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