Intel® 64 and IA-32 Architectures Optimization Reference Manual

GENERAL OPTIMIZATION GUIDELINESFor example, loading data from memory with MOVSS or MOVSD causes an extramicro-op for zeroing the upper part of the XMM register.On Pentium M, Intel Core Solo, and Intel Core Duo processors, this penalty can beavoided by using MOVLPD. However, using MOVLPD causes a performance penalty onPentium 4 processors.Another situation occurs when mixing single-precision and double-precision code. Onprocessors based on Intel NetBurst microarchitecture, using CVTSS2SD has performancepenalty relative to the alternative sequence:XORPS XMM1, XMM1MOVSS XMM1, XMM2CVTPS2PD XMM1, XMM1On Intel Core Solo and Intel Core Duo processors, using CVTSS2SD is more desirablethan the alternative sequence.3.8.4.2 x87 Floating-point Operations with Integer OperandsFor processors based on Intel NetBurst microarchitecture, splitting floating-pointoperations (FIADD, FISUB, FIMUL, and FIDIV) that take 16-bit integer operands intotwo instructions (FILD and a floating-point operation) is more efficient. However, forfloating-point operations with 32-bit integer operands, using FIADD, FISUB, FIMUL,and FIDIV is equally efficient compared with using separate instructions.Assembly/Compiler Coding Rule 64. (M impact, L generality) Try to use32-bit operands rather than 16-bit operands for FILD. However, do not do so at theexpense of introducing a store-forwarding problem by writing the two halves of the32-bit memory operand separately.3.8.4.3 x87 Floating-point Comparison InstructionsThe FCOMI and FCMOV instructions should be used when performing x87 floatingpointcomparisons. Using the FCOM, FCOMP, and FCOMPP instructions typicallyrequires additional instruction like FSTSW. The latter alternative causes more μops tobe decoded, and should be avoided.3.8.4.4 Transcendental FunctionsIf an application needs to emulate math functions in software for performance orother reasons (see Section 3.8.1, “Guidelines for Optimizing Floating-point Code”), itmay be worthwhile to inline math library calls because the CALL and theprologue/epilogue involved with such calls can significantly affect the latency ofoperations.Note that transcendental functions are supported only in x87 floating point, not inStreaming SIMD Extensions or Streaming SIMD Extensions 2.3-86