| 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. |