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OxCaml changes from minus23 to minus25
OXCAML-CHANGELOG-23-25.md
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OxCaml Changes: 5.2.0minus-23 to 5.2.0minus-25#

This document summarizes the key changes between OxCaml versions 5.2.0minus-23 and 5.2.0minus-25.

Major New Features#

The Dissector: Linking Large Executables (#5146)#

A new compiler pass called the "dissector" enables linking very large executables with the small code model by:

  • Analyzing all object files to compute total ELF section sizes
  • Partitioning object files to prevent relocation overflow
  • Partially linking each partition
  • Creating an intermediate PLT/GOT for cross-partition calls

Enable with -dissector. Additional flags:

  • -dissector-partition-size <gb> - Set partition threshold (default varies)
  • -ddissector - Verbose logging
  • -ddissector-sizes - Dump section sizes per file
  • -ddissector-partitions - Keep partition files for debugging

Untagged Int Arrays (#4643)#

Full support for packed arrays of small integers:

(* New array types - tightly packed *)
let bytes : int8# array = [| #0s; #1s; #255s |]      (* 1 byte/element *)
let shorts : int16# array = [| #0S; #1S; #32767S |]  (* 2 bytes/element *)
let ints : int# array = [| #0; #1; #42 |]            (* native word/element *)
let chars : char# array = [| #'a'; #'b'; #'c' |]     (* 1 byte/element *)

Pattern matching now works with char# ranges:

match c with
| #'a'..#'z' -> `lowercase
| #'A'..#'Z' -> `uppercase
| _ -> `other

Unboxed Elements at Module Top-Level (#4020, #5064)#

Unboxed types can now appear at the top-level of modules:

module M = struct
  let pi : float# = #3.14159
  let answer : int32# = #42l
end

Previously this was prohibited.

SIMD Enhancements#

AVX2 Gather Intrinsics (#5040)#

Gather operations for loading values from non-contiguous memory addresses:

(* Gather using index vector - loads arr[indices[0]], arr[indices[1]], etc. *)
gather_int32x4 ~base ~indices ~scale ~mask
gather_float64x2 ~base ~indices ~scale ~mask

BMI/BMI2 Intrinsics (#5065)#

Complete set of bit manipulation instructions:

  • BMI: andn, bextr, blsi, blsmsk, blsr, tzcnt
  • BMI2: bzhi, mulx, pdep, pext, rorx, sarx, shrx, shlx
  • POPCNT: popcnt_int32, popcnt_int64
  • LZCNT: lzcnt_int32, lzcnt_int64

SIMD Load/Store Intrinsics (#4994)#

Direct memory operations with explicit alignment handling:

(* Aligned/unaligned loads and stores *)
vec128_load_aligned, vec128_store_aligned
vec128_load_unaligned, vec128_store_unaligned
vec256_load_aligned, vec256_store_aligned

(* Non-temporal (streaming) stores *)
vec128_store_aligned_uncached

(* Partial loads/stores *)
vec128_load_low64, vec128_load_low32
vec128_store_low64, vec128_store_low32

128-bit Integer Arithmetic (#5025)#

Support for wide integer arithmetic using register pairs.

Float64/Int64 Cast Builtins (#5114)#

Bitwise reinterpretation between float64 and int64.

Mode System Changes#

New shareable Portability Mode#

The portability axis now has three values instead of two:

nonportable → shareable → portable
  • nonportable: Functions capturing uncontended mutable state
  • shareable: Functions capturing shared state (may execute in parallel)
  • portable: Functions capturing all values at contended (may execute concurrently)

@@ global Implies @@ aliased#

For modalities, @@ global now always implies @@ aliased. Using @@ global unique together is forbidden to ensure soundness of borrowing:

(* OK *)
type t = { field : 'a @@ global aliased }

(* ERROR - forbidden *)
type t = { field : 'a @@ global unique }

Improved Modal Inclusion Errors (#5112)#

Better error messages when mode constraints are violated.

CFG Backend Improvements#

CFG Reducibility Checking (#4920, #4921)#

The compiler now checks for and handles irreducible control flow graphs, with safeguards to prevent optimizations that could create them.

CFG Value Propagation (#4879, #4807)#

Extended value propagation to float values and improved terminator simplification. New flags:

  • -cfg-value-propagation / -no-cfg-value-propagation
  • -cfg-value-propagation-float / -no-cfg-value-propagation-float

Register Allocation Affinity (#5059)#

Basic support for register affinity hints in the allocator.

CFG Peephole: Neutral Element Removal (#4932)#

Removes operations whose operand is a neutral element (e.g., adding 0).

Flambda2 / Optimizer Improvements#

Reaper Enhancements#

  • Auto mode for direct call preservation (#5081): New -reaper-preserve-direct-calls auto option that preserves direct calls only when the reaper cannot identify called functions.
  • Type rewriting (#5043): The reaper can now rewrite types.
  • Local field handling: New -reaper-local-fields flag.

Improved Inlining Metrics (#5116)#

Added profiling counters for inlining decisions that don't decrease code size.

to_cmm Safety Improvements (#4941)#

Prevents illegal re-orderings when converting to Cmm representation.

Runtime Metaprogramming (Experimental)#

Slambda and Quotes (#4776, #5023, #5077)#

Initial support for compile-time metaprogramming:

  • runtime_metaprogramming language extension
  • Slambda splices in Lambda
  • Quote printing and AST mapper fixes

Probes Support#

Unoptimized Implementation (#5007)#

Basic probe support for runtime instrumentation. Closure middle-end support was removed (#4990) - probes now require Flambda 2.

Type System#

With-Bounds for GADTs Re-enabled (#5046)#

Support for with-bounds constraints on GADTs has been restored.

Looser Checking for Staticity (#5075)#

More permissive checking for static expressions.

Compiler Flags#

New Flags#

  • -dissector and related debug flags
  • -cfg-value-propagation[-float]
  • -reaper-preserve-direct-calls auto
  • -reaper-local-fields
  • -reaper-unbox
  • -reaper-change-calling-conventions
  • -flambda2-expert-cmm-safe-subst
  • -ddwarf-metrics-output-file
  • -no-locs (for test output)

Build System Changes#

  • Removed boot directory (#5067)
  • Added make clean and make distclean targets (#5035)
  • Manifest files support (#4986)

Bug Fixes#

  • Fixed computation of code size for Boolean_not switches (#5121)
  • Fixed code size computation when converting switch to lookup table (#5120)
  • Fixed over-estimation of removed primitives due to canonicalization (#5118)
  • Race condition fix for -dump-dir with nonexistent path (#5010)
  • Fixed fiber cache leak (#5017)
  • Don't access globals in tight marking loop (#4997)
  • Avoid touching global variables in sweep loop (#5022)
  • Application type error now preempts mode error (#5073)
  • Fixed simplify terminator pass for irreducible graphs (#5174)
  • Small int SIMD casts no longer sign-extend incorrectly (#4987)

Array Tag Changes (#5126)#

Rearranged array tags to restore backwards compatibility with existing code.

Documentation Updates#

  • Parallelism tutorials now use #(...) syntax for unboxed tuples
  • Clarified portability mode documentation
  • Updated small numbers documentation with array support
  • Fixed contended/uncontended field projection documentation