Re: [PATCH 2/8] i386: bitops: Rectify bogus "Ir" constraints

[H. Peter Anvin]

H. Peter Anvin wrote:
> 
> "I" is correct.  The Intel documentation on this is highly confusing
> (and has bugs in it), but it does unambiguously state:
> 
> "Some assemblers support immediate bit offsets larger than 31 by using
> the immediate bit offset field in combination with the displacement
> field in the memory operand ... The processor will ignore the high-order
> bits if they are not zero."  AMD processors might be different for all I
> know.
> 
> So unless gas is capable of doing this transformation (and it's not as
> of binutils-2.17.50.0.6) "I" is what's needed here.
> 

Just tested it on a K8 machine; AMD behaves the same way.  So "I" is
correct, and changing it to "N" would introduce a bug.

The only way to optimize this is by using __builtin_constant_p() and
adjust the offset appropriately.

	-hpa
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