* [PATCH v2] powerpc/lib/sstep: Fix count leading zeros instructions
@ 2017-10-10 6:45 Sandipan Das
2017-10-10 6:54 ` Naveen N. Rao
2017-10-12 0:20 ` [v2] " Michael Ellerman
0 siblings, 2 replies; 3+ messages in thread
From: Sandipan Das @ 2017-10-10 6:45 UTC (permalink / raw)
To: mpe; +Cc: naveen.n.rao, paulus, anton, segher, linuxppc-dev
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 <sandipan@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
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH v2] powerpc/lib/sstep: Fix count leading zeros instructions
2017-10-10 6:45 [PATCH v2] powerpc/lib/sstep: Fix count leading zeros instructions Sandipan Das
@ 2017-10-10 6:54 ` Naveen N. Rao
2017-10-12 0:20 ` [v2] " Michael Ellerman
1 sibling, 0 replies; 3+ messages in thread
From: Naveen N. Rao @ 2017-10-10 6:54 UTC (permalink / raw)
To: Sandipan Das; +Cc: mpe, paulus, anton, segher, linuxppc-dev
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 <sandipan@linux.vnet.ibm.com>
Reviewed-by: Naveen N. Rao <naveen.n.rao@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
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [v2] powerpc/lib/sstep: Fix count leading zeros instructions
2017-10-10 6:45 [PATCH v2] powerpc/lib/sstep: Fix count leading zeros instructions Sandipan Das
2017-10-10 6:54 ` Naveen N. Rao
@ 2017-10-12 0:20 ` Michael Ellerman
1 sibling, 0 replies; 3+ messages in thread
From: Michael Ellerman @ 2017-10-12 0:20 UTC (permalink / raw)
To: Sandipan Das; +Cc: linuxppc-dev, naveen.n.rao, paulus, anton
On Tue, 2017-10-10 at 06:45:30 UTC, 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 <sandipan@linux.vnet.ibm.com>
> Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Applied to powerpc fixes, thanks.
https://git.kernel.org/powerpc/c/b0490a04e736356e427e227902b17f
cheers
^ permalink raw reply [flat|nested] 3+ messages in thread
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2017-10-10 6:54 ` Naveen N. Rao
2017-10-12 0:20 ` [v2] " Michael Ellerman
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