Emit check for memory intrinsics for WebAssembly

src/codegen/llvm.zig

@@ -8511,6 +8511,14 @@ pub const FuncGen = struct {
         const dest_ptr = self.sliceOrArrayPtr(dest_slice, ptr_ty);
         const is_volatile = ptr_ty.isVolatilePtr(mod);
 
+        // Any WebAssembly runtime will trap when the destination pointer is out-of-bounds, regardless
+        // of the length. This means we need to emit a check where we skip the memset when the length
+        // is 0 as we allow for undefined pointers in 0-sized slices.
+        // This logic can be removed once https://github.com/ziglang/zig/issues/16360 is done.
+        const intrinsic_len0_traps = o.target.isWasm() and
+            ptr_ty.isSlice(mod) and
+            std.Target.wasm.featureSetHas(o.target.cpu.features, .bulk_memory);
+
         if (try self.air.value(bin_op.rhs, mod)) |elem_val| {
             if (elem_val.isUndefDeep(mod)) {
                 // Even if safety is disabled, we still emit a memset to undefined since it conveys
@@ -8521,7 +8529,11 @@ pub const FuncGen = struct {
                 else
                     u8_llvm_ty.getUndef();
                 const len = self.sliceOrArrayLenInBytes(dest_slice, ptr_ty);
-                _ = self.builder.buildMemSet(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+                if (intrinsic_len0_traps) {
+                    try self.safeWasmMemset(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+                } else {
+                    _ = self.builder.buildMemSet(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+                }
 
                 if (safety and mod.comp.bin_file.options.valgrind) {
                     self.valgrindMarkUndef(dest_ptr, len);
@@ -8539,7 +8551,12 @@ pub const FuncGen = struct {
                     .val = byte_val,
                 });
                 const len = self.sliceOrArrayLenInBytes(dest_slice, ptr_ty);
-                _ = self.builder.buildMemSet(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+
+                if (intrinsic_len0_traps) {
+                    try self.safeWasmMemset(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+                } else {
+                    _ = self.builder.buildMemSet(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+                }
                 return null;
             }
         }
@@ -8551,7 +8568,12 @@ pub const FuncGen = struct {
             // In this case we can take advantage of LLVM's intrinsic.
             const fill_byte = try self.bitCast(value, elem_ty, Type.u8);
             const len = self.sliceOrArrayLenInBytes(dest_slice, ptr_ty);
-            _ = self.builder.buildMemSet(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+
+            if (intrinsic_len0_traps) {
+                try self.safeWasmMemset(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+            } else {
+                _ = self.builder.buildMemSet(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+            }
             return null;
         }
 
