tint: Add PreservePadding transform

This is used to ensure that assignments to host-visible memory do not
modify padding bytes in structures and arrays. We decompose
assignments of whole structure and array types into member-wise or
element-wise copies, using helper functions.

This is used in all backends except HLSL, which already decomposes
memory accesses.

Bug: tint:1571
Change-Id: Id6de2f917fb80151cc654a7e1c8413ae956f0d61
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/112720
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>

src/dawn/tests/end2end/ComputeLayoutMemoryBufferTests.cpp

@@ -526,6 +526,9 @@ std::string AlignDeco(uint32_t value) {
 
 // Test different types used as a struct member
 TEST_P(ComputeLayoutMemoryBufferTests, StructMember) {
+    // TODO(crbug.com/dawn/1606): find out why these tests fail on Windows for OpenGL.
+    DAWN_TEST_UNSUPPORTED_IF(IsOpenGLES() && IsWindows());
+
     // Sentinel value markers codes used to check that the start and end of
     // structures are correctly aligned. Each of these codes are distinct and
     // are not likely to be confused with data.
@@ -694,6 +697,9 @@ fn main() {
 
 // Test different types that used directly as buffer type
 TEST_P(ComputeLayoutMemoryBufferTests, NonStructMember) {
+    // TODO(crbug.com/dawn/1606): find out why these tests fail on Windows for OpenGL.
+    DAWN_TEST_UNSUPPORTED_IF(IsOpenGLES() && IsWindows());
+
     auto params = GetParam();
 
     Field& field = params.mField;

src/tint/BUILD.gn

@@ -529,6 +529,8 @@ libtint_source_set("libtint_core_all_src") {
     "transform/packed_vec3.h",
     "transform/pad_structs.cc",
     "transform/pad_structs.h",
+    "transform/preserve_padding.cc",
+    "transform/preserve_padding.h",
     "transform/promote_initializers_to_let.cc",
     "transform/promote_initializers_to_let.h",
     "transform/promote_side_effects_to_decl.cc",
@@ -1244,6 +1246,7 @@ if (tint_build_unittests) {
       "transform/num_workgroups_from_uniform_test.cc",
       "transform/packed_vec3_test.cc",
       "transform/pad_structs_test.cc",
+      "transform/preserve_padding_test.cc",
       "transform/promote_initializers_to_let_test.cc",
       "transform/promote_side_effects_to_decl_test.cc",
       "transform/remove_continue_in_switch_test.cc",

src/tint/CMakeLists.txt

@@ -453,6 +453,8 @@ list(APPEND TINT_LIB_SRCS
   transform/packed_vec3.h
   transform/pad_structs.cc
   transform/pad_structs.h
+  transform/preserve_padding.cc
+  transform/preserve_padding.h
   transform/promote_initializers_to_let.cc
   transform/promote_initializers_to_let.h
   transform/promote_side_effects_to_decl.cc
@@ -1212,6 +1214,7 @@ if(TINT_BUILD_TESTS)
       transform/num_workgroups_from_uniform_test.cc
       transform/packed_vec3_test.cc
       transform/pad_structs_test.cc
+      transform/preserve_padding_test.cc
       transform/promote_initializers_to_let_test.cc
       transform/promote_side_effects_to_decl_test.cc
       transform/remove_continue_in_switch_test.cc

src/tint/transform/preserve_padding.cc

@@ -0,0 +1,226 @@
+// Copyright 2022 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/tint/transform/preserve_padding.h"
+
+#include <unordered_set>
+#include <utility>
+
+#include "src/tint/program_builder.h"
+#include "src/tint/sem/reference.h"
+#include "src/tint/sem/struct.h"
+#include "src/tint/utils/map.h"
+#include "src/tint/utils/vector.h"
+
+TINT_INSTANTIATE_TYPEINFO(tint::transform::PreservePadding);
+
+using namespace tint::number_suffixes;  // NOLINT
+
+namespace tint::transform {
+
+PreservePadding::PreservePadding() = default;
+
+PreservePadding::~PreservePadding() = default;
+
+/// The PIMPL state for the PreservePadding transform
+struct PreservePadding::State {
+    /// Constructor
+    /// @param src the source Program
+    explicit State(const Program* src) : ctx{&b, src, /* auto_clone_symbols */ true} {}
+
+    /// The main function for the transform.
+    /// @returns the ApplyResult
+    ApplyResult Run() {
+        // Gather a list of assignments that need to be transformed.
+        std::unordered_set<const ast::AssignmentStatement*> assignments_to_transform;
+        for (auto* node : ctx.src->ASTNodes().Objects()) {
+            Switch(
+                node,  //
+                [&](const ast::AssignmentStatement* assign) {
+                    auto* ty = sem.Get(assign->lhs)->Type();
+                    if (assign->lhs->Is<ast::PhonyExpression>()) {
+                        // Ignore phony assignment.
+                        return;
+                    }
+                    if (ty->As<sem::Reference>()->AddressSpace() != ast::AddressSpace::kStorage) {
+                        // We only care about assignments that write to variables in the storage
+                        // address space, as nothing else is host-visible.
+                        return;
+                    }
+                    if (HasPadding(ty->UnwrapRef())) {
+                        // The assigned type has padding bytes, so we need to decompose the writes.
+                        assignments_to_transform.insert(assign);
+                    }
+                },
+                [&](const ast::Enable* enable) {
+                    // Check if the full pointer parameters extension is already enabled.
+                    if (enable->extension ==
+                        ast::Extension::kChromiumExperimentalFullPtrParameters) {
+                        ext_enabled = true;
+                    }
+                });
+        }
+        if (assignments_to_transform.empty()) {
+            return SkipTransform;
+        }
+
+        // Replace all assignments that include padding with decomposed versions.
+        ctx.ReplaceAll([&](const ast::AssignmentStatement* assign) -> const ast::Statement* {
+            if (!assignments_to_transform.count(assign)) {
+                return nullptr;
+            }
+            auto* ty = sem.Get(assign->lhs)->Type()->UnwrapRef();
+            return MakeAssignment(ty, ctx.Clone(assign->lhs), ctx.Clone(assign->rhs));
+        });
+
+        ctx.Clone();
+        return Program(std::move(b));
+    }
+
+    /// Create a statement that will perform the assignment `lhs = rhs`, creating and using helper
+    /// functions to decompose the assignment into element-wise copies if needed.
+    /// @param ty the type of the assignment
+    /// @param lhs the lhs expression (in the destination program)
+    /// @param rhs the rhs expression (in the destination program)
+    /// @returns the statement that performs the assignment
+    const ast::Statement* MakeAssignment(const sem::Type* ty,
+                                         const ast::Expression* lhs,
+                                         const ast::Expression* rhs) {
+        if (!HasPadding(ty)) {
+            // No padding - use a regular assignment.
+            return b.Assign(lhs, rhs);
+        }
+
+        // Call (and create if necessary) a helper function that assigns a composite using the
+        // statements in `body`. The helper will have the form:
+        //   fn assign_helper_T(dest : ptr<storage, T, read_write>, value : T) {
+        //     <body>
+        //   }
+        // It will be called by passing a pointer to the original LHS:
+        //   assign_helper_T(&lhs, rhs);
+        //
+        // Since this requires passing pointers to the storage address space, this will also enable
+        // the chromium_experimental_full_ptr_parameters extension.
+        constexpr const char* kDestParamName = "dest";
+        constexpr const char* kValueParamName = "value";
+        auto call_helper = [&](auto&& body) {
+            EnableExtension();
+            auto helper = helpers.GetOrCreate(ty, [&]() {
+                auto helper_name = b.Symbols().New("assign_and_preserve_padding");
+                utils::Vector<const ast::Parameter*, 2> params = {
+                    b.Param(kDestParamName,
+                            b.ty.pointer(CreateASTTypeFor(ctx, ty), ast::AddressSpace::kStorage,
+                                         ast::Access::kReadWrite)),
+                    b.Param(kValueParamName, CreateASTTypeFor(ctx, ty)),
+                };
+                b.Func(helper_name, params, b.ty.void_(), body());
+                return helper_name;
+            });
+            return b.CallStmt(b.Call(helper, b.AddressOf(lhs), rhs));
+        };
+
+        return Switch(
+            ty,  //
+            [&](const sem::Array* arr) {
+                // Call a helper function that uses a loop to assigns each element separately.
+                return call_helper([&]() {
+                    utils::Vector<const ast::Statement*, 8> body;
+                    auto* idx = b.Var("i", b.Expr(0_u));
+                    body.Push(
+                        b.For(b.Decl(idx), b.LessThan(idx, u32(arr->ConstantCount().value())),
+                              b.Assign(idx, b.Add(idx, 1_u)),
+                              b.Block(MakeAssignment(arr->ElemType(),
+                                                     b.IndexAccessor(b.Deref(kDestParamName), idx),
+                                                     b.IndexAccessor(kValueParamName, idx)))));
+                    return body;
+                });
+            },
+            [&](const sem::Struct* str) {
+                // Call a helper function that assigns each member separately.
+                return call_helper([&]() {
+                    utils::Vector<const ast::Statement*, 8> body;
+                    for (auto member : str->Members()) {
+                        auto name = sym.NameFor(member->Declaration()->symbol);
+                        body.Push(MakeAssignment(member->Type(),
+                                                 b.MemberAccessor(b.Deref(kDestParamName), name),
+                                                 b.MemberAccessor(kValueParamName, name)));
+                    }
+                    return body;
+                });
+            },
+            [&](Default) {
+                TINT_ICE(Transform, b.Diagnostics()) << "unhandled type with padding";
+                return nullptr;
+            });
+    }
+
+    /// Checks if a type contains padding bytes.
+    /// @param ty the type to check
+    /// @returns true if `ty` (or any of its contained types) have padding bytes
+    bool HasPadding(const sem::Type* ty) {
+        return Switch(
+            ty,  //
+            [&](const sem::Array* arr) {
+                auto* elem_ty = arr->ElemType();
+                if (elem_ty->Size() % elem_ty->Align() > 0) {
+                    return true;
+                }
+                return HasPadding(elem_ty);
+            },
+            [&](const sem::Struct* str) {
+                uint32_t current_offset = 0;
+                for (auto* member : str->Members()) {
+                    if (member->Offset() > current_offset) {
+                        return true;
+                    }
+                    if (HasPadding(member->Type())) {
+                        return true;
+                    }
+                    current_offset += member->Type()->Size();
+                }
+                return (current_offset < str->Size());
+            },
+            [&](Default) { return false; });
+    }
+
+    /// Enable the full pointer parameters extension, if we have not already done so.
+    void EnableExtension() {
+        if (!ext_enabled) {
+            b.Enable(ast::Extension::kChromiumExperimentalFullPtrParameters);
+            ext_enabled = true;
+        }
+    }
+
+  private:
+    /// The program builder
+    ProgramBuilder b;
+    /// The clone context
+    CloneContext ctx;
+    /// Alias to the semantic info in ctx.src
+    const sem::Info& sem = ctx.src->Sem();
+    /// Alias to the symbols in ctx.src
+    const SymbolTable& sym = ctx.src->Symbols();
+    /// Flag to track whether we have already enabled the full pointer parameters extension.
+    bool ext_enabled = false;
+    /// Map of semantic types to their assignment helper functions.
+    utils::Hashmap<const sem::Type*, Symbol, 8> helpers;
+};
+
+Transform::ApplyResult PreservePadding::Apply(const Program* program,
+                                              const DataMap&,
+                                              DataMap&) const {
+    return State(program).Run();
+}
+
+}  // namespace tint::transform

