[LLVMdev] Removing dead code

[Andrew Lenharth]

On Fri, 2006-06-30 at 10:26 -0700, Fernando Magno Quintao Pereira wrote:
> 
> > > It seems to me that the only instructions
> > > with dead definitions that I should not remove are the calls. Is it true?
> > > I would like to know if a code like this below is safe, that is, besides
> > > call instructions, is there other instructions that must stay in the code
> > > even if their definitions are dead?
> > >
> > > MachineInstr * mi = iter;
> > > opCode = // get the opcode of mi
> > > if(!mi.isCall(opCode)) {
> > >    mbb.remove(iter);
> > > }
> >
> > You can't do that unless you can prove the instructions don't have side
> > effects, which you can't.  Higher-level passes will remove dead code.  Are
> > you seeing a case where dead code is making it down to the codegen level?
> >
> > -Chris
> >
> 
> I think so. LLVM is producing code like this one here, before RA:

That is because the physical register has to be copied to a virtual
register.  Without the copy, physical registers could be alive across
basic blocks.  There is a function you can call for each target to tell
you if an instruction is a copy.

Andrew

> ----------------------------------------------------------------------------
> entry (0x8605ba0, LLVM BB @0x8602d30):
>         %reg1024 = OR4 %r3, %r3
>         %reg1025 = OR4 %r4, %r4
>         %reg1026 = LWZ 0, %reg1025
>         %reg1027 = LIS <ga:.str_1>
>         %reg1028 = LIS <ga:.str_2>
>         %reg1029 = LBZ 0, %reg1026
>         ADJCALLSTACKDOWN 56
>         %reg1030 = IMPLICIT_DEF_GPR
>         %reg1031 = LA %reg1027, <ga:.str_1>
>         %r3 = OR4 %reg1031, %reg1031
>         BL <ga:printf>, %r3
>         %reg1032 = OR4 %r3, %r3      <-------------------
>         %reg1033 = EXTSB %reg1029
>         %reg1034 = LA %reg1028, <ga:.str_2>
>         ADJCALLSTACKUP 56
>         ADJCALLSTACKDOWN 56
>         %r3 = OR4 %reg1034, %reg1034
>         %r4 = OR4 %reg1033, %reg1033
>         BL <ga:printf>, %r3, %r4
>         %reg1035 = OR4 %r3, %r3
>         ADJCALLSTACKUP 56
>         %r3 = OR4 %reg1030, %reg1030
>         BLR
> ----------------------------------------------------------- produced from:
> #include <stdio.h>
> int main(int argc, char ** argv) {
>     int i;
>     i = argv[0][0];
>     printf("Hello, world\n");
>     printf("i = %d\n", i);
> }
> --------------------------------------------------------------------------
> 
> where %reg1032 is dead. I'm removing these instructions. In Linear scan,
> they are removed too. I'm removing all the dead
> definitions from instructions that are not function calls, and the
> resulting programs seem to work fine. The ratio of these instructions is
> about 1:20, that is, for each 20 instructions (in PPC) the RA could remove
> 1 dead definition.
> 
> Fernando
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