bearophile wrote:
Travis Boucher:
Although it's design
promotes all sorta of optimization techniques, its still pretty young
(compared to gcc) and just doesn't have all of the optimization stuff
gcc has.
I have already done hundred of tests and benchmarks with LDC and llvm-gcc, and
I'm starting to understand its optimizations. I am mostly ignorant of LLVM
still, but I'm giving a bit of help tuning it, this improvement was motivated
by me:
http://blog.llvm.org/2009/12/advanced-topics-in-redundant-load.html
Compared to GCC LLVM lacks vectorization (this can be important for certain
heavy numerical computing code), profile-guided optimization (this is usually
less important, it's uncommon that it gives more than 5-25% performance
improvement), but it has a link-time optimizations that gcc lacks (about as
important as profile-guided optimization or a little more).
LLVM produces bad X86 floating point code still, but its int/FP SSE code is
about as good as GCC one or better (but it's not vectorized, so far).
GCC is older and it knows few extra small/tiny optimization tricks, but in most
situations they don't create a large difference in performance, they are often
quite specific.
So overall LLVM may sometime produce a little slower code, but in many
situations it's about as good or even better (I can show a large amount of
cases where LLVM is better). So the asm quality difference is smaller than you
seem to imply. If the size of such performance differences are important for
you, then you may want to use the Intel compiler instead of GCC, because it's
sometimes better than GCC.
Bye,
bearophile
I am not trying to get into the benchmark game, for every example of gcc
generating better code then llvm, there could be an example of llvm
generating better code then gcc.
What I was trying to state is the overall differences between the two:
- ldc supports newer versions of the dmd front end then gcc.
- gdc tend to generate better code then ldc (in many cases)
- gdc supports more targets (the code generator, not the runtime)
I personally use an old-ass gdc because it works for what I need. I'd
like to switch to ldc, but there is limited support for my target platform.