src/tint/transform/preserve_padding.h

@@ -0,0 +1,47 @@
+// Copyright 2022 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SRC_TINT_TRANSFORM_PRESERVE_PADDING_H_
+#define SRC_TINT_TRANSFORM_PRESERVE_PADDING_H_
+
+#include "src/tint/transform/transform.h"
+
+namespace tint::transform {
+
+/// Decompose assignments of whole structure and array types to preserve padding bytes.
+///
+/// WGSL states that memory operations on structures and arrays will not access padding bytes. To
+/// avoid overwriting padding bytes when writing to buffers, this transform decomposes those
+/// assignments into element-wise assignments via helper functions.
+///
+/// @note Assumes that the DirectVariableTransform will be run afterwards for backends that need it.
+class PreservePadding final : public Castable<PreservePadding, Transform> {
+  public:
+    /// Constructor
+    PreservePadding();
+    /// Destructor
+    ~PreservePadding() override;
+
+    /// @copydoc Transform::Apply
+    ApplyResult Apply(const Program* program,
+                      const DataMap& inputs,
+                      DataMap& outputs) const override;
+
+  private:
+    struct State;
+};
+
+}  // namespace tint::transform
+
+#endif  // SRC_TINT_TRANSFORM_PRESERVE_PADDING_H_

