AltiVec is a floating point and integer SIMD instruction set designed and owned by Apple, IBM and Freescale Semiconductor, formerly the Semiconductor Products Sector of Motorola, (the AIM alliance), and implemented on versions of the PowerPC including Motorola's G4, IBM's G5 and POWER6 processors, and P.A. Semi's PWRficient PA6T. AltiVec is a trademark owned solely by Freescale, so the system is also referred to as Velocity Engine by Apple and VMX by IBM and P.A. Semi, although IBM has recently begun using AltiVec as well.
While AltiVec refers to an instruction set, the implementations in CPUs produced by IBM and Motorola are separate in terms of logic design. To date, no IBM core has included an AltiVec logic design licensed from Motorola or vice-versa.
AltiVec is a standard part of the new Power ISA v.2.03 specification. It was never formally a part of the PowerPC architecture until this specification although it used PowerPC instruction formats and syntax and occupied the opcode space expressly allocated for such purposes.
Comparison to x86-64 SSE
Both AltiVec and SSE feature 128-bit vector registers that can represent sixteen 8-bit signed or unsigned chars, eight 16-bit signed or unsigned shorts, four 32-bit ints or four 32-bit floating point variables. Both provide cache-control instructions intended to minimize cache pollution when working on streams of data.
They also exhibit important differences. Unlike SSE2, AltiVec supports a special RGB "pixel" data type, but it does not operate on 64-bit double precision floats, and there is no way to move data directly between scalar and vector registers. In keeping with the "load/store" model of the PowerPC's RISC design, the vector registers, like the scalar registers, can only be loaded from and stored to memory. However, AltiVec provides a much more complete set of "horizontal" operations that work across all the elements of a vector; the allowable combinations of data type and operations are much more complete. Thirty-two 128-bit vector registers are provided, compared to eight for SSE and SSE2 (extended to 16 in x86-64), and most AltiVec instructions take three register operands compared to only two register/register or register/memory operands on IA-32.
AltiVec is also unique in its support for a flexible vector permute instruction, in which each byte of a resulting vector value can be taken from any byte of either of two other vectors, parametrized by yet another vector. This allows for sophisticated manipulations in a single instruction.
Recent versions of the GNU Compiler Collection (GCC), IBM VisualAge compiler and other compilers provide intrinsics to access AltiVec instructions directly from C and C++ programs. As of version 4, the GCC also includes auto-vectorization capabilities that attempt to intelligently create Altivec accelerated binaries without the need for the programmer to use intrinsics directly. The "vector" type keyword is introduced to permit the declaration of native vector types, e.g., "
vector unsigned char foo;" declares a 128-bit vector variable named "foo" containing sixteen 8-bit unsigned chars. The full complement of arithmetic and binary operators is defined on vector types so that the normal C expression language can be used to manipulate vector variables. There are also overloaded intrinsic functions such as "
vec_add" that emit the appropriate op code based on the type of the elements within the vector, and very strong type checking is enforced. In contrast, the Intel-defined data types for IA-32 SIMD registers declare only the size of the vector register (128 or 64 bits) and in the case of a 128-bit register, whether it contains integers or floating point values. The programmer must select the appropriate intrinsic for the data types in use, e.g., "
_mm_add_epi16(x,y)" for adding two vectors containing eight 16-bit integers.
AltiVec was developed between 1996 and 1998 by a collaborative project between Apple, IBM, and Motorola. Apple was the primary customer for AltiVec until Apple switched to Intel-made, x86-based CPUs in 2006. They used it to accelerate multimedia applications such as QuickTime, iTunes and key parts of Apple's Mac OS X including in the Quartz graphics compositor. Other companies such as Adobe used AltiVec to optimize their image-processing programs such as Adobe Photoshop. Motorola was the first to supply AltiVec enabled processors starting with their G4 line. AltiVec was also used in some embedded systems for high-performance digital signal processing.
IBM consistently left VMX out of their earlier POWER microprocessors, which were intended for server applications where it was not very useful. The POWER6 microprocessor, introduced in 2007, implements AltiVec. The implementation is similar to the one in 970 and Cell. The last desktop microprocessor from IBM, the PowerPC 970 (dubbed the "G5" by Apple) also implemented AltiVec with hardware similar to that of the PowerPC 7400.
IBM enhanced VMX for use in Xenon (Xbox 360) and called this enhancement VMX128. The enhancements comprise new routines targeted at gaming (accelerating 3D graphics and game physics) and a total of 128 registers. VMX128 is not entirely compatible with VMX/Altivec, as a number of integer operations were removed to make space for the larger register file and additional application-specific operations.
Power ISA v2.06 introduces the new VSX vector-scalar instructions which extend SIMD processing for the Power ISA to support up to 64 registers, with support for regular floating point, decimal floating point and vector execution. POWER7 is the first Power Architecture processor to implement Power ISA v2.06.
In C++, the standard way of accessing AltiVec support is mutually exclusive with use of the Standard Template Library
vector<> class template due to the treatment of "vector" as a reserved word when the compiler does not implement the context sensitive keyword version of vector. However, it may be possible to combine them using compiler-specific workarounds; for instance, in GCC one may do
#undef vector to remove the
vector keyword, and then use the GCC-specific
__vector keyword in its place.
The following processors have AltiVec, VMX or VMX128 included
- QorIQ T4240
- PowerPC 970
- PowerPC 970FX
- PowerPC 970MP
- Cell B.E.
- PowerXCell 8i
- Introducing the PowerPC SIMD unit
- Freescale's AltiVec page
- Using data-parallel SIMD architecture in video games and supercomputers
- Apple's Velocity Engine page
- SIMD history and performance comparison