Commit b0490a04 authored Oct 10, 2017 by Sandipan Das Committed by Michael Ellerman Oct 10, 2017

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: 3cdfcbfd ("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>

parent e36a82ee

arch/powerpc/lib/sstep.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -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 */

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