src/tint/transform/preserve_padding_test.cc

@@ -0,0 +1,677 @@
+// Copyright 2022 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/tint/transform/preserve_padding.h"
+
+#include <utility>
+
+#include "src/tint/transform/test_helper.h"
+
+namespace tint::transform {
+namespace {
+
+using PreservePaddingTest = TransformTest;
+
+TEST_F(PreservePaddingTest, ShouldRun_EmptyModule) {
+    auto* src = R"()";
+
+    EXPECT_FALSE(ShouldRun<PreservePadding>(src));
+}
+
+TEST_F(PreservePaddingTest, ShouldRun_NonStructVec3) {
+    auto* src = R"(
+@group(0) @binding(0) var<storage, read_write> v : vec3<u32>;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = vec3<u32>();
+}
+    )";
+
+    EXPECT_FALSE(ShouldRun<PreservePadding>(src));
+}
+
+TEST_F(PreservePaddingTest, ShouldRun_StructWithoutPadding) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : u32,
+  c : u32,
+  d : u32,
+  e : vec3<u32>,
+  f : u32,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+    )";
+
+    EXPECT_FALSE(ShouldRun<PreservePadding>(src));
+}
+
+TEST_F(PreservePaddingTest, ShouldRun_ArrayWithoutPadding) {
+    auto* src = R"(
+@group(0) @binding(0) var<storage, read_write> v : array<vec4<u32>, 4>;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = array<vec4<u32>, 4>();
+}
+    )";
+
+    EXPECT_FALSE(ShouldRun<PreservePadding>(src));
+}
+
+TEST_F(PreservePaddingTest, EmptyModule) {
+    auto* src = R"()";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, StructTrailingPadding) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : u32,
+  c : u32,
+  d : u32,
+  e : vec3<u32>,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+struct S {
+  a : u32,
+  b : u32,
+  c : u32,
+  d : u32,
+  e : vec3<u32>,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+fn assign_and_preserve_padding(dest : ptr<storage, S, read_write>, value : S) {
+  (*(dest)).a = value.a;
+  (*(dest)).b = value.b;
+  (*(dest)).c = value.c;
+  (*(dest)).d = value.d;
+  (*(dest)).e = value.e;
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), S());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, StructInternalPadding) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : vec4<u32>,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+struct S {
+  a : u32,
+  b : vec4<u32>,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+fn assign_and_preserve_padding(dest : ptr<storage, S, read_write>, value : S) {
+  (*(dest)).a = value.a;
+  (*(dest)).b = value.b;
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), S());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, StructExplicitSize_TrailingPadding) {
+    auto* src = R"(
+struct S {
+  @size(16) a : u32,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+struct S {
+  @size(16)
+  a : u32,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+fn assign_and_preserve_padding(dest : ptr<storage, S, read_write>, value : S) {
+  (*(dest)).a = value.a;
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), S());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, StructExplicitSize_InternalPadding) {
+    auto* src = R"(
+struct S {
+  @size(16) a : u32,
+  b : u32,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+struct S {
+  @size(16)
+  a : u32,
+  b : u32,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+fn assign_and_preserve_padding(dest : ptr<storage, S, read_write>, value : S) {
+  (*(dest)).a = value.a;
+  (*(dest)).b = value.b;
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), S());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NestedStructs) {
+    auto* src = R"(
+struct S1 {
+  a1 : u32,
+  b1 : vec3<u32>,
+  c1 : u32,
+}
+
+struct S2 {
+  a2 : u32,
+  b2 : S1,
+  c2 : S1,
+}
+
+struct S3 {
+  a3 : S1,
+  b3 : S2,
+  c3 : S2,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S3;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S3();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+struct S1 {
+  a1 : u32,
+  b1 : vec3<u32>,
+  c1 : u32,
+}
+
+struct S2 {
+  a2 : u32,
+  b2 : S1,
+  c2 : S1,
+}
+
+struct S3 {
+  a3 : S1,
+  b3 : S2,
+  c3 : S2,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S3;
+
+fn assign_and_preserve_padding_1(dest : ptr<storage, S1, read_write>, value : S1) {
+  (*(dest)).a1 = value.a1;
+  (*(dest)).b1 = value.b1;
+  (*(dest)).c1 = value.c1;
+}
+
+fn assign_and_preserve_padding_2(dest : ptr<storage, S2, read_write>, value : S2) {
+  (*(dest)).a2 = value.a2;
+  assign_and_preserve_padding_1(&((*(dest)).b2), value.b2);
+  assign_and_preserve_padding_1(&((*(dest)).c2), value.c2);
+}
+
+fn assign_and_preserve_padding(dest : ptr<storage, S3, read_write>, value : S3) {
+  assign_and_preserve_padding_1(&((*(dest)).a3), value.a3);
+  assign_and_preserve_padding_2(&((*(dest)).b3), value.b3);
+  assign_and_preserve_padding_2(&((*(dest)).c3), value.c3);
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), S3());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, ArrayOfVec3) {
+    auto* src = R"(
+@group(0) @binding(0) var<storage, read_write> v : array<vec3<u32>, 4>;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = array<vec3<u32>, 4>();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+@group(0) @binding(0) var<storage, read_write> v : array<vec3<u32>, 4>;
