Intel® 64 and IA-32 Architectures Optimization Reference Manual

SUMMARY OF RULES AND SUGGESTIONSfour µops and require multiple cycles to decode. Use sequences of simpleinstructions instead. ............................................................................3-25Assembler/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..............................3-26Assembler/Compiler Coding Rule 33. (ML impact, L generality) If an LEAinstruction using the scaled index is on the critical path, a sequence with ADDsmay be better. If code density and bandwidth out of the trace cache are thecritical factor, then use the LEA instruction. ............................................3-27Assembler/Compiler Coding Rule 34. (ML impact, L generality) Avoid ROTATEby register or ROTATE by immediate instructions. If possible, replace with aROTATE by 1 instruction.......................................................................3-27Assembler/Compiler Coding Rule 35. (M impact, ML generality) Usedependency-breaking-idiom instructions to set a register to 0, or to break a falsedependence chain resulting from re-use of registers. In contexts where thecondition codes must be preserved, move 0 into the register instead. Thisrequires more code space than using XOR and SUB, but avoids setting thecondition codes...................................................................................3-28Assembler/Compiler Coding Rule 36. (M impact, MH generality) Breakdependences on portions of registers between instructions by operating on 32-bitregisters instead of partial registers. For moves, this can be accomplished with32-bit moves or by using MOVZX. .........................................................3-29Assembler/Compiler Coding Rule 37. (M impact, M generality) Try to use zeroextension or operate on 32-bit operands instead of using moves with signextension. ..........................................................................................3-29Assembler/Compiler Coding Rule 38. (ML impact, L generality) Avoid placinginstructions that use 32-bit immediates which cannot be encoded as signextended16-bit immediates near each other. Try to schedule µops that have noimmediate immediately before or after µops with 32-bit immediates. .........3-29Assembler/Compiler Coding Rule 39. (ML impact, M generality) Use the TESTinstruction instead of AND when the result of the logical AND is not used. Thissaves µops in execution. Use a TEST if a register with itself instead of a CMP ofthe register to zero, this saves the need to encode the zero and saves encodingspace. Avoid comparing a constant to a memory operand. It is preferable to loadthe memory operand and compare the constant to a register. ...................3-30Assembler/Compiler Coding Rule 40. (ML impact, M generality) Eliminateunnecessary compare with zero instructions by using the appropriate conditionaljump instruction when the flags are already set by a preceding arithmeticinstruction. If necessary, use a TEST instruction instead of a compare. Be certainE-4