Intel® 64 and IA-32 Architectures Optimization Reference Manual

APPENDIX ESUMMARY OF RULES AND SUGGESTIONSThis appendix summarizes the rules and suggestions specified in this manual. Pleasebe reminded that coding recommendations are ranked in importance according tothese two criteria:• Local impact (referred to earlier as “impact”) – the difference that a recommendationmakes to performance for a given instance.• Generality – how frequently such instances occur across all application domains.Again, understand that this ranking is intentionally very approximate, and can varydepending on coding style, application domain, and other factors. Throughout thechapter you observed references to these criteria using the high, medium and lowpriorities for each recommendation. In places where there was no priority assigned,the local impact or generality has been determined not to be applicable.E.1 ASSEMBLY/COMPILER CODING RULESAssembler/Compiler Coding Rule 1. (MH impact, M generality) Arrange codeto make basic blocks contiguous and eliminate unnecessary branches. .........3-7Assembler/Compiler Coding Rule 2. (M impact, ML generality) Use the SETCCand CMOV instructions to eliminate unpredictable conditional branches wherepossible. Do not do this for predictable branches. Do not use these instructions toeliminate all unpredictable conditional branches (because using these instructionswill incur execution overhead due to the requirement for executing both paths ofa conditional branch). In addition, converting a conditional branch to SETCC orCMOV trades off control flow dependence for data dependence and restricts thecapability of the out-of-order engine. When tuning, note that all Intel 64 andIA-32 processors usually have very high branch prediction rates. Consistentlymispredicted branches are generally rare. Use these instructions only if theincrease in computation time is less than the expected cost of a mispredictedbranch.................................................................................................3-7Assembler/Compiler Coding Rule 3. (M impact, H generality) Arrange code tobe consistent with the static branch prediction algorithm: make the fall-throughcode following a conditional branch be the likely target for a branch with a forwardtarget, and make the fall-through code following a conditional branch be theunlikely target for a branch with a backward target. ................................ 3-10Assembler/Compiler Coding Rule 4. (MH impact, MH generality) Near callsmust be matched with near returns, and far calls must be matched with farE-1