From 1e8b82f6b4852211b76a54da14fc8cf0351ada09 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20R=C3=B8nne=20Petersen?= <alex@alexrp.com>
Date: Sat, 25 Jan 2025 14:47:14 +0100
Subject: [PATCH] compiler: Rework LTO settings for some Zig-provided
 libraries.

* compiler-rt and mingw32 have both run into LLD bugs, and LLVM disables LTO for
  its compiler-rt, so disable LTO for these.
* While we haven't run into any bugs in it, LLVM disables LTO for its libtsan,
  so follow suit just to be safe.
* Allow LTO for libfuzzer as LLVM does.
---
 src/Compilation.zig | 19 +++++++++++++++----
 src/libtsan.zig     |  2 ++
 src/mingw.zig       |  2 ++
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/src/Compilation.zig b/src/Compilation.zig
index 1a056c1c04..4a57fdcc9d 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -3695,15 +3695,19 @@ fn performAllTheWorkInner(
     // In case it failed last time, try again. `clearMiscFailures` was already
     // called at the start of `update`.
     if (comp.queued_jobs.compiler_rt_lib and comp.compiler_rt_lib == null) {
-        comp.link_task_wait_group.spawnManager(buildRt, .{ comp, "compiler_rt.zig", .compiler_rt, .Lib, &comp.compiler_rt_lib, main_progress_node });
+        // LLVM disables LTO for its compiler-rt and we've had various issues with LTO of our
+        // compiler-rt due to LLD bugs as well, e.g.:
+        //
+        // https://github.com/llvm/llvm-project/issues/43698#issuecomment-2542660611
+        comp.link_task_wait_group.spawnManager(buildRt, .{ comp, "compiler_rt.zig", .compiler_rt, .Lib, false, &comp.compiler_rt_lib, main_progress_node });
     }
 
     if (comp.queued_jobs.compiler_rt_obj and comp.compiler_rt_obj == null) {
-        comp.link_task_wait_group.spawnManager(buildRt, .{ comp, "compiler_rt.zig", .compiler_rt, .Obj, &comp.compiler_rt_obj, main_progress_node });
+        comp.link_task_wait_group.spawnManager(buildRt, .{ comp, "compiler_rt.zig", .compiler_rt, .Obj, false, &comp.compiler_rt_obj, main_progress_node });
     }
 
     if (comp.queued_jobs.fuzzer_lib and comp.fuzzer_lib == null) {
-        comp.link_task_wait_group.spawnManager(buildRt, .{ comp, "fuzzer.zig", .libfuzzer, .Lib, &comp.fuzzer_lib, main_progress_node });
+        comp.link_task_wait_group.spawnManager(buildRt, .{ comp, "fuzzer.zig", .libfuzzer, .Lib, true, &comp.fuzzer_lib, main_progress_node });
     }
 
     if (comp.queued_jobs.glibc_shared_objects) {
@@ -4635,12 +4639,14 @@ fn buildRt(
     root_source_name: []const u8,
     misc_task: MiscTask,
     output_mode: std.builtin.OutputMode,
+    allow_lto: bool,
     out: *?CrtFile,
     prog_node: std.Progress.Node,
 ) void {
     comp.buildOutputFromZig(
         root_source_name,
         output_mode,
+        allow_lto,
         out,
         misc_task,
         prog_node,
@@ -4748,6 +4754,7 @@ fn buildZigLibc(comp: *Compilation, prog_node: std.Progress.Node) void {
     comp.buildOutputFromZig(
         "c.zig",
         .Lib,
+        true,
         &comp.libc_static_lib,
         .zig_libc,
         prog_node,
@@ -6453,6 +6460,7 @@ fn buildOutputFromZig(
     comp: *Compilation,
     src_basename: []const u8,
     output_mode: std.builtin.OutputMode,
+    allow_lto: bool,
     out: *?CrtFile,
     misc_task_tag: MiscTask,
     prog_node: std.Progress.Node,
@@ -6481,6 +6489,7 @@ fn buildOutputFromZig(
         .root_strip = strip,
         .link_libc = comp.config.link_libc,
         .any_unwind_tables = comp.root_mod.unwind_tables != .none,
+        .lto = if (allow_lto) comp.config.lto else .none,
     });
 
     const root_mod = try Package.Module.create(arena, .{
@@ -6581,6 +6590,8 @@ pub const CrtFileOptions = struct {
     unwind_tables: ?std.builtin.UnwindTables = null,
     pic: ?bool = null,
     no_builtin: ?bool = null,
+
+    allow_lto: bool = true,
 };
 
 pub fn build_crt_file(
@@ -6619,7 +6630,7 @@ pub fn build_crt_file(
         .link_libc = false,
         .any_unwind_tables = options.unwind_tables != .none,
         .lto = switch (output_mode) {
-            .Lib => comp.config.lto,
+            .Lib => if (options.allow_lto) comp.config.lto else .none,
             .Obj, .Exe => .none,
         },
     });
diff --git a/src/libtsan.zig b/src/libtsan.zig
index 8b2427df1b..d5fffd2b6c 100644
--- a/src/libtsan.zig
+++ b/src/libtsan.zig
@@ -65,6 +65,8 @@ pub fn buildTsan(comp: *Compilation, prog_node: std.Progress.Node) BuildError!vo
         .root_strip = strip,
         .link_libc = true,
         .link_libcpp = link_libcpp,
+        // LLVM disables LTO for its libtsan.
+        .lto = .none,
     }) catch |err| {
         comp.setMiscFailure(
             .libtsan,
diff --git a/src/mingw.zig b/src/mingw.zig
index 47aff05133..17c0a49b11 100644
--- a/src/mingw.zig
+++ b/src/mingw.zig
@@ -175,6 +175,8 @@ pub fn buildCrtFile(comp: *Compilation, crt_file: CrtFile, prog_node: std.Progre
 
             return comp.build_crt_file("mingw32", .Lib, .@"mingw-w64 mingw32.lib", prog_node, c_source_files.items, .{
                 .unwind_tables = unwind_tables,
+                // https://github.com/llvm/llvm-project/issues/43698#issuecomment-2542660611
+                .allow_lto = false,
             });
         },
     }