Too many load/store in Machine code represtation

There are a lot of load/store instructions for accessing variable in stack slot. The following message is use “llc -march=ppc32” command and dump from MachineFunction.

%reg16384 = LWZ 0, <fi#6>; mem:LD4[%b] GPRC:%reg16384
%reg16385 = LWZ 0, <fi#5>; mem:LD4[%c] GPRC:%reg16385
%reg16386 = ADD4 %reg16384, %reg16385; GPRC:%reg16386,16384,16385
STW %reg16386, 0, <fi#7>; mem:ST4[%a] GPRC:%reg16386

I am interesting in this method because it seems smart and different from the compiler textbook. For understanding how it generated the LWZ and STW machine instruction, I search whole the LLVM2.8 and find somewhere about LWZ/STW in Target/PowerPC/ ,but there is nothing through here. Is there any other special way to generate those MachineInstr ?

Thanks & Regards,