From bddf138e7285eefb86ef880e1200a929193da40b Mon Sep 17 00:00:00 2001
From: Luuk de Gram <luuk@degram.dev>
Date: Wed, 8 Mar 2023 06:27:03 +0100
Subject: [PATCH] wasm-link: fix storing decls in the right segment

When a decl is `undefined` is must be stored in the data segment when
the build mode is safe. For unsafe optimize modes, it must be stored
in the bss segment instead.

For mutable decls where the atom contains all zeroes, it must always
be stored in the bss segment. All other values will result in the
atom being stored in the data segment.
---
 src/link/Wasm.zig | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/link/Wasm.zig b/src/link/Wasm.zig
index e998a8d50e..01100add4b 100644
--- a/src/link/Wasm.zig
+++ b/src/link/Wasm.zig
@@ -2828,22 +2828,31 @@ pub fn flushModule(wasm: *Wasm, comp: *Compilation, prog_node: *std.Progress.Nod
             const decl = mod.declPtr(entry.key_ptr.*);
             if (decl.isExtern()) continue;
             const atom_index = entry.value_ptr.*;
+            const atom = wasm.getAtomPtr(atom_index);
             if (decl.ty.zigTypeTag() == .Fn) {
                 try wasm.parseAtom(atom_index, .function);
             } else if (decl.getVariable()) |variable| {
                 if (!variable.is_mutable) {
                     try wasm.parseAtom(atom_index, .{ .data = .read_only });
                 } else if (variable.init.isUndefDeep()) {
-                    try wasm.parseAtom(atom_index, .{ .data = .uninitialized });
+                    // for safe build modes, we store the atom in the data segment,
+                    // whereas for unsafe build modes we store it in bss.
+                    const is_initialized = wasm.base.options.optimize_mode == .Debug or
+                        wasm.base.options.optimize_mode == .ReleaseSafe;
+                    try wasm.parseAtom(atom_index, .{ .data = if (is_initialized) .initialized else .uninitialized });
                 } else {
-                    try wasm.parseAtom(atom_index, .{ .data = .initialized });
+                    // when the decl is all zeroes, we store the atom in the bss segment,
+                    // in all other cases it will be in the data segment.
+                    const is_zeroes = for (atom.code.items) |byte| {
+                        if (byte != 0) break false;
+                    } else true;
+                    try wasm.parseAtom(atom_index, .{ .data = if (is_zeroes) .uninitialized else .initialized });
                 }
             } else {
                 try wasm.parseAtom(atom_index, .{ .data = .read_only });
             }
 
             // also parse atoms for a decl's locals
-            const atom = wasm.getAtomPtr(atom_index);
             for (atom.locals.items) |local_atom_index| {
                 try wasm.parseAtom(local_atom_index, .{ .data = .read_only });
             }