On Tue, Apr 05, 2011 at 08:56:22AM +0100, Russell King - ARM Linux wrote: > On Tue, Apr 05, 2011 at 09:43:21AM +0200, Jan Weitzel wrote: > > parameter "u32 mask" type cast befor inversion s/befor/before/ > Nak. I want a 32-bit all ones quantity. > > unsigned long long vali = (unsigned short)~0; > unsigned long long vall = ~(unsigned short)0; > BTW, the definiton of vall is equivalent to unsigned long long valu = ~(unsigned int)0; because ~ converts the unsigned short to unsigned int. > compiles to: > > vali: > .word 65535 > .word 0 > > vall: > .word -1 > .word -1 I really wonder about that. If I take a value of 0xffffffff (i.e. a 32 bit wide int == ~0U) and assign that to an 64-bit unsigned long long I'd expect it to get the value 0x00000000ffffffffULL, not 0xffffffffffffffffULL. What's wrong? > So moving the ~ to be evaluated after the cast has the effect of making > the cast pointless, and produces wrong values. (u32)~0 does the 32-bit My C-Book tells that using ~ on a signed it produces implementation defined behaviour. That's what I pointed out to Jan and I guess that's the reason why he created the patch. > cast _after_ the inversion which ensures that its always truncated to > a 32-bit value. > > As the function is declared as taking a u32, the cast isn't needed. If > the function gets changed to take a u64, the casts will need to be > re-added. So, (u32)~0 makes the fact that we want a 32-bit all-ones > mask explicit. Actually this only results in a 32-bit all-ones value if int is at least 32 bits long, so technically using ~(u32)0 is better. (Obviously this is given on ARM and I guess even on all platforms that Linux runs on.) Best regards Uwe PS: this mail is not about trolling. -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ | -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
