powerpc/lib/sstep: Fix count leading zeros instructions

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>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 4 insertions, 2 deletions

diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
index 5e8418c28bd8..f208f560aecd 100644
--- a/arch/powerpc/lib/sstep.c
+++ b/arch/powerpc/lib/sstep.c
@@ -1684,11 +1684,13 @@ 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 */