On Mon, Apr 22, 2013 at 11:38:10AM +0200, Borislav Petkov wrote: > On Mon, Apr 22, 2013 at 10:53:42AM +0200, Paolo Bonzini wrote: > > Il 21/04/2013 14:23, Borislav Petkov ha scritto: > > > On Sun, Apr 21, 2013 at 01:46:50PM +0200, Borislav Petkov wrote: > > >> We probably need something with copying values to a temp variable or so. > > > > > > Basically something like that: > > > > > > case 2: > > > /* > > > * From MOVBE definition: "...When the operand size is 16 bits, > > > * the upper word of the destination register remains unchanged > > > * ..." > > > * > > > * Both casting ->valptr and ->val to u16 breaks strict aliasing > > > * rules so we have to do the operation almost per hand. > > > */ > > > tmp = (u16)ctxt->src.val; > > > ctxt->dst.val &= ~0xffffUL; > > > ctxt->dst.val |= (unsigned long)swab16(tmp); > > > break; > > > > > > This passes all gcc checks, even the stricter ones when building with W=3. > > > > I thought the valptr one was ok. > > Yep, it looked like that too. And, it could actually really be ok and > the gcc's warning here is bogus. I'll try to talk to gcc people about > it. Ok, I did and here's the explanation, as far as I understood it. Micha, please correct me if I'm talking bullsh*t. So basically, gcc screams because there's a type incompatibility according to the ICO C standard. IOW, valptr is declared as char[] and we are casting it to "unsigned short *" and then dereffing it, and both types are not compatible. So, I'm looking at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf, section 6.2.7 says "Two types have compatible type if their types are the same." Then, section "6.7.2 Type specifiers" talks about the different types and on the next page, in sentence 5 it says: "Each of the comma-separated multisets designates the same type,... " And, no wonder there, char and unsigned short are in different multisets so... So, what gcc actually warns about is, something which has been declared as char[] should not be subsequently accessed through "unsigned short *" because the two types are incompatible. No wonder we're building the kernel with -fno-strict-aliasing :). -- Regards/Gruss, Boris. Sent from a fat crate under my desk. Formatting is fine. -- -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
