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>
---
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
