Intel® 64 and IA-32 Architectures Optimization Reference Manual

GENERAL OPTIMIZATION GUIDELINESInstead, software should follow these additional decoder guidelines:• If you need to use multiple μop, non-microsequenced instructions, try toseparate by a few single μop instructions. The following instructions areexamples of multiple-μop instruction not requiring micro-sequencer:ADC/SBBCMOVccRead-modify-write instructions• If a series of multiple-μop instructions cannot be separated, try breaking theseries into a different equivalent instruction sequence. For example, a series ofread-modify-write instructions may go faster if sequenced as a series of readmodify+ store instructions. This strategy could improve performance even if thenew code sequence is larger than the original one.3.5.1.1 Use of the INC and DEC InstructionsThe INC and DEC instructions modify only a subset of the bits in the flag register. Thiscreates a dependence on all previous writes of the flag register. This is especiallyproblematic when these instructions are on the critical path because they are used tochange an address for a load on which many other instructions depend.Assembly/Compiler Coding Rule 32. (M impact, H generality) INC and DECinstructions should be replaced with ADD or SUB instructions, because ADD andSUB overwrite all flags, whereas INC and DEC do not, therefore creating falsedependencies on earlier instructions that set the flags.3.5.1.2 Integer DivideTypically, an integer divide is preceded by a CWD or CDQ instruction. Depending onthe operand size, divide instructions use DX:AX or EDX:EAX for the dividend. TheCWD or CDQ instructions sign-extend AX or EAX into DX or EDX, respectively. Theseinstructions have denser encoding than a shift and move would be, but they generatethe same number of micro-ops. If AX or EAX is known to be positive, replace theseinstructions with:xor dx, dxorxor edx, edx3.5.1.3 Using LEAIn some cases with processor based on Intel NetBurst microarchitecture, the LEAinstruction or a sequence of LEA, ADD, SUB and SHIFT instructions can replaceconstant multiply instructions. The LEA instruction can also be used as a multipleoperand addition instruction, for example:LEA ECX, [EAX + EBX + 4 + A]3-26