On 11/20/2013 12:13 PM, Linus Torvalds wrote:
> On Wed, Nov 20, 2013 at 11:28 AM, H. Peter Anvin <h...@linux.intel.com> wrote:
>>>
>>> .section .fixup,"ax"
>>> 11: lea (%rdx,%rcx,8),%rcx
>>> 12: movl %ecx,%edx /* ecx is zerorest also */
>>
>> -> Even if %rdx+%rcx*8 > 2^32 we end up truncating at 12: -- not that it
>> matters, since both arguments are prototyped as "unsigned" and therefore
>> the C compiler is supposed to guarantee the upper 32 bits are ignored.
>
> Ahh. That was the one I thought was broken, but yes, while the upper
> bits of %rcx are calculated and not zeroed, they end up not actually
> getting used. So yeah, I'll believe it's correct.
>
That being said, "lea (%rdx,%rcx,8),%ecx" (leal, as opposed to leaq) is
a perfectly legitimate instruction and actually one byte shorter. The
big question is if some broken version of gas will choke on it.
-hpa
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