From f45f9649e3fc2aa2b6a76476f2467f02ffc7d461 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Carl=20=C3=85stholm?= <carl@astholm.se>
Date: Sun, 23 Mar 2025 13:12:45 +0100
Subject: [PATCH] Lower `@returnAddress` to a constant 0 in Emscripten release
 builds

Emscripten currently implements `emscripten_return_address()` by calling
out into JavaScript and parsing a stack trace, which introduces
significant overhead that we would prefer to avoid in release builds.

This is especially problematic for allocators because the generic parts
of `std.mem.Allocator` make frequent use of `@returnAddress`, even
though very few allocator implementations even observe the return
address, which makes allocators nigh unusable for performance-critical
applications like games if the compiler is unable to devirtualize the
allocator calls.
---
 lib/std/debug.zig    | 4 +++-
 src/codegen/llvm.zig | 2 +-
 src/target.zig       | 9 +++++++--
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index 77b619a056..929ab69bf0 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -183,9 +183,11 @@ pub const sys_can_stack_trace = switch (builtin.cpu.arch) {
 
     // `@returnAddress()` in LLVM 10 gives
     // "Non-Emscripten WebAssembly hasn't implemented __builtin_return_address".
+    // On Emscripten, Zig only supports `@returnAddress()` in debug builds
+    // because Emscripten's implementation is very slow.
     .wasm32,
     .wasm64,
-    => native_os == .emscripten,
+    => native_os == .emscripten and builtin.mode == .Debug,
 
     // `@returnAddress()` is unsupported in LLVM 13.
     .bpfel,
diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index 167e0d1607..f4e4c1cdd6 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -9525,7 +9525,7 @@ pub const FuncGen = struct {
         _ = inst;
         const o = self.ng.object;
         const llvm_usize = try o.lowerType(Type.usize);
-        if (!target_util.supportsReturnAddress(o.pt.zcu.getTarget())) {
+        if (!target_util.supportsReturnAddress(o.pt.zcu.getTarget(), self.ng.ownerModule().optimize_mode)) {
             // https://github.com/ziglang/zig/issues/11946
             return o.builder.intValue(llvm_usize, 0);
         }
diff --git a/src/target.zig b/src/target.zig
index 76eec4fa6e..5c8f7895b5 100644
--- a/src/target.zig
+++ b/src/target.zig
@@ -248,9 +248,14 @@ pub fn libcProvidesStackProtector(target: std.Target) bool {
     return !target.isMinGW() and target.os.tag != .wasi and !target.cpu.arch.isSpirV();
 }
 
-pub fn supportsReturnAddress(target: std.Target) bool {
+/// Returns true if `@returnAddress()` is supported by the target and has a
+/// reasonably performant implementation for the requested optimization mode.
+pub fn supportsReturnAddress(target: std.Target, optimize: std.builtin.OptimizeMode) bool {
     return switch (target.cpu.arch) {
-        .wasm32, .wasm64 => target.os.tag == .emscripten,
+        // Emscripten currently implements `emscripten_return_address()` by calling
+        // out into JavaScript and parsing a stack trace, which introduces significant
+        // overhead that we would prefer to avoid in release builds.
+        .wasm32, .wasm64 => target.os.tag == .emscripten and optimize == .Debug,
         .bpfel, .bpfeb => false,
         .spirv, .spirv32, .spirv64 => false,
         else => true,