1By Chris: 2 3LLVM has been designed with two primary goals in mind. First we strive to 4enable the best possible division of labor between static and dynamic 5compilers, and second, we need a flexible and powerful interface 6between these two complementary stages of compilation. We feel that 7providing a solution to these two goals will yield an excellent solution 8to the performance problem faced by modern architectures and programming 9languages. 10 11A key insight into current compiler and runtime systems is that a 12compiler may fall in anywhere in a "continuum of compilation" to do its 13job. On one side, scripting languages statically compile nothing and 14dynamically compile (or equivalently, interpret) everything. On the far 15other side, traditional static compilers process everything statically and 16nothing dynamically. These approaches have typically been seen as a 17tradeoff between performance and portability. On a deeper level, however, 18there are two reasons that optimal system performance may be obtained by a 19system somewhere in between these two extremes: Dynamic application 20behavior and social constraints. 21 22From a technical perspective, pure static compilation cannot ever give 23optimal performance in all cases, because applications have varying dynamic 24behavior that the static compiler cannot take into consideration. Even 25compilers that support profile guided optimization generate poor code in 26the real world, because using such optimization tunes that application 27to one particular usage pattern, whereas real programs (as opposed to 28benchmarks) often have several different usage patterns. 29 30On a social level, static compilation is a very shortsighted solution to 31the performance problem. Instruction set architectures (ISAs) continuously 32evolve, and each implementation of an ISA (a processor) must choose a set 33of tradeoffs that make sense in the market context that it is designed for. 34With every new processor introduced, the vendor faces two fundamental 35problems: First, there is a lag time between when a processor is introduced 36to when compilers generate quality code for the architecture. Secondly, 37even when compilers catch up to the new architecture there is often a large 38body of legacy code that was compiled for previous generations and will 39not or can not be upgraded. Thus a large percentage of code running on a 40processor may be compiled quite sub-optimally for the current 41characteristics of the dynamic execution environment. 42 43For these reasons, LLVM has been designed from the beginning as a long-term 44solution to these problems. Its design allows the large body of platform 45independent, static, program optimizations currently in compilers to be 46reused unchanged in their current form. It also provides important static 47type information to enable powerful dynamic and link time optimizations 48to be performed quickly and efficiently. This combination enables an 49increase in effective system performance for real world environments. 50