Eygene Ryabinkin schrieb:
Christoph, good day.
Thu, Nov 27, 2008 at 09:39:45PM +0100, Christoph Mallon wrote:
A few days ago libFIRM[1] and cparser were added to the ports tree. If
you want to see, what other compilers besides GCC have to offer, this
might be of interest for you. libFIRM is a modern optimizing
intermediate representation (IR) library. cparser is a C compiler
providing many useful warnings and uses libFIRM for optimization and
code generation.
[...]
The whole description looks like that of LLVM GCC port, http://llvm.org/
Do you know something about that project and if yes, could you, please,
provide brief comparison of these two?
Both LLVM and FIRM use SSA as an important aspect of their IR. FIRM uses
it more extensively in backend. The whole FIRM backend operates on
program graphs in SSA form. Spilling and register allocation are
decoupled: First spilling is done and after that registers are
allocated. There are several spilling algorithms available. The best so
far is one based on the idea of Belady's algorithm and it produces very
good results. Our novel approach to register allocation and copy
coalescing was developed as PhD thesis by Sebastian hack. LLVM uses a
more classical approach. It has a combined register allocation and
spilling phase using a modified linear scan algorithm. LLVM not only
needs an instruction schedule, but also a block schedule. FIRM does
block scheduling after register allocation is done.
The SSA based spilling of FIRM leads to better code (i.e. less spill and
reloads) in most cases. A paper by Matthias Braun and Sebastian Hack
submitted to the next CC elaborates on this subject.
In the middle end both systems are quite similar, but there are some
notable differences. LLVM uses instructions scheduled in basic blocks
whereas FIRM resembles the "sea of nodes" proposed by Click and has no
schedule in the "middleend". Actually a schedule is introduced after
instruction selection. This is necessary, so spilling and register
allocation can operator. To be more precise our representation is called
an "explicit dependency graph": All necessary dependencies (we use
dependencies, not (data) flow) are modelled as edges in the program
graph. We use no temporary variables, but every node in the graph
represents a value. If a node produces several results, projection nodes
are used to get the different aspects. Memory is modelled as SSA value,
too. You can have a look at this in our online demo of the compiler. The
link is on our website. You can also read a bit more about FIRM there, too.
You mentioned LLVM GCC. FIRM only has a stale GCC frontend, which is not
maintained anymore. Currently there are three frontends available: C and
Java using the Edison Design Group frontend and our own C frontend
called "cparser". The latter is probably the more interesting option for
you.
Did you find the frontend examples interesting or are you mainly
interested in optimization and backend aspects?
Regards
Christoph
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