[LLVMdev] RegAllocFast uses too much stack

[Jakob Stoklund Olesen]

On Jul 11, 2011, at 1:48 PM, Nick Lewycky wrote:

> I discovered recently that RegAllocFast spills all the registers before every function call. This is the root cause of one of our recursive functions that takes about 150 bytes of stack when built with gcc (same at -O0 and -O2, or 120 bytes at llc -O2) taking 960 bytes of stack when built by llc -O0. That's pretty bad for situations where you have small stacks, which is not uncommon for threaded software on a 32-bit architecture, or a signal handler.
> 
> I realize that some of this is intentional and we don't want to do optimization at -O0, but I'm really hoping there's something we could sensibly do to improve this. Here's an example:
> 
>   extern "C" void foo(int);
>   
>   void test() {
>     foo(0);
>     foo(1);
>     foo(2);
>   }
> 
> This doesn't just spill out all the registers to the stack before each call, we also set up 0, 1 and 2 into regs first, then spill them and don't even get a chance to reuse stack slots. That's just bad:
> 
>         pushq   %rax
>         movl    $2, %edi
>         movl    $1, %eax
>         movl    $0, %ecx
>         movl    %edi, 4(%rsp)           # 4-byte Spill
>         movl    %ecx, %edi
>         movl    %eax, (%rsp)            # 4-byte Spill
>         callq   foo
>         movl    (%rsp), %edi            # 4-byte Reload
>         callq   foo
>         movl    4(%rsp), %edi           # 4-byte Reload
>         callq   foo
>         popq    %rax
>         ret
> 
> Does anyone have any ideas what we could do that doesn't add to the compile time?

We probably won't be having virtual registers live across calls. That requires quite a bit of analysis which would be too expensive compile time wise.

Which means that we should avoid creating live ranges spanning calls. This is what fast isel does now:

# After Instruction Selection:
# Machine code for function test:

BB#0: derived from LLVM BB %entry
        %vreg0<def> = MOV32ri 2; GR32:%vreg0
        %vreg2<def> = MOV32ri 1; GR32:%vreg2
        %vreg4<def> = MOV32ri 0; GR32:%vreg4
        ADJCALLSTACKDOWN64 0, %RSP<imp-def>, %EFLAGS<imp-def>, %RSP<imp-use>
        %EDI<def> = COPY %vreg4; GR32:%vreg4
        %AL<def> = MOV8ri 0
        CALL64pcrel32 <ga:@foo>, %AL, %EDI, %RAX<imp-def>, %RDI<imp-def,dead>, %RSP<imp-use>, ...
        ADJCALLSTACKUP64 0, 0, %RSP<imp-def>, %EFLAGS<imp-def>, %RSP<imp-use>
        %vreg5<def> = COPY %EAX; GR32:%vreg5
        ADJCALLSTACKDOWN64 0, %RSP<imp-def>, %EFLAGS<imp-def>, %RSP<imp-use>
        %EDI<def> = COPY %vreg2; GR32:%vreg2
        %AL<def> = MOV8ri 0
        CALL64pcrel32 <ga:@foo>, %AL, %EDI, %RAX<imp-def>, %RDI<imp-def,dead>, %RSP<imp-use>, ...
        ADJCALLSTACKUP64 0, 0, %RSP<imp-def>, %EFLAGS<imp-def>, %RSP<imp-use>
        %vreg3<def> = COPY %EAX; GR32:%vreg3
        ADJCALLSTACKDOWN64 0, %RSP<imp-def>, %EFLAGS<imp-def>, %RSP<imp-use>
        %EDI<def> = COPY %vreg0; GR32:%vreg0
        %AL<def> = MOV8ri 0
        CALL64pcrel32 <ga:@foo>, %AL, %EDI, %RAX<imp-def>, %RDI<imp-def,dead>, %RSP<imp-use>, ...
        ADJCALLSTACKUP64 0, 0, %RSP<imp-def>, %EFLAGS<imp-def>, %RSP<imp-use>
        %vreg1<def> = COPY %EAX; GR32:%vreg1
        RET

I don't know why it materializes those constants at the top of the block. I think they should just be materialized immediately before they are used.

/jakob