bpf: improve dead Defs check for XADD

bpf: improve dead Defs check for XADD

BPF XADD semantics require all Defs of XADD are dead, meaning any result of
XADD insn is not used.

However, BPF backend hasn't enabled sub-register liveness track, so when
the source and destination operands of XADD are GPR32, there is no
sub-register dead info. If we rely on the generic
MachineInstr::allDefsAreDead, then we will raise false alarm on GPR32 Def.
This was fine as there was no sub-register code-gen support for XADD which
will be added by the next patch.

To support GPR32 Def, ideally we could just enable sub-registr liveness
track on BPF backend, then allDefsAreDead could work on GPR32 Def. This
requires implementing TargetSubtargetInfo::enableSubRegLiveness on BPF.

However, sub-register liveness tracking module inside LLVM is actually
designed for the situation where one register could be split into more
than one sub-registers for which case each sub-register could have their
own liveness and kill one of them doesn't kill others. So, tracking
liveness for each make sense.

For BPF, each 64-bit register could only have one 32-bit sub-register. This
is exactly the case which LLVM think brings no benefits for doing
sub-register tracking, because the live range of sub-register must always
equal to its parent register, therefore liveness tracking is disabled even
the back-end has implemented enableSubRegLiveness. The detailed information
is at r232695:

  Author: Matthias Braun <matze@braunis.de>
  Date:   Thu Mar 19 00:21:58 2015 +0000
  Do not track subregister liveness when it brings no benefits

Hence, for BPF, we enhance MachineInstr::allDefsAreDead. Given the solo
sub-register always has the same liveness as its parent register, LLVM is
already attaching a implicit 64-bit register Def whenever the there is
a sub-register Def. The liveness of the implicit 64-bit Def is available.
For example, for "lock *(u32 *)(r0 + 4) += w9", the MachineOperand info
could be:

  $w9 = XADDW32 killed $r0, 4, $w9(tied-def 0),
                       implicit killed $r9, implicit-def dead $r9

Even though w9 is not marked as Dead, the parent register r9 is marked as
Dead correctly, and it is safe to use such information or our purpose.

v1 -> v2:
 - Simplified code logic inside hasLiveDefs. (Yonghong)

Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
llvm-svn: 355124