From 84254a9d8cb7d8bd28852933484be3f499cfbeee Mon Sep 17 00:00:00 2001
From: Riccardo Mazzarini <me@noib3.dev>
Date: Sat, 25 Jul 2026 16:35:50 +0200
Subject: [PATCH] build: avoid MSVC C++ runtime in no-libcxx builds

AI-assisted: Codex
---
 pkg/highway/build.zig      |  7 +++++++
 pkg/simdutf/build.zig      | 14 ++++++++++----
 src/build/GhosttyLibVt.zig | 11 ++++++++---
 src/build/SharedDeps.zig   |  5 +++++
 src/lib_vt.zig             | 11 +++++++++++
 5 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/pkg/highway/build.zig b/pkg/highway/build.zig
index fce823bf3..af3300eb3 100644
--- a/pkg/highway/build.zig
+++ b/pkg/highway/build.zig
@@ -99,6 +99,13 @@ pub fn build(b: *std.Build) !void {
             "-fmath-errno",
             "-fno-exceptions",
         });
+    } else if (target.result.abi == .msvc) {
+        try flags.appendSlice(b.allocator, &.{
+            // -fno-autolink also drops UCRT's /alternatename fallback.
+            "-D_Avx2WmemEnabledWeakValue=_Avx2WmemEnabled",
+            "-fno-autolink",
+            "-fno-stack-protector",
+        });
     }
 
     lib.root_module.addCSourceFiles(.{ .flags = flags.items, .files = &.{
diff --git a/pkg/simdutf/build.zig b/pkg/simdutf/build.zig
index a92fa3a07..3efc08263 100644
--- a/pkg/simdutf/build.zig
+++ b/pkg/simdutf/build.zig
@@ -50,10 +50,16 @@ pub fn build(b: *std.Build) !void {
 
     if (no_libcxx) {
         try flags.append(b.allocator, "-DSIMDUTF_NO_LIBCXX");
-        if (target.result.abi != .msvc) {
-            // Clang/GCC-only flags; MSVC doesn't accept these.
-            try flags.append(b.allocator, "-fno-exceptions");
-            try flags.append(b.allocator, "-fno-rtti");
+        try flags.append(b.allocator, "-fno-exceptions");
+        try flags.append(b.allocator, "-fno-rtti");
+        if (target.result.abi == .msvc) {
+            try flags.appendSlice(b.allocator, &.{
+                "-D_USE_STD_VECTOR_ALGORITHMS=0",
+                // -fno-autolink also drops UCRT's /alternatename fallback.
+                "-D_Avx2WmemEnabledWeakValue=_Avx2WmemEnabled",
+                "-fno-autolink",
+                "-fno-stack-protector",
+            });
         }
 
         lib.root_module.addCMacro("SIMDUTF_NO_LIBCXX", "1");
diff --git a/src/build/GhosttyLibVt.zig b/src/build/GhosttyLibVt.zig
index 389be4ef5..0ef7d6937 100644
--- a/src/build/GhosttyLibVt.zig
+++ b/src/build/GhosttyLibVt.zig
@@ -249,9 +249,14 @@ fn initLib(
         lib.bundle_ubsan_rt = false;
 
         if (kind == .static) {
-            // compiler_rt expects ntdll to provide _fltused when libc is
-            // linked, and the Zig standard library uses other NT and kernel32
-            // symbols.
+            if (target.result.abi == .msvc) {
+                // Zig's compiler runtime doesn't provide MSVC's security
+                // cookie symbols when libc is linked. Disable stack-protector
+                // generation so static consumers don't need BufferOverflowU.
+                lib.root_module.stack_protector = false;
+            }
+
+            // The Zig standard library uses NT and kernel32 symbols.
             lib.root_module.linkSystemLibrary("ntdll", .{});
             lib.root_module.linkSystemLibrary("kernel32", .{});
         }
diff --git a/src/build/SharedDeps.zig b/src/build/SharedDeps.zig
index f1ec7c9d5..0c1589842 100644
--- a/src/build/SharedDeps.zig
+++ b/src/build/SharedDeps.zig
@@ -991,6 +991,11 @@ pub fn addSimd(
             "-fno-sanitize=undefined",
             "-fno-sanitize-trap=undefined",
         });
+        if (target.result.abi == .msvc) try flags.appendSlice(b.allocator, &.{
+            // -fno-autolink also drops UCRT's /alternatename fallback.
+            "-D_Avx2WmemEnabledWeakValue=_Avx2WmemEnabled",
+            "-fno-autolink",
+        });
 
         m.addCSourceFiles(.{
             .files = &.{
diff --git a/src/lib_vt.zig b/src/lib_vt.zig
index b4a2722bf..e49427409 100644
--- a/src/lib_vt.zig
+++ b/src/lib_vt.zig
@@ -12,6 +12,8 @@ const lib = @This();
 const std = @import("std");
 const builtin = @import("builtin");
 
+var msvc_fltused: c_int = 1;
+
 // The public API below reproduces a lot of terminal/main.zig but
 // is separate because (1) we need our root file to be in `src/`
 // so we can access other directories and (2) we may want to withhold
@@ -145,6 +147,15 @@ comptime {
     // If we're building the C library (vs. the Zig module) then
     // we want to reference the C API so that it gets exported.
     if (@import("root") == lib) {
+        // MSVC requires this marker whenever floating-point code is present.
+        // Zig's compiler_rt only provides it when libc is not linked.
+        if (builtin.os.tag == .windows and
+            builtin.abi == .msvc and
+            builtin.link_mode == .static)
+        {
+            @export(&msvc_fltused, .{ .name = "_fltused" });
+        }
+
         // Force-reference our memset override so its export is
         // emitted. This must stay inside the root guard so that
         // downstream Zig module consumers don't get the override