+
+fn assign_and_preserve_padding(dest : ptr<storage, array<vec3<u32>, 4u>, read_write>, value : array<vec3<u32>, 4u>) {
+  for(var i = 0u; (i < 4u); i = (i + 1u)) {
+    (*(dest))[i] = value[i];
+  }
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), array<vec3<u32>, 4>());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, ArrayOfArray) {
+    auto* src = R"(
+type Array = array<array<vec3<u32>, 4>, 3>;
+
+@group(0) @binding(0) var<storage, read_write> v : Array;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = Array();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+type Array = array<array<vec3<u32>, 4>, 3>;
+
+@group(0) @binding(0) var<storage, read_write> v : Array;
+
+fn assign_and_preserve_padding_1(dest : ptr<storage, array<vec3<u32>, 4u>, read_write>, value : array<vec3<u32>, 4u>) {
+  for(var i = 0u; (i < 4u); i = (i + 1u)) {
+    (*(dest))[i] = value[i];
+  }
+}
+
+fn assign_and_preserve_padding(dest : ptr<storage, array<array<vec3<u32>, 4u>, 3u>, read_write>, value : array<array<vec3<u32>, 4u>, 3u>) {
+  for(var i = 0u; (i < 3u); i = (i + 1u)) {
+    assign_and_preserve_padding_1(&((*(dest))[i]), value[i]);
+  }
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), Array());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, ArrayOfStructOfArray) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : array<vec3<u32>, 4>,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : array<S, 3>;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = array<S, 3>();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+struct S {
+  a : u32,
+  b : array<vec3<u32>, 4>,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : array<S, 3>;
+
+fn assign_and_preserve_padding_2(dest : ptr<storage, array<vec3<u32>, 4u>, read_write>, value : array<vec3<u32>, 4u>) {
+  for(var i = 0u; (i < 4u); i = (i + 1u)) {
+    (*(dest))[i] = value[i];
+  }
+}
+
+fn assign_and_preserve_padding_1(dest : ptr<storage, S, read_write>, value : S) {
+  (*(dest)).a = value.a;
+  assign_and_preserve_padding_2(&((*(dest)).b), value.b);
+}
+
+fn assign_and_preserve_padding(dest : ptr<storage, array<S, 3u>, read_write>, value : array<S, 3u>) {
+  for(var i = 0u; (i < 3u); i = (i + 1u)) {
+    assign_and_preserve_padding_1(&((*(dest))[i]), value[i]);
+  }
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), array<S, 3>());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NoModify_Vec3) {
+    auto* src = R"(
+@group(0) @binding(0) var<storage, read_write> v : vec3<u32>;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = vec3<u32>();
+}
+)";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, AvoidDuplicateEnables) {
+    auto* src = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+struct S {
+  @size(16) a : u32,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+)";
+
+    auto* expect = R"(
+enable chromium_experimental_full_ptr_parameters;
+
+struct S {
+  @size(16)
+  a : u32,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+fn assign_and_preserve_padding(dest : ptr<storage, S, read_write>, value : S) {
+  (*(dest)).a = value.a;
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  assign_and_preserve_padding(&(v), S());
+}
+)";
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NoModify_Mat3x3) {
+    auto* src = R"(
+@group(0) @binding(0) var<storage, read_write> v : mat3x3<f32>;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = mat3x3<f32>();
+}
+)";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NoModify_StructNoPadding) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : u32,
+  c : u32,
+  d : u32,
+  e : vec4<u32>,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+)";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NoModify_ArrayNoPadding) {
+    auto* src = R"(
+@group(0) @binding(0) var<storage, read_write> v : array<vec4<u32>, 4>;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = array<vec4<u32>, 4>();
+}
+)";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NoModify_ArrayOfStructNoPadding) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : u32,
+  c : u32,
+  d : u32,
+  e : vec4<u32>,
+}
+
+@group(0) @binding(0) var<storage, read_write> v : array<S, 4>;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = array<S, 4>();
+}
+)";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NoModify_Workgroup) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : vec3<u32>,
+}
+
+var<workgroup> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+)";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NoModify_Private) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : vec3<u32>,
+}
+
+var<private> v : S;
+
+@compute @workgroup_size(1)
+fn foo() {
+  v = S();
+}
+)";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+TEST_F(PreservePaddingTest, NoModify_Function) {
+    auto* src = R"(
+struct S {
+  a : u32,
+  b : vec3<u32>,
+}
+
+@compute @workgroup_size(1)
+fn foo() {
+  var<function> v : S;
+  v = S();
+}
+)";
+
+    auto* expect = src;
+
+    auto got = Run<PreservePadding>(src);
+
+    EXPECT_EQ(expect, str(got));
+}
+
+}  // namespace
+}  // namespace tint::transform

