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- Subject: Re: [ANN] Lua 5.2.0 (work1) now available
- From: Doug Rogers <doug.rogers@...>
- Date: Thu, 14 Jan 2010 19:46:27 -0500
Mike Pall wrote:
> Doug Rogers wrote:
>> I'll need to go back and read your original reasoning on (*) 'Shift
>> counts should be masked by the bit width', because I would think the
>> masking should occur only for rotates. For shifts, the return value of a
>> overshift should be zero (or all ones, a.k.a. -1 according to the
>> Summary, for a sign-extended right shift with a sign bit of one).
> Just check your own code base for usage examples of bit shifts.
Maybe I misunderstand you here. In all my bit-busting code, if the shift
is greater than the word size, I return 0 for the result of the shift.
It is as if the bits are simply shifted out of the register as in an LFSR.
For 32-bit entities, I would like ((w >> 20) >> 20) == (w >> 40),
whereas my reading of your requirement would make (w >> 40) == (w >> 8).
> On some CPUs (e.g. x86) the mask operation can even be optimized
> away by the C compiler or a Lua compiler. And as I've mentioned,
> it doesn't need a branch ...
Many, many years ago I found to my surprise and consternation that x86
shifting did nothing (or was undefined?) when the count is greater than
or equal to the operand size. I remember singlestepping a debugger,
thinking the result was crazy. I opened the data book and was surprised
(well, not all that surprised) to see that I was the crazy one! Since
then I always check the size and make it behave as I describe above. I
realize it causes a branch to appear in the pipeline, but I prefer that
to the alternative, non-commutative masking of the shift.
I recognize that in some contexts it might be appropriate to mask the
count, but to me it seems like an issue of mathematical correctness.
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