Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()

[Scott Lurndal]

On Fri, Mar 26, 2010 at 10:23:46AM -0700, Linus Torvalds wrote:
> 
> 
> On Fri, 26 Mar 2010, David Howells wrote:
> >
> > fls(N), ffs(N) and fls64(N) can be optimised on x86/x86_64.  Currently they
> > perform checks against N being 0 before invoking the BSR/BSF instruction, or
> > use a CMOV instruction afterwards.  Either the check involves a conditional
> > jump which we'd like to avoid, or a CMOV, which we'd also quite like to avoid.
> > 
> > Instead, we can make use of the fact that BSR/BSF doesn't modify its output
> > register if its input is 0.  By preloading the output with -1 and incrementing
> > the result, we achieve the desired result without the need for a conditional
> > check.
> 
> This is totally incorrect.
> 
> Where did you find that "doesn't modify its output" thing? It's not true. 
> The truth is that the destination is undefined. Just read the dang Intel 
> documentation, it's very clearly stated right there.

While this is true for the current (253666-031US) Intel documentation,
the AMD documentation (rev 3.14) for the same instruction states that the
destination register is unchanged (as opposed to Intel's undefined).

I wonder if Intel's EM64 stuff makes this more deterministic, perhaps
David's implementation would work for x86_64 only?

scott
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