Merge pull request #14838 from Luukdegram/bss-fix

wasm-linker: fix storing atoms in the correct segment

3 files changed, 91 insertions, 37 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 });
             }
diff --git a/test/link/wasm/bss/build.zig b/test/link/wasm/bss/build.zig
index bba2e7c602..4a26e78a12 100644
--- a/test/link/wasm/bss/build.zig
+++ b/test/link/wasm/bss/build.zig
@@ -6,38 +6,78 @@ pub fn build(b: *std.Build) void {
     const test_step = b.step("test", "Test");
     b.default_step = test_step;
 
-    const lib = b.addSharedLibrary(.{
-        .name = "lib",
-        .root_source_file = .{ .path = "lib.zig" },
-        .target = .{ .cpu_arch = .wasm32, .os_tag = .freestanding },
-        .optimize = .Debug,
-    });
-    lib.use_llvm = false;
-    lib.use_lld = false;
-    lib.strip = false;
-    // to make sure the bss segment is emitted, we must import memory
-    lib.import_memory = true;
-    lib.install();
-
-    const check_lib = lib.checkObject();
-
-    // since we import memory, make sure it exists with the correct naming
-    check_lib.checkStart("Section import");
-    check_lib.checkNext("entries 1");
-    check_lib.checkNext("module env"); // default module name is "env"
-    check_lib.checkNext("name memory"); // as per linker specification
-
-    // since we are importing memory, ensure it's not exported
-    check_lib.checkNotPresent("Section export");
-
-    // validate the name of the stack pointer
-    check_lib.checkStart("Section custom");
-    check_lib.checkNext("type data_segment");
-    check_lib.checkNext("names 2");
-    check_lib.checkNext("index 0");
-    check_lib.checkNext("name .rodata");
-    check_lib.checkNext("index 1"); // bss section always last
-    check_lib.checkNext("name .bss");
-
-    test_step.dependOn(&check_lib.step);
+    add(b, test_step, .Debug, true);
+    add(b, test_step, .ReleaseFast, false);
+    add(b, test_step, .ReleaseSmall, false);
+    add(b, test_step, .ReleaseSafe, true);
+}
+
+fn add(b: *std.Build, test_step: *std.Build.Step, optimize_mode: std.builtin.OptimizeMode, is_safe: bool) void {
+    {
+        const lib = b.addSharedLibrary(.{
+            .name = "lib",
+            .root_source_file = .{ .path = "lib.zig" },
+            .target = .{ .cpu_arch = .wasm32, .os_tag = .freestanding },
+            .optimize = optimize_mode,
+        });
+        lib.use_llvm = false;
+        lib.use_lld = false;
+        lib.strip = false;
+        // to make sure the bss segment is emitted, we must import memory
+        lib.import_memory = true;
+
+        const check_lib = lib.checkObject();
+
+        // since we import memory, make sure it exists with the correct naming
+        check_lib.checkStart("Section import");
+        check_lib.checkNext("entries 1");
+        check_lib.checkNext("module env"); // default module name is "env"
+        check_lib.checkNext("name memory"); // as per linker specification
+
+        // since we are importing memory, ensure it's not exported
+        check_lib.checkNotPresent("Section export");
+
+        // validate the name of the stack pointer
+        check_lib.checkStart("Section custom");
+        check_lib.checkNext("type data_segment");
+        check_lib.checkNext("names 2");
+        check_lib.checkNext("index 0");
+        check_lib.checkNext("name .rodata");
+        // for safe optimization modes `undefined` is stored in data instead of bss.
+        if (is_safe) {
+            check_lib.checkNext("index 1");
+            check_lib.checkNext("name .data");
+            check_lib.checkNotPresent("name .bss");
+        } else {
+            check_lib.checkNext("index 1"); // bss section always last
+            check_lib.checkNext("name .bss");
+        }
+        test_step.dependOn(&check_lib.step);
+    }
+
+    // verify zero'd declaration is stored in bss for all optimization modes.
+    {
+        const lib = b.addSharedLibrary(.{
+            .name = "lib",
+            .root_source_file = .{ .path = "lib2.zig" },
+            .target = .{ .cpu_arch = .wasm32, .os_tag = .freestanding },
+            .optimize = optimize_mode,
+        });
+        lib.use_llvm = false;
+        lib.use_lld = false;
+        lib.strip = false;
+        // to make sure the bss segment is emitted, we must import memory
+        lib.import_memory = true;
+
+        const check_lib = lib.checkObject();
+        check_lib.checkStart("Section custom");
+        check_lib.checkNext("type data_segment");
+        check_lib.checkNext("names 2");
+        check_lib.checkNext("index 0");
+        check_lib.checkNext("name .rodata");
+        check_lib.checkNext("index 1");
+        check_lib.checkNext("name .bss");
+
+        test_step.dependOn(&check_lib.step);
+    }
 }
diff --git a/test/link/wasm/bss/lib2.zig b/test/link/wasm/bss/lib2.zig
new file mode 100644
index 0000000000..9f43128880
--- /dev/null
+++ b/test/link/wasm/bss/lib2.zig
@@ -0,0 +1,5 @@
+pub var bss: u32 = 0;
+
+export fn foo() void {
+    _ = bss;
+}