wiki:MemcpyOptimizations

Version 2 (modified by tibbe, 3 years ago) (diff)

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memcpy/memmove/memset optimizations

Starting with version 7.2 GHC has three new primitives for copying/setting blocks of memory, corresponding to the C functions memcpy, memmove, and memset. GHC optimizes occurrences of these primitives into efficient unrolled loops whenever possible (and calls the corresponding C functions otherwise.)

Implementation

The primitives are implemented as three CallishMachOps, defined in compiler/cmm/CmmMachOp. The code generator generates calls to these CallishMachOps using three utility functions: emitMemcpyCall, emitMemmoveCall, and emitMemsetCall, defined in compiler/codeGen/CgPrimOp.hs (old code generator) and compiler/codeGen/StgCmmPrim.hs (new code generator). The helper functions take an extra parameter that indicates the alignment of the arguments, which is used as a hint by the backends.

The reason the primitives are unrolled in the backends, instead of in the code generator, is to allow us to make use of LLVM's memcpy/memmove/memset intrinsics, which LLVM optimizes well. In the x86/x86-64 backend we unroll the primitives ourselves.

Unrolling heuristics

We currently only unroll a primitive if

  • the address is at least DWORD aligned, and
  • we're operating on at most 128 bytes.

The unrolled primitive uses the widest possible move instruction available on the platform and allowed by the address alignment. The 128 byte threshold is the same as the one used by GCC and LLVM when unrolling the C memcpy/memmove/memset functions.

User API

These primitives are exposed to the user as a set of primitive operations on boxed arrays:

  • copyArray#
  • copyMutableArray#
  • cloneArray#
  • cloneMutableArray#
  • freezeArray#
  • thawArray#

The latter four allow the user to efficiently clone an array without first setting all elements to some dummy element, which would be required to e.g. implement cloneArray# in terms of newArray# and copyArray#. The implementation of these primitive operations are in compiler/cmm/CgPrimOps.hs (old code generator) and compiler/codeGen/StgCmmPrim.hs (new code generator)