Devlog ⚡ Zig Programming Language

This page contains a curated list of recent changes to main branch Zig. This page contains entries for the year 2026. Other years are available in the Devlog archive page. June 26, 2026SPIR-V Backend Progress Author: Ali CheraghiThere’s quite a bit to cover. The SPIR-V backend had bitrotted in a number of places after the recent compiler changes, so I spent the past several weeks dragging it into a better state.@SpirvTypeSPIR-V has a handful of types that couldn’t be expressed in Zig’s type system. The new @SpirvType builtin has been introduced to address the longest-standing blocker for writing shaders. See #20550, #23326 and #35461 to trace the background.const Sampler = @SpirvType(.sampler); const Image = @SpirvType(.{ .image = .{ .usage = .{ .sampled = u32 }, .format = .unknown, .dim = .@"2d", .depth = .unknown, .arrayed = false, .multisampled = false, .access = .unknown, } }); const SampledImage = @SpirvType(.{ .sampled_image = Image }); const RuntimeArray = @SpirvType(.{ .runtime_array = u32 }); const sampled_image = @extern(*addrspace(.constant) const SampledImage, .{ .name = "sampled_image", .decoration = .{ .descriptor = .{ .set = 0, .binding = 1 } }, }); Execution Mode on the Calling ConventionExecution mode info (workgroup size, fragment origin, etc.) is now carried by the calling convention instead of being emitted via inline assembly OpExecutionMode. The old std.gpu.executionMode() helper is gone, and the SPIR-V assembler now rejects manual OpExecutionMode instructions. Two new calling conventions, spirv_task and spirv_mesh, were also added for mesh shading pipelines.export fn vert() callconv(.spirv_vertex) void {} export fn frag() callconv(.{ .spirv_fragment = .{ .depth_assumption = .greater } }) void {} export fn comp() callconv(.{

.spirv_kernel = .{ .x = 8, .y = 8, .z = 1 } }) void {} export fn task() callconv(.{ .spirv_task = .{ .x = 1, .y = 1, .z = 1 } }) void {} export fn mesh() callconv(.{ .spirv_mesh = .{ .stage_output = .output_lines, .max_primitives = 1, .max_vertices = 2 } }) void {} Capabilities and Extensions from CPU FeaturesCapabilities and extensions used to be emitted ad hoc by codegen or via inline assembly. They’re now driven entirely by the CPU feature set like other targets, with dependency chains extracted from SPIRV-Headers (excluding external vendors for now), and the assembler now rejects any attempt to emit OpCapability or OpExtension directly.Multi-Threaded CodegenFrom day one, the SPIR-V backend ran codegen single-threaded inside the linker thread. Each codegen job now produces an Mir value just like every other self-hosted backend, and gets scheduled on the compiler’s thread pool.The same change brought back two ISel passes that had been removed during earlier refactors: dedup_types (which merges equivalent type instructions) and prune_unused (which strips dead code from the final module). These had originally been deleted back when codegen was single-threaded.Object File Linking.spv files are now recognised as object files. You can compile multiple .zig files (or external .spv objects) and have the SPIR-V linker stitch them into a single module.Tens of bugs have also been fixed along the way with a nearly 10% increase in total passing behavior tests (49% now) on the spirv64-vulkan target, std.gpu was renamed to std.spirv and the SPIR-V backend is meaningfully more useful than it was a month ago, but there’s still a long way to go. Plenty of behavior tests remain skipped on SPIR-V. That said, if you’ve been on the fence about trying Zig for shaders or compute kernels, this is a good time to give it a shot. Bug reports are very welcome on Codeberg.

Happy hacking! June 25, 2026New @bitCast Semantics and LLVM Backend Improvements Author: Matthew Lugg(Quite long devlog coming up, apologies—I got a little carried away with this one!)A few weeks ago, I began working on a branch implementing an improvement to the LLVM backend which had been planned for a long time. This ended up snowballing into a bigger change which implemented a few language proposals you might be interested to hear about.LLVM Backend Integer LoweringZig has always lowered arbitrary bit-width integer types (e.g. u4, i13, u40) directly to LLVM IR’s bit-int types (i4, i13, i40). However, we’ve known for a long time that this lowering is not optimal, because LLVM’s documented semantics for representing these types in memory are unnecessarily restrictive to the optimizer. Perhaps more importantly, because Clang never emits LLVM IR like this, these code paths in LLVM have never been properly tested, and so are poorly supported in practice—over the past few years, we have observed many instances of trivial optimizations being missed and even straight-up miscompilations.So, the original goal of the PR was to only use these bit-int types when manipulating values in SSA form, and to zero- or sign-extend them to ABI-sized types (i8, i16, i32, etc) when storing them in memory. This should be well-supported, not least because it matches how Clang lowers C’s _BitInt(N)!That change was actually fairly straightforward, but I hit one issue which led me down a bit of a rabbit-hole.The Problem with @bitCast@bitCast is an interesting builtin. In the past, it was defined as being equivalent to the following sequence of operations:Take a pointer to the operand valueCast it to a pointer to the destination typeLoad from that pointerIn other words, it was essentially syntax sugar for reinterpreting bytes of memory. However, over time, we diverged from this definition—for instance, it became allowed to use @bitCast to reinterpret a [3]u8 as a u24, even though on most targets @sizeOf(u24) is greater than @sizeOf([3]u8) so the above definition would invoke Illegal Behavior.

Up to now, the LLVM backend had implemented these underspecified semantics for the @bitCast builtin. However, because that definition involved reinterpreting memory, changing how we store integer types in memory ended up impacting the implementation of @bitCast, and introducing Illegal Behavior which led to crashes in the compiler test suite.The easiest solution to this would probably have been to implement logic in the LLVM backend to approximately match the old behavior. I instead opted for a better solution—implement a new definition of @bitCast.Redefining @bitCastIn 2024, Jacob Young wrote up language proposal #19755 which aimed to solve the problems with @bitCast by precisely specifying a new set of semantics for it. This proposal was accepted shortly after it was submitted, and in fact, the semantics it details are already implemented by the self-hosted x86_64 backend! So to solve the LLVM backend’s problems, I didn’t necessarily need to match the old @bitCast semantics—instead, this seemed like a good time to finally get the new semantics implemented everywhere.As an aside, another advantage to doing this is that we could take advantage of the compiler’s Legalize pass, which takes difficult-to-lower operations and rewrites them in terms of simpler operations, so that compiler backends only need to support those simple operations. Legalize already had functionality, used by the self-hosted x86_64 backend, which converted complex @bitCast operations into simpler ones, and it could be easily adapted to aid the other compiler backends too (mainly the LLVM and C backends)—but only if they implemented the new semantics.Regardless, the point is, I set out on a side quest (which ended up being harder than the original quest) to implement these new semantics throughout the compiler. This includes not only the LLVM and C backends, but also comptime execution—after all, Zig allows you to do almost any operation at comptime, @bitCast included! Because the new semantics are meaningfully different from the old (more on this later), I also had to audit a lot of uses of @bitCast across the standard library, compiler, and supporting libraries (e.g. compiler_rt).

But after a few mostly-painless fixes for CI failures, I was able to finally get my PR green, and landed it in master yesterday (closing a good few issues in the process!).The New @bitCast SemanticsNow that we’ve gotten through all of the background, it’s finally time for me to actually explain new @bitCast behavior. Instead of being based on reinterpreting bytes in memory like before, the builtin is now defined in terms of the bits which logically represent a type.Every type which supports @bitCast has a “logical bit layout”—a representation of that type as an ordered sequence of bits. For instance, u5 is composed of 5 logical bits, which we order from least-significant to most-significant. [2]u5 is composed of 10 logical bits—the 5 from the first element, followed by the 5 from the second element. The new definition of @bitCast is that it reinterprets the logical bits of one type as the logical bits of a different type.The simplest example is to take an unsigned integer, say a u8, and convert it to a signed integer of the same size, in this case i8. This operation does exactly what you’d expect—the bits are unchanged, and we just reinterpret the most-significant bit as a sign bit. Also unchanged are the semantics of @bitCast between an integer type and a packed struct/packed union type.The place where the new semantics differ from the old is when you get aggregate types (arrays and vectors) involved.Consider, for instance, bitcasting a [2]u8 to a u16. Under the old semantics, the result of this operation depends on the target endian: on big-endian targets, the first array element became the 8 most significant bits, whereas on little-endian targets, the first array element became the 8 least significant bits. Under the new semantics, because we only care about logical bit representation (which is endian-agnostic), the operation behaves identically on every target: the first array element becomes the 8 least significant bits. As a general rule, the new semantics tend to match the behavior of the old semantics on little-endian targets.