Re: [PATCH v2] powerpc/lib/sstep: Fix count leading zeros instructions
From: Naveen N. Rao <hidden>
Date: 2017-10-10 06:55:11
On 2017/10/10 06:45AM, Sandipan Das wrote:
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 <redacted>Reviewed-by: Naveen N. Rao <redacted>
quoted hunk ↗ jump to hunk
--- 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