Re: c compilation - gcc vs visual c
- From: "cr88192" <cr88192@xxxxxxxxxxxxxxxxxx>
- Date: Sat, 10 May 2008 08:41:39 +1000
<kumarchi@xxxxxxxxx> wrote in message
news:4780a4b0-8034-4a20-9058-8d53c89bfdd4@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
hello:
I recently compiled a numerically intensive c project under cygwin gcc
3.4.4 and microsoft visual c. The platform is intel T2400 1.83 ghz
dual core lap top.
the numerical stuff is both floating point and integer intensive
The gcc optimized (-O3) vs non optimized improved the performance 30 %
visual c optimized (standard , check 'release; under visual c 2005)
vs non optimized ('build') was a whopping 8x performance gain
but the most surprising thing was visual c optimized was 2x
performance over gcc optimized.
likely reason:
MS focuses a lot more on specific optimizations, and tweaking performance
for specific targets;
gcc, however, targets many targets, and tends to use far more generic code
generation (they try more to leverage fancy general purpose optimizations,
rather than arch-specific tweaks for various special cases).
in any case, gcc tends to, fairly often, produce fairly silly code (even
with optimizations), and, sadly, even with a very braindead lower-compiler
design (a hacked over stack machine), and optimizations focusing more on
"common special cases", it is not too hard to match or somewhat exceed gcc's
performance...
IMO, the 'O' options may well be Obfuscate rather than Optimize...
actually, one of the better ways at optimizing, would be likely to implement
a kind of abstract combinatorial tester, which would basically search the
space of possible optimizations and look for the ones with the lowest
simulated cost. sadly though, this will not work so well in the face of
usage patterns, which require actually using the code (the general option
could treat a very common case like an uncommon case, ...).
in something like a VM, it could be possible to use a kind of genetic
evolver for adapting functions (initially, it compiles functions
generically, and any functions it detects are using a significant portion of
the time, it starts mutating in an attempt to improve the general
performance). later, if/when a "final" version is desired, it uses the
versions of the functions found to be most effective.
note that this would likely be confined to the realms of low-level
optimization, with what are typically the biggest time wasters (general
algorithmic issues), being beyond the scope of such a tool...
the simplest approach, however (and the one I currently use in my compiler),
is to basically just test the compiler, and any obvious issues in the output
(silly code), are ones I focus on fixing.
the compiler machinery itself in my case, as this level, is little more than
just a very large and elaborate mass of decision trees (no fancy transforms
or general optimizer machinery, more just operations dispatched through a
maze of function calls).
this approach seems to work good enough IME...
is anybody else seeing the same thing. if this is true microsoft c
compiler is in a different league altogether
that, or, most of us are not that concerned with raw performance (vs having
a compiler we are not obligated to pay for...).
none the less, MS has at least a decent compiler in these regards...
i was not successful so far compiling under mingw. will it make dent ?
2x is hard to overcome
well, with gcc, it is hard to do much better...
as noted, MSVC and Intel are good options...
.
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