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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: 3cdfcbfd32 ("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>
This commit is contained in:
Sandipan Das 2017-10-10 12:15:30 +05:30 committed by Michael Ellerman
parent e36a82ee4c
commit b0490a04e7
1 changed files with 4 additions and 2 deletions
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6
arch/powerpc/lib/sstep.c
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@ -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 */