Intel® 64 and IA-32 Architectures Optimization Reference Manual

GENERAL OPTIMIZATION GUIDELINESAssembly/Compiler Coding Rule 20. (M impact, ML generality) Software canenable macro fusion when it can be logically determined that a variable is nonnegativeat the time of comparison; use TEST appropriately to enable macro-fusionwhen comparing a variable with 0.Example 3-13. Macro-fusion, Signed VariableWithout Macro-fusionWith Macro-fusiontest ecx, ecxtest ecx, ecxjle OutSideTheIFjle OutSideTheIFcmp ecx, 64Hcmp ecx, 64Hjge OutSideTheIFjae OutSideTheIFOutSideTheIF:OutSideTheIF:For either signed or unsigned variable ‘a’; “CMP a,0” and “TEST a,a” produce thesame result as far as the flags are concerned. Since TEST can be macro-fused moreoften, software can use “TEST a,a” to replace “CMP a,0” for the purpose of enablingmacro-fusion.Example 3-14. Macro-fusion, Signed ComparisonC Code Without Macro-fusion With Macro-fusionif (a == 0) cmp a, 0jne lbl...lbl:if ( a >= 0) cmp a, 0jl lbl;...lbl:test a, ajne lbl...lbl:test a, ajl lbl...lbl:3.4.2.3 Length-Changing Prefixes (LCP)The length of an instruction can be up to 15 bytes in length. Some prefixes candynamically change the length of an instruction that the decoder must recognize.Typically, the pre-decode unit will estimate the length of an instruction in the bytestream assuming the absence of LCP. When the predecoder encounters an LCP in thefetch line, it must use a slower length decoding algorithm. With the slower lengthdecoding algorithm, the predecoder decodes the fetch in 6 cycles, instead of theusual 1 cycle. Normal queuing throughout of the machine pipeline generally cannothide LCP penalties.The prefixes that can dynamically change the length of a instruction include:• operand size prefix (0x66)• address size prefix (0x67)3-21