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

From: Segher Boessenkool

Sent: 09 October 2017 15:21
On Mon, Oct 09, 2017 at 01:49:26PM +0000, David Laight wrote:

quoted

From: Sandipan Das

quoted

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.

=20
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.
=20
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