On Saturday 06 August 2005 18:57, Jeff Garzik wrote:
> 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.
But we are talking about one-bit field here:
+ unsigned int no_msi:1; /* device may not use msi */
If _this_ isn't optimized nicely into ANDs, ORs, etc, then
bug report is in order and gcc should be fixed.
--
vda
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