http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53399
Bug #: 53399
Summary: "*ffs" pattern generates wrong code with BMI enabled
(for corner cases)
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: kirill.yuk...@intel.com
We have in GCC int. (__ffs description):
These functions return the index of the least significant 1-bit in a, or the
value zero if a is zero.
and in i386.md:
(define_insn "*ffs<mode>_1"
[(set (reg:CCZ FLAGS_REG)
(compare:CCZ (match_operand:SWI48 1 "nonimmediate_operand" "rm")
(const_int 0)))
(set (match_operand:SWI48 0 "register_operand" "=r")
(ctz:SWI48 (match_dup 1)))]
""
{
if (TARGET_BMI)
return "tzcnt{<imodesuffix>}\t{%1, %0|%0, %1}";
else
This pattern works fine for bsf insn (although the result with zero input is
undefined)
But for tzcnt with 0 as input we have (operand_size+1) as output.
That contradicts with GCC int, right?
It also seems to fail gcc.c-torture/execute/builtin-bitops-1.c
