index b3204eea0..cd098e651 100644
--- a/vlib/v/builder/parallel_cc_failure_test.v
+++ b/vlib/v/builder/parallel_cc_failure_test.v
@@ -10,6 +10,11 @@ fn run_parallel_cc_case(case_name string, files map[string]string) os.Result {
 }
 
 fn run_parallel_cc_case_with_env(case_name string, files map[string]string, env_overrides map[string]string) os.Result {
+	return run_parallel_cc_case_with_args(case_name, ['-parallel-cc', 'main.v'], files,
+		env_overrides)
+}
+
+fn run_parallel_cc_case_with_args(case_name string, args []string, files map[string]string, env_overrides map[string]string) os.Result {
 	case_dir := os.join_path(parallel_cc_test_path, case_name)
 	os.rmdir_all(case_dir) or {}
 	os.mkdir_all(case_dir) or { panic(err) }
@@ -27,7 +32,7 @@ fn run_parallel_cc_case_with_env(case_name string, files map[string]string, env_
 	}
 	mut p := os.new_process(parallel_cc_vexe)
 	p.set_work_folder(case_dir)
-	p.set_args(['-parallel-cc', 'main.v'])
+	p.set_args(args)
 	p.set_environment(env)
 	p.set_redirect_stdio()
 	p.wait()
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index ff4603101..24c50dbc2 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -1229,6 +1229,9 @@ pub fn (mut g Gen) init() {
 	if g.pref.autofree {
 		g.comptime_definitions.writeln('#define _VAUTOFREE (1)')
 	}
+	if g.pref.parallel_cc {
+		g.comptime_definitions.writeln('#define _VPARALLELCC (1)')
+	}
 	if g.pref.prealloc {
 		g.comptime_definitions.writeln('#define _VPREALLOC (1)')
 	}
diff --git a/vlib/v/gen/c/cheaders.v b/vlib/v/gen/c/cheaders.v
index 1904b36d0..04da01808 100644
--- a/vlib/v/gen/c/cheaders.v
+++ b/vlib/v/gen/c/cheaders.v
@@ -179,7 +179,11 @@ const c_common_macros = '
 #endif
 #if defined(_WIN32) || defined(__CYGWIN__)
 	#define VV_EXP extern __declspec(dllexport)
-	#define VV_LOC static
+	#ifdef _VPARALLELCC
+		#define VV_LOC
+	#else
+		#define VV_LOC static
+	#endif
 #else
 	// 4 < gcc < 5 is used by some older Ubuntu LTS and Centos versions,
 	// and does not support __has_attribute(visibility) ...
diff --git a/vlib/v/gen/c/link_generated_c_files_test.v b/vlib/v/gen/c/link_generated_c_files_test.v
index cc4370b88..d23183f17 100644
--- a/vlib/v/gen/c/link_generated_c_files_test.v
+++ b/vlib/v/gen/c/link_generated_c_files_test.v
@@ -1,6 +1,8 @@
 module c
 
 import os
+import v.builder
+import v.pref
 
 const test_vexe = os.quoted_path(@VEXE)
 
@@ -53,3 +55,38 @@ int main(void) {
 	assert res.exit_code == 0, res.output
 	assert res.output.trim_space() == '1 2'
 }
+
+fn test_parallel_cc_windows_header_keeps_vv_loc_external() {
+	tmp_dir := os.join_path(os.vtmp_dir(), 'parallel_cc_windows_linkage_${os.getpid()}')
+	os.mkdir_all(tmp_dir)!
+	defer {
+		os.rmdir_all(tmp_dir) or {}
+	}
+	source_path := os.join_path(tmp_dir, 'main.v')
+	os.write_file(source_path, 'module main
+fn helper() string {
+	return "ok"
+}
+
+fn main() {
+	println(helper())
+}
+')!
+	mut prefs, _ := pref.parse_args_and_show_errors([], [
+		'',
+		'-parallel-cc',
+		'-os',
+		'windows',
+		source_path,
+	], false)
+	mut b := builder.new_builder(prefs)
+	mut files := b.get_builtin_files()
+	files << b.get_user_files()
+	b.set_module_lookup_paths()
+	b.front_and_middle_stages(files)!
+	result := gen(b.parsed_files, mut b.table, b.pref)
+	header := result.header.replace('\r\n', '\n')
+	assert header.contains('#define _VPARALLELCC (1)'), header
+	assert header.contains('#ifdef _VPARALLELCC\n\t\t#define VV_LOC\n\t#else\n\t\t#define VV_LOC static\n\t#endif'), header
+	assert header.contains('VV_LOC string main__helper(void);'), header
+}
-- 
2.39.5