Intel® 64 and IA-32 Architectures Optimization Reference Manual

GENERAL OPTIMIZATION GUIDELINESThere are several cases in which data is passed through memory, and the store mayneed to be separated from the load:• Spills, save and restore registers in a stack frame• Parameter passing• Global and volatile variables• Type conversion between integer and floating point• When compilers do not analyze code that is inlined, forcing variables that areinvolved in the interface with inlined code to be in memory, creating morememory variables and preventing the elimination of redundant loadsAssembly/Compiler Coding Rule 51. (H impact, MH generality) Where it ispossible to do so without incurring other penalties, prioritize the allocation ofvariables to registers, as in register allocation and for parameter passing, tominimize the likelihood and impact of store-forwarding problems. Try not to storeforwarddata generated from a long latency instruction - for example, MUL or DIV.Avoid store-forwarding data for variables with the shortest store-load distance.Avoid store-forwarding data for variables with many and/or long dependencechains, and especially avoid including a store forward on a loop-carried dependencechain.Example 3-34 shows an example of a loop-carried dependence chain.Example 3-34. Loop-carried Dependence Chainfor ( i = 0; i < MAX; i++ ) {a[i] = b[i] * foo;foo = a[i] / 3;} // foo is a loop-carried dependence.Assembly/Compiler Coding Rule 52. (M impact, MH generality) Calculatestore addresses as early as possible to avoid having stores block loads.3.6.5 Data Layout OptimizationsUser/Source Coding Rule 6. (H impact, M generality) Pad data structuresdefined in the source code so that every data element is aligned to a naturaloperand size address boundary.If the operands are packed in a SIMD instruction, align to the packed element size(64-bit or 128-bit).Align data by providing padding inside structures and arrays. Programmers can reorganizestructures and arrays to minimize the amount of memory wasted by padding.However, compilers might not have this freedom. The C programming language, forexample, specifies the order in which structure elements are allocated in memory. Formore information, see Section 4.4, “Stack and Data Alignment,” and Appendix D,“Stack Alignment.”3-56