On Sat, Aug 06, 2005 at 09:50:13AM +0100, Matthew Wilcox wrote:
> On Fri, Aug 05, 2005 at 11:34:55PM -0400, Jeff Garzik wrote:
> > FWIW, compilers generate AWFUL code for bitfields. Bitfields are
> > really tough to do optimally, whereas bit flags ["unsigned int flags &
> > bitmask"] are the familiar ints and longs that the compiler is well
> > tuned to optimize.
>
> I'm sure the GCC developers would appreciate a good bug report with a
> test-case that generates worse code. If you don't want to mess with their
> bug tracking system, just send me a test case and I'll add it for you.
Its an order-of-complexity issue. No matter how hard you try,
bitfields will -always- be tougher to optimize, than machine ints.
Bitfields are weirdly-sized, weirdly-aligned integers. A simple look at
the generated asm from gcc on ARM or MIPS demonstrates the explosion of
code that can sometimes occur, versus a simple 'and' test of a machine
int and a mask. x86 is a tiny bit better, but still more expensive to
do bitfields than machine ints.
Jeff
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