Messages in this thread |  | | | Date | Sat, 6 Aug 2005 11:57:55 -0400 | | From | Jeff Garzik <> | | Subject | Re: [PATCH] 6700/6702PXH quirk |
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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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