src/tint/writer/glsl/generator_impl.cc

@@ -59,6 +59,7 @@
 #include "src/tint/transform/expand_compound_assignment.h"
 #include "src/tint/transform/manager.h"
 #include "src/tint/transform/pad_structs.h"
+#include "src/tint/transform/preserve_padding.h"
 #include "src/tint/transform/promote_initializers_to_let.h"
 #include "src/tint/transform/promote_side_effects_to_decl.h"
 #include "src/tint/transform/remove_phonies.h"
@@ -210,6 +211,9 @@ SanitizedResult Sanitize(const Program* in,
     manager.Add<transform::Renamer>();
     data.Add<transform::Renamer::Config>(transform::Renamer::Target::kGlslKeywords,
                                          /* preserve_unicode */ false);
+
+    manager.Add<transform::PreservePadding>();  // Must come before DirectVariableAccess
+
     manager.Add<transform::Unshadow>();  // Must come before DirectVariableAccess
     manager.Add<transform::DirectVariableAccess>();
 

src/tint/writer/msl/generator_impl.cc

@@ -67,6 +67,7 @@
 #include "src/tint/transform/manager.h"
 #include "src/tint/transform/module_scope_var_to_entry_point_param.h"
 #include "src/tint/transform/packed_vec3.h"
+#include "src/tint/transform/preserve_padding.h"
 #include "src/tint/transform/promote_initializers_to_let.h"
 #include "src/tint/transform/promote_side_effects_to_decl.h"
 #include "src/tint/transform/remove_phonies.h"
@@ -219,6 +220,8 @@ SanitizedResult Sanitize(const Program* in, const Options& options) {
     }
     manager.Add<transform::MultiplanarExternalTexture>();
 
+    manager.Add<transform::PreservePadding>();
+
     manager.Add<transform::Unshadow>();
 
     if (!options.disable_workgroup_init) {

src/tint/writer/spirv/generator_impl.cc

@@ -28,6 +28,7 @@
 #include "src/tint/transform/for_loop_to_loop.h"
 #include "src/tint/transform/manager.h"
 #include "src/tint/transform/merge_return.h"
+#include "src/tint/transform/preserve_padding.h"
 #include "src/tint/transform/promote_side_effects_to_decl.h"
 #include "src/tint/transform/remove_phonies.h"
 #include "src/tint/transform/remove_unreachable_statements.h"
@@ -78,6 +79,8 @@ SanitizedResult Sanitize(const Program* in, const Options& options) {
     }
     manager.Add<transform::MultiplanarExternalTexture>();
 
+    manager.Add<transform::PreservePadding>();  // Must come before DirectVariableAccess
+
     manager.Add<transform::Unshadow>();  // Must come before DirectVariableAccess
     bool disable_workgroup_init_in_sanitizer =
         options.disable_workgroup_init || options.use_zero_initialize_workgroup_memory_extension;