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

[David Laight]

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.

Here you are emulating the cpu instruction - so executing one will
give the same answer as it the 'real' one were execucted.

Indeed it might be worth an asm statement that definitely executes
the cpu instruction?

	David

> 
> Signed-off-by: Sandipan Das <sandipan at linux.vnet.ibm.com>
> ---
>  arch/powerpc/lib/sstep.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
> index 0f7e41bd7e88..ebbc0b92650c 100644
> --- a/arch/powerpc/lib/sstep.c
> +++ b/arch/powerpc/lib/sstep.c
> @@ -1717,11 +1717,19 @@ 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];
> +			if (val == 0)
> +				op->val = 32;
> +			else
> +				op->val = __builtin_clz(val);
>  			goto logical_done;
>  #ifdef __powerpc64__
>  		case 58:	/* cntlzd */
> -			op->val = __builtin_clzl(regs->gpr[rd]);
> +			val = regs->gpr[rd];
> +			if (val == 0)
> +				op->val = 64;
> +			else
> +				op->val = __builtin_clzl(val);
>  			goto logical_done;
>  #endif
>  		case 28:	/* and */
> --
> 2.13.6