julia

8ffcedf6 - codegen: Add `blackbox` and `compilerbarrier(:blackbox, ...)` for benchmarking (#61438)

codegen: Add `blackbox` and `compilerbarrier(:blackbox, ...)` for benchmarking (#61438) Adds `Base.blackbox(x)` and the corresponding `:blackbox` setting for `compilerbarrier`, motivated by the LICM-hoisting issue seen in #61394. `Base.blackbox(x)` returns `x` but treats the output as opaque to the optimizer: it cannot be constant-folded, CSE'd, or treated as loop-invariant. This is Julia's equivalent of Google Benchmark's `DoNotOptimize`. https://google.github.io/benchmark/user_guide.html#preventing-optimization `blackbox` is implemented as `compilerbarrier(:blackbox, x)`, keeping all compiler-barrier functionality consolidated in one builtin rather than introducing a new one. ## Implementation **Julia-level (inference):** `:blackbox` has `consistent=ALWAYS_FALSE` and returns `widenconst(val)`, preventing constant propagation and CSE at the abstract interpretation level. **Codegen level**, three cases based on the value's LLVM type: - **Non-pointer scalars** (integers, floats — anything that fits in a register): emits `asm "" "=r,0"` directly, tying the output register to the input. - **Boxed/GC-tracked pointers**: cannot use register-tied asm on GC-tracked address spaces. Instead, emits a `julia.blackbox` intrinsic call that is lowered to `asm "" "=r,0"` on the raw untracked pointer after GC frame expansion. - **Unboxed aggregates / unboxed pointers** (e.g., structs, `Ptr{T}`): uses `asm sideeffect "" "~{memory}"` to clobber memory, making the value appear non-invariant without needing a register constraint. Ghost types (`Nothing`, etc.) pass through unchanged with no code emitted. --------- Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: Valentin Churavy <v.churavy@gmail.com> Co-authored-by: Keno Fischer <1291671+Keno@users.noreply.github.com>