Commit f45f9649e3fc2aa2b6a76476f2467f02ffc7d461 | zig

author	Carl Åstholm <carl@astholm.se> 2025-03-23 13:12:45+01:00
committer	Andrew Kelley <andrew@ziglang.org> 2025-03-23 17:13:19-04:00
log	f45f9649e3fc2aa2b6a76476f2467f02ffc7d461
tree	4d10ac814ef9d7d7e9f06fffac796217336bfbc5
parent	9f235a105b29c6f1c0c773f66c40e7ee655da559

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.

3 files changed, 11 insertions(+), 4 deletions(-)

lib
- std
  - debug.zig
src
- codegen
  - llvm.zig
- target.zig

lib/std/debug.zig+3-1

 @@ -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,

src/codegen/llvm.zig+1-1

...	@@ -9525,7 +9525,7 @@ pub const FuncGen = struct {	...	@@ -9525,7 +9525,7 @@ pub const FuncGen = struct {
9525	_ = inst;	9525	_ = inst;
9526	const o = self.ng.object;	9526	const o = self.ng.object;
9527	const llvm_usize = try o.lowerType(Type.usize);	9527	const llvm_usize = try o.lowerType(Type.usize);
9528	if (!target_util.supportsReturnAddress(o.pt.zcu.getTarget())) {	9528	if (!target_util.supportsReturnAddress(o.pt.zcu.getTarget(), self.ng.ownerModule().optimize_mode)) {
9529	// https://github.com/ziglang/zig/issues/11946	9529	// https://github.com/ziglang/zig/issues/11946
9530	return o.builder.intValue(llvm_usize, 0);	9530	return o.builder.intValue(llvm_usize, 0);
9531	}	9531	}

src/target.zig+7-2

 @@ -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,