wiki:SIMD/Implementation/Status

The prototype implementation of the above specification is vailable as the simd branch of GHC.

General plan

Vector types

Vectors of the following types are implemented: Int32, Int64, Float, and Double.

Fixed and variable sized vectors

For each type, currently only one vector width is implemented, namely the width that is appropriate for SSE2. This means that vectors are currently all 16 bytes in size.

Code generators

Only the LLVM code generator is supported.

Cmm layer

Our CmmType representation for vectors differs slightly from the proposal. See cmm/CmmType.hs.

See cmm/CmmMachOp.hs for the new vector MachOps?.

Core layer

The implementation differs from the proposal in its naming scheme. We wanted to avoid overloading the term "vector," so, e.g., a 4-wide SIMD vector of Float#s is a FloatX4#.

See compiler/prelude/primops.txt.pp for the new primops. Not everything in the proposal is implemented, but we do have a useful subset.

Native vector sizes

This is unimplemented. Instead we define a higher-level Multi data family whose instance is platform-dependent. For example, a Multi Int is represented using an Int32X4# on a 32-bit platform, and by a Int64X2# on a 64-bit platform.

ABIs and calling conventions

Integrating variable-sized vectors with GHC's calling convention is a challenge. How many new registers do we add? Do we add registers for each vector type? The correct approach is unclear, so the current implementation passes all SIMD vectors on the stack.

Memory alignment for vectors

The implementation does not attempt to align memory containing SIMD vectors. SIMD vector loads and stores do not assume alignment.

Last modified 2 years ago Last modified on Apr 16, 2012 12:36:13 PM