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.