[PATCH v2] powerpc/lib/sstep: Fix count leading zeros instructions

[Sandipan Das]

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 at 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