On 2019-02-18 05:23:28, Laszlo Ersek wrote: > generic comment (applies to all NASM usage I guess): > > On 02/18/19 11:10, Jordan Justen wrote: > > > + mov eax, cr0 > > + and eax, ~(1 << 30) > > + mov cr0, eax > > > + mov rax, cr0 > > + and eax, ~(1 << 30) > > + mov cr0, rax > > I've read up on the << and ~ operators in the NASM documentation, and I > think the above build-time calculations of the masks are well-defined > and correct. > > - bit shifts are always unsigned > - given bit position 30, ~(1 << 30) will be a value with 32 bits > - bit-neg simply flips bits (one's complement) > > On the other hand, I find these NASM specifics counter-intuitive. The > expression ~(1 << 30) looks like valid C, but in C, it means a quite > different thing.
Can you elaborate? I guess there might be something subtly different, but for the most part it means the same thing, right? > I think calculating the mask with "strict dword" somehow (not exactly > sure how) would make this more readable; Oh, are you saying that (1 << 30) doesn't necessarily mean we are operating on a 32-bit value? > or else the BTR instruction would. Yeah, I guess this works. > Opinions? (Again, pertaining to all NASM usage in edk2.) As always, my opinion is to avoid writing assembly code. :) We actually had a version that set this just before the decompress in SecMain.c. Then I noted that we were initializing temp-ram here, so I moved it, even though the memory init doesn't take a significant amount of time compared to the decompress. Maybe we should just do that instead? -Jordan _______________________________________________ edk2-devel mailing list edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel
