Intel ARCHITECTURE IA-32 User Manual

Optimizing for SIMD Integer Applications 4
4-5
• Don’t empty when already empty: If the next instruction uses an
MMX register,
_mm_empty() incurs a cost with no benefit.
• Group Instructions: Try to partition regions that use x87 FP
instructions from those that use 64-bit SIMD integer instructions.
This eliminates needing an
emms instruction within the body of a
critical loop.
• Runtime initialization: Use _mm_empty() during runtime
initialization of
__m64 and x87 FP data types. This ensures
resetting the register between data type transitions. See Example 4-1
for coding usage.
Further, you must be aware that your code generates an MMX
instruction, which uses the MMX registers with the Intel C++ Compiler,
in the following situations:
• when using a 64-bit SIMD integer intrinsic from MMX technology,
SSE, or SSE2
• when using a 64-bit SIMD integer instruction from MMX
technology, SSE, or SSE2 through inline assembly
• when referencing an __m64 data type variable
Additional information on the x87 floating-point programming model
can be found in the IA-32 Intel® Architecture Software Developer’s
Manual, Volume 1. For more documentation on
emms, visit
h
ttp://developer.intel.com.
Example 4-1 Resetting the Register between __m64 and FP Data Types
Incorrect Usage Correct Usage
__m64 x = _m_paddd(y, z); __m64 x = _m_paddd(y, z);
float f = init(); float f = (_mm_empty(), init());

Intel ARCHITECTURE IA-32 Specifications

General

Instruction Set	x86
Instruction Set Type	CISC
Memory Segmentation	Supported
Operating Modes	Real mode, Protected mode, Virtual 8086 mode
Max Physical Address Size	36 bits (with PAE)
Max Virtual Address Size	32 bits
Architecture	IA-32 (Intel Architecture 32-bit)
Addressable Memory	4 GB (with Physical Address Extension up to 64 GB)
Floating Point Registers	8 x 80-bit
MMX Registers	8 x 64-bit
SSE Registers	8 x 128-bit
Registers	General-purpose registers (EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP), Segment registers (CS, DS, SS, ES, FS, GS), Instruction pointer (EIP), Flags register (EFLAGS)
Floating Point Unit	Yes (x87)