On 2017/10/10 06:45AM, Sandipan Das wrote:
> 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.
>
> Fixes: 3cdfcbfd32b9d ("powerpc: Change analyse_instr so it doesn't modify
> *regs")
> Signed-off-by: Sandipan Das <sandi...@linux.vnet.ibm.com>
Reviewed-by: Naveen N. Rao <naveen.n....@linux.vnet.ibm.com>
> ---
> v2: Make zero-checking condition more compact.
> Add details of original commit that is being fixed here.
> ---
> arch/powerpc/lib/sstep.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
> index 5118110c3983..8c3955e183d4 100644
> --- a/arch/powerpc/lib/sstep.c
> +++ b/arch/powerpc/lib/sstep.c
> @@ -1699,11 +1699,13 @@ int analyse_instr(struct instruction_op *op, const
> struct pt_regs *regs,
> * Logical instructions
> */
> case 26: /* cntlzw */
> - op->val = __builtin_clz((unsigned int) regs->gpr[rd]);
> + val = (unsigned int) regs->gpr[rd];
> + op->val = ( val ? __builtin_clz(val) : 32 );
> goto logical_done;
> #ifdef __powerpc64__
> case 58: /* cntlzd */
> - op->val = __builtin_clzl(regs->gpr[rd]);
> + val = regs->gpr[rd];
> + op->val = ( val ? __builtin_clzl(val) : 64 );
> goto logical_done;
> #endif
> case 28: /* and */
> --
> 2.13.6
>
