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    SubjectRe: kernel/microcode.c error from new 64bit code
    Date
    Followup to:  <Pine.LNX.4.58.0402210914530.3301@ppc970.osdl.org>
    By author: Linus Torvalds <torvalds@osdl.org>
    In newsgroup: linux.dev.kernel
    >
    >
    >
    > On Sat, 21 Feb 2004, Pavel Machek wrote:
    > >
    > > > + wrmsr(MSR_IA32_UCODE_WRITE,
    > > > + (unsigned long) uci->mc->bits,
    > > > + (unsigned long) uci->mc->bits >> 16 >> 16);
    > > ~~~~~~~~~~~~
    > >
    > > I see what you are doing, but this is evil. At least comment /* ">> 32"
    > > is undefined on i386 */ ?
    >
    > Sorry, but you're wrong.
    >
    > ">> 32" is underfined PERIOD! It has nothing to do with x86, it's a C
    > standards issue. It's undefined on any 32-bit architecture. (shifting by
    > the wordsize or bigger is simply not a defined C operation).
    >
    > The above is not evil. The above is the standard way of doing this in C if
    > you know the word-size is 32-bits or bigger.
    >

    Actually, what is undefined is shifting with the size of the data type or higher.

    If the cast of uci->mc->bits had been an uint64_t (unsigned long
    long), >> 32 would have been perfectly well-defined.

    The above code is just plain wrong: the cast to (unsigned long) has
    higher precedence than the shift, so on i386 (which I presume this is)
    it will become an unsigned long, and the shifts will bring it down to
    zero.

    You might as well write zero if that's what you mean.

    -hpa
    -
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