Zig overhauls @bitCast semantics, rethinks integer lowering for LLVM, advances SPIR-V

Zig’s latest devlog covers two substantial compiler efforts. The first is a long-planned fix to how the LLVM backend lowers arbitrary-width integers (u4, i13, u40 and the like). Zig had mapped these directly onto LLVM’s bit-int types, but those code paths are poorly tested—Clang never emits them—and have produced missed optimizations and outright miscompilations. The new approach keeps bit-int types only for SSA values and zero- or sign-extends them to ABI-sized integers in memory, matching how Clang lowers C’s _BitInt(N). That seemingly narrow change exposed problems in @bitCast, whose old definition amounted to reinterpreting raw bytes through a pointer. Rather than patch the legacy behavior, the team implemented the precise replacement semantics from accepted proposal #19755 across every backend plus comptime execution, which also lets the compiler’s Legalize pass rewrite tricky bitcasts into simpler operations. The work required auditing @bitCast uses throughout the standard library and compiler.