@@ -8622,6 +8644,25 @@ pub const FuncGen = struct {
         return null;
     }
 
+    fn safeWasmMemset(
+        self: *FuncGen,
+        dest_ptr: *llvm.Value,
+        fill_byte: *llvm.Value,
+        len: *llvm.Value,
+        dest_ptr_align: u32,
+        is_volatile: bool,
+    ) !void {
+        const llvm_usize_ty = self.context.intType(self.dg.object.target.ptrBitWidth());
+        const cond = try self.cmp(len, llvm_usize_ty.constInt(0, .False), Type.usize, .neq);
+        const memset_block = self.context.appendBasicBlock(self.llvm_func, "MemsetTrapSkip");
+        const end_block = self.context.appendBasicBlock(self.llvm_func, "MemsetTrapEnd");
+        _ = self.builder.buildCondBr(cond, memset_block, end_block);
+        self.builder.positionBuilderAtEnd(memset_block);
+        _ = self.builder.buildMemSet(dest_ptr, fill_byte, len, dest_ptr_align, is_volatile);
+        _ = self.builder.buildBr(end_block);
+        self.builder.positionBuilderAtEnd(end_block);
+    }
+
     fn airMemcpy(self: *FuncGen, inst: Air.Inst.Index) !?*llvm.Value {
         const o = self.dg.object;
         const mod = o.module;
@@ -8634,6 +8675,35 @@ pub const FuncGen = struct {
         const len = self.sliceOrArrayLenInBytes(dest_slice, dest_ptr_ty);
         const dest_ptr = self.sliceOrArrayPtr(dest_slice, dest_ptr_ty);
         const is_volatile = src_ptr_ty.isVolatilePtr(mod) or dest_ptr_ty.isVolatilePtr(mod);
+
+        // When bulk-memory is enabled, this will be lowered to WebAssembly's memory.copy instruction.
+        // This instruction will trap on an invalid address, regardless of the length.
+        // For this reason we must add a check for 0-sized slices as its pointer field can be undefined.
+        // We only have to do this for slices as arrays will have a valid pointer.
+        // This logic can be removed once https://github.com/ziglang/zig/issues/16360 is done.
+        if (o.target.isWasm() and
+            std.Target.wasm.featureSetHas(o.target.cpu.features, .bulk_memory) and
+            dest_ptr_ty.isSlice(mod))
+        {
+            const llvm_usize_ty = self.context.intType(self.dg.object.target.ptrBitWidth());
+            const cond = try self.cmp(len, llvm_usize_ty.constInt(0, .False), Type.usize, .neq);
+            const memcpy_block = self.context.appendBasicBlock(self.llvm_func, "MemcpyTrapSkip");
+            const end_block = self.context.appendBasicBlock(self.llvm_func, "MemcpyTrapEnd");
+            _ = self.builder.buildCondBr(cond, memcpy_block, end_block);
+            self.builder.positionBuilderAtEnd(memcpy_block);
+            _ = self.builder.buildMemCpy(
+                dest_ptr,
+                dest_ptr_ty.ptrAlignment(mod),
+                src_ptr,
+                src_ptr_ty.ptrAlignment(mod),
+                len,
+                is_volatile,
+            );
+            _ = self.builder.buildBr(end_block);
+            self.builder.positionBuilderAtEnd(end_block);
+            return null;
+        }
+
         _ = self.builder.buildMemCpy(
             dest_ptr,
             dest_ptr_ty.ptrAlignment(mod),

test/standalone.zig

@@ -230,6 +230,10 @@ pub const build_cases = [_]BuildCase{
         .build_root = "test/standalone/cmakedefine",
         .import = @import("standalone/cmakedefine/build.zig"),
     },
+    .{
+        .build_root = "test/standalone/zerolength_check",
+        .import = @import("standalone/zerolength_check/build.zig"),
+    },
 };
 
 const std = @import("std");

test/standalone/zerolength_check/build.zig

@@ -0,0 +1,27 @@
+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+    const test_step = b.step("test", "Test it");
+    b.default_step = test_step;
+
+    add(b, test_step, .Debug);
+    add(b, test_step, .ReleaseFast);
+    add(b, test_step, .ReleaseSmall);
+    add(b, test_step, .ReleaseSafe);
+}
+
+fn add(b: *std.Build, test_step: *std.Build.Step, optimize: std.builtin.OptimizeMode) void {
+    const unit_tests = b.addTest(.{
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = .{
+            .os_tag = .wasi,
+            .cpu_arch = .wasm32,
+            .cpu_features_add = std.Target.wasm.featureSet(&.{.bulk_memory}),
+        },
+        .optimize = optimize,
+    });
+
+    const run_unit_tests = b.addRunArtifact(unit_tests);
+    run_unit_tests.skip_foreign_checks = true;
+    test_step.dependOn(&run_unit_tests.step);
+}

test/standalone/zerolength_check/src/main.zig

@@ -0,0 +1,23 @@
+const std = @import("std");
+
+test {
+    var dest = foo();
+    var source = foo();
+
+    @memcpy(dest, source);
+    @memset(dest, 4);
+    @memset(dest, undefined);
+
+    var dest2 = foo2();
+    @memset(dest2, 0);
+}
+
+fn foo() []u8 {
+    const ptr = comptime std.mem.alignBackward(usize, std.math.maxInt(usize), 1);
+    return @as([*]align(1) u8, @ptrFromInt(ptr))[0..0];
+}
+
+fn foo2() []u64 {
+    const ptr = comptime std.mem.alignBackward(usize, std.math.maxInt(usize), 1);
+    return @as([*]align(1) u64, @ptrFromInt(ptr))[0..0];
+}