Hi Grigori,
Hope my question will not completely divert the topic of this discussion - just curious what do you mean by better code? Better execution time, code size, compilation time?..
this depends on each persons needs of course. The dragonegg plugin makes it easy for people to see if the LLVM optimizers and code generators are helpful for their projects. Evaluating whether replacing whole-sale the gcc middle and backends with LLVM (which I consider pretty unlikely) is an overall win is much harder, but I doubt anyone on this mailing list needs to be told that.
If yes, then why not to compare different compilers by just compiling multiple programs with GCC, LLVM, Open64, ICC, etc, separately to compare those characteristics and then find missing optimizations or better combinations of optimizations to achieve the result?
how do you compile a program with LLVM? It's not a compiler, it's a set of optimization and codegen libraries. You also need a front-end, which takes the users code and turns it into the LLVM intermediate representation [IR]. The dragonegg plugin takes the output of the gcc-4.5 front-ends, turns it into LLVM IR and runs the LLVM optimizers and code generators on it. In other words, it is exactly what you need in order to compile programs with LLVM. There is also llvm-gcc, which is a hacked version of gcc-4.2 that does much the same thing, and for C and C++ there is now the clang front-end to LLVM. The big advantage of dragonegg is that it isolates the effect of the LLVM optimizers and code generators by removing the effect of having a different front-end. For example, if llvm-gcc produces slower code than gcc-4.5, this might be due to front-end changes between gcc-4.2 and gcc-4.5 rather than because the gcc optimizers are doing a better job. This confounding factor goes away with the dragonegg plugin. Ciao, Duncan.
