[PATCH] powerpc/lib/sstep: Fix count leading zeros instructions

[David Laight]

From: Segher Boessenkool
> Sent: 09 October 2017 15:21
> On Mon, Oct 09, 2017 at 01:49:26PM +0000, David Laight wrote:
> > From: Sandipan Das
> > > Sent: 09 October 2017 12:07
> > > According to the GCC documentation, the behaviour of __builtin_clz()
> > > and __builtin_clzl() is undefined if the value of the input argument
> > > is zero. Without handling this special case, these builtins have been
> > > used for emulating the following instructions:
> > >   * Count Leading Zeros Word (cntlzw[.])
> > >   * Count Leading Zeros Doubleword (cntlzd[.])
> > >
> > > This fixes the emulated behaviour of these instructions by adding an
> > > additional check for this special case.
> >
> > Presumably the result is undefined because the underlying cpu
> > instruction is used - and it's return value is implementation defined.
> 
> It is undefined because the result is undefined, and the compiler
> optimises based on that.  The return value of the builtin is undefined,
> not implementation defined.
> 
> The patch is correct.

But the code you are emulating might be relying on the (un)defined value
the cpu instruction gives for zero input rather than the input width.

Or, put another way, if the return value for a clz instruction with zero
argument is undefined (as it is on x86 - intel and amd may differ) then the
emulation can return any value since the code can't care.
So the conditional is not needed.

	David