Sema: correct AIR semantics around array concat optimization

Author: Rexicon226

src/Sema.zig

@@ -14562,8 +14562,9 @@ fn zirArrayCat(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Ai
 
         const mutable_alloc = try block.addTy(.alloc, alloc_ty);
 
-        // if both the source and destination are arrays
-        // we can hotpath via a memcpy.
+        // there's nothing to copy
+        if (result_len == 0 and res_sent_val == null) return mutable_alloc;
+
         if (lhs_ty.zigTypeTag(zcu) == .pointer and
             rhs_ty.zigTypeTag(zcu) == .pointer)
         {
@@ -14576,48 +14577,79 @@ fn zirArrayCat(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Ai
             });
 
             const many_ty = slice_ty.slicePtrFieldType(zcu);
-            const many_alloc = try block.addBitCast(many_ty, mutable_alloc);
+            const many_ty_ref = Air.internedToRef(many_ty.toIntern());
+            // only used when both are non-zero lengths
+            const many_alloc = if (lhs_len != 0 and rhs_len != 0)
+                try block.addBitCast(many_ty, mutable_alloc)
+            else
+                undefined;
 
-            // lhs_dest_slice = dest[0..lhs.len]
-            const slice_ty_ref = Air.internedToRef(slice_ty.toIntern());
             const lhs_len_ref = try pt.intRef(.usize, lhs_len);
-            const lhs_dest_slice = try block.addInst(.{
-                .tag = .slice,
-                .data = .{ .ty_pl = .{
-                    .ty = slice_ty_ref,
-                    .payload = try sema.addExtra(Air.Bin{
-                        .lhs = many_alloc,
-                        .rhs = lhs_len_ref,
-                    }),
-                } },
-            });
-
-            _ = try block.addBinOp(.memcpy, lhs_dest_slice, lhs);
+            const slice_ty_ref = Air.internedToRef(slice_ty.toIntern());
 
-            // rhs_dest_slice = dest[lhs.len..][0..rhs.len]
-            const rhs_len_ref = try pt.intRef(.usize, rhs_len);
-            const rhs_dest_offset = try block.addInst(.{
-                .tag = .ptr_add,
-                .data = .{ .ty_pl = .{
-                    .ty = Air.internedToRef(many_ty.toIntern()),
-                    .payload = try sema.addExtra(Air.Bin{
-                        .lhs = many_alloc,
-                        .rhs = lhs_len_ref,
-                    }),
-                } },
-            });
-            const rhs_dest_slice = try block.addInst(.{
-                .tag = .slice,
-                .data = .{ .ty_pl = .{
-                    .ty = slice_ty_ref,
-                    .payload = try sema.addExtra(Air.Bin{
-                        .lhs = rhs_dest_offset,
-                        .rhs = rhs_len_ref,
+            if (lhs_len != 0) {
+                const lhs_dest_slice = switch (rhs_len) {
+                    0 => try block.addTyOp(.array_to_slice, slice_ty, mutable_alloc),
+                    else => try block.addInst(.{
+                        .tag = .slice,
+                        .data = .{ .ty_pl = .{
+                            .ty = slice_ty_ref,
+                            .payload = try sema.addExtra(Air.Bin{ .lhs = many_alloc, .rhs = lhs_len_ref }),
+                        } },
                     }),
-                } },
-            });
-
-            _ = try block.addBinOp(.memcpy, rhs_dest_slice, rhs);
+                };
+                const lhs_src_pointer = if (lhs_ty.isSlice(zcu))
+                    try block.addInst(.{
+                        .tag = .slice_ptr,
+                        .data = .{ .ty_op = .{
+                            .ty = many_ty_ref,
+                            .operand = lhs,
+                        } },
+                    })
+                else
+                    lhs;
+                _ = try block.addBinOp(.memcpy, lhs_dest_slice, lhs_src_pointer);
+            }
+
+            if (rhs_len != 0) {
+                const rhs_dest_slice = switch (lhs_len) {
+                    0 => try block.addTyOp(.array_to_slice, slice_ty, mutable_alloc),
+                    else => s: {
+                        const rhs_len_ref = try pt.intRef(.usize, rhs_len);
+                        const rhs_dest_offset = switch (lhs_len) {
+                            0 => many_alloc,
+                            else => try block.addInst(.{
+                                .tag = .ptr_add,
+                                .data = .{ .ty_pl = .{
+                                    .ty = many_ty_ref,
+                                    .payload = try sema.addExtra(Air.Bin{ .lhs = many_alloc, .rhs = lhs_len_ref }),
+                                } },
+                            }),
+                        };
+                        break :s try block.addInst(.{
+                            .tag = .slice,
+                            .data = .{ .ty_pl = .{
+                                .ty = slice_ty_ref,
+                                .payload = try sema.addExtra(Air.Bin{
+                                    .lhs = rhs_dest_offset,
+                                    .rhs = rhs_len_ref,
+                                }),
+                            } },
+                        });
+                    },
+                };
+                const rhs_src_pointer = if (rhs_ty.isSlice(zcu))
+                    try block.addInst(.{
+                        .tag = .slice_ptr,
+                        .data = .{ .ty_op = .{
+                            .ty = many_ty_ref,
+                            .operand = rhs,
+                        } },
+                    })
+                else
+                    rhs;
+                _ = try block.addBinOp(.memcpy, rhs_dest_slice, rhs_src_pointer);
+            }
 
             if (res_sent_val) |sent_val| {
                 const elem_index = try pt.intRef(.usize, result_len);

test/behavior/array.zig

@@ -92,6 +92,12 @@ test "array init with concat" {
     try expect(std.mem.eql(u8, &i, "abcd"));
 }
 
+test "array concat zero length pointers" {
+    var x: *const [0]u8 = &.{};
+    _ = &x;
+    _ = @as([]const u8, "") ++ x;
+}
+
 test "array init with mult" {
     if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO