SSE2

Differences between x87 FPU and SSE2

FPU (x87) instructions provide higher precision by calculating intermediate results with 80 bits of precision, by default, to minimise roundoff error in numerically unstable algorithms (see IEEE 754 design rationale and references therein). However, the x87 FPU is a scalar unit only whereas SSE2 can process a small vector of operands in parallel.

If code designed for x87 is ported to the lower precision double precision SSE2 floating point, certain combinations of math operations or input datasets can result in measurable numerical deviation, which can be an issue in reproducible scientific computations, e.g. if the calculation results must be compared against results generated from a different machine architecture. A related issue is that, historically, language standards and compilers had been inconsistent in their handling of the x87 80-bit registers implementing double extended precision variables, compared with the double and single precision formats implemented in SSE2: the rounding of extended precision intermediate values to double precision variables was not fully defined and was dependent on implementation details such as when registers were spilled to memory.

Differences between MMX and SSE2

SSE2 extends MMX instructions to operate on XMM registers. Therefore, it is possible to convert all existing MMX code to an SSE2 equivalent. Since an SSE2 register is twice as long as an MMX register, loop counters and memory access may need to be changed to accommodate this. However, 8 byte loads and stores to XMM are available, so this is not strictly required.

Although one SSE2 instruction can operate on twice as much data as an MMX instruction, performance might not increase significantly. Two major reasons are: accessing SSE2 data in memory not aligned to a 16-byte boundary can incur significant penalty, and the throughput of SSE2 instructions in older x86 implementations was half that for MMX instructions. Intel addressed the first problem by adding an instruction in SSE3 to reduce the overhead of accessing unaligned data and improving the overall performance of misaligned loads, and the last problem by widening the execution engine in their Core microarchitecture in Core 2 Duo and later products.

Since MMX and x87 register files alias one another, using MMX will prevent x87 instructions from working as desired. Once MMX has been used, the programmer must use the emms instruction (C: _mm_empty()) to restore operation to the x87 register file. On some operating systems, x87 is not used very much, but may still be used in some critical areas like pow() where the extra precision is needed. In such cases, the corrupt floating-point state caused by failure to emit emms may go undetected for millions of instructions before ultimately causing the floating-point routine to fail, returning NaN. Since the problem is not locally apparent in the MMX code, finding and correcting the bug can be very time consuming. As SSE2 does not have this problem and it usually provides much better throughput and provides more registers in 64-bit code, it should be preferred for nearly all vectorization work.

Compiler usage

When introduced in 2000, SSE2 was not supported by software development tools. For example, to use SSE2 in a Microsoft Visual Studio project, the programmer had to either manually write inline-assembly or import object-code from an external source. Later the Visual C++ Processor Pack added SSE2 support to Visual C++ and MASM.

The Intel C++ Compiler can automatically generate SSE4, SSSE3, SSE3, SSE2, and SSE code without the use of hand-coded assembly.

Since GCC 3, GCC can automatically generate SSE/SSE2 scalar code when the target supports those instructions. Automatic vectorization for SSE/SSE2 has been added since GCC 4.

The Sun Studio Compiler Suite can also generate SSE2 instructions when the compiler flag -xvector=simd is used.

Since Microsoft Visual C++ 2012, the compiler option to generate SSE2 instructions is turned on by default.

CPU support

SSE2 is an extension of the IA-32 architecture, based on the x86 instruction set. Therefore, only x86 processors can include SSE2. The AMD64 architecture supports the IA-32 as a compatibility mode and includes the SSE2 in its specification. It also doubles the number of XMM registers, allowing for better performance. SSE2 is also a requirement for installing Windows 8 (and later) or Microsoft Office 2013 (and later) "to enhance the reliability of third-party apps and drivers running in Windows 8".

The following IA-32 CPUs support SSE2:

• Intel NetBurst-based CPUs (Pentium 4, Xeon, Celeron, Pentium D, Celeron D)
• Intel Pentium M and Celeron M
• Intel Atom
• AMD Athlon 64
• Transmeta Efficeon
• VIA C7

The following IA-32 CPUs were released after SSE2 was developed, but did not implement it:

• AMD CPUs prior to Athlon 64, such as Athlon XP
• VIA C3
• Transmeta Crusoe
• Intel Quark