Re: [PATCH] 6700/6702PXH quirk

[Jeff Garzik <jgarzik@pobox.com>]

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