mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-24 11:42:57 +01:00
50 lines
2.9 KiB
Plaintext
50 lines
2.9 KiB
Plaintext
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.
|