EMMS Usage Guidelines

Here are guidelines for when to use the EMMS instruction:

  • Use _mm_empty() after an MMX™ instruction if the next instruction is a floating-point (FP) instruction. For example, you should use the EMMS instruction before performing calculations on float, double or long double. You must be aware of all situations in which your code generates an MMX™ instruction:

    • when using an MMX™ technology intrinsic

    • when using Intel® Streaming SIMD Extensions (Intel® SSE) integer intrinsics that use the __m64 data type

    • when referencing an __m64 data type variable

    • when using an MMX™ instruction through inline assembly

  • Use different functions for operations that use floating point instructions and those that use MMX™ instructions. This action eliminates the need to empty the multimedia state within the body of a critical loop.

  • Use _mm_empty() during runtime initialization of __m64 and FP data types. This ensures resetting the register between data type transitions.

  • Do not use _mm_empty() before an MMX™ instruction, since using _mm_empty() before an MMX™ instruction incurs an operation with no benefit (no-op).

  • See the Correct Usage and Incorrect Usage coding examples in the following table.

Incorrect Usage

Correct Usage

__m64 x = _m_paddd(y, z);

float f = init();

__m64 x = _m_paddd(y, z);

float f = ( _mm_empty(), init());

Para obtener información más completa sobre las optimizaciones del compilador, consulte nuestro Aviso de optimización.