[LLVMdev] Not enough optimisations in the SelectionDAG phase?

Evan Cheng
Sun, 29 Apr 2012 18:19:15 -0000

On Apr 24, 2012, at 11:48 PM, Fan Dawei wrote:

> For the following code fragment,
>
> ; :27 ; preds = %27,
%entry
>   %28 = load volatile i32* inttoptr (i64 2149581832 to i32*), align 8
>   %29 = icmp slt i32 %28, 0
>   br i1 %29, label %27, label %loop.exit
>
> loop.exit:                                  ; preds = %27
>
> llc will generate following MIPS code,
>
> $BB0_1:
>   lui $3, 32800
>   ori $3, $3, 1032
>   lw  $3, 0($3)
>   bltz  $3, $BB0_1
>   nop
> # BB#2:
>
>
> The two operation lui and ori which are used to calculate memory address
actually are loop invariants. They supposed to be moved out of the loop.  I
thought it might be a limitation of the MIPS backend.  Then I tried the ARM
backend,
>
>  .LBB1_1:
>   ldr r2, .LCPI1_2
>   ldr r2, [r2]
>   cmp r2, #0
>   blt .LBB1_1
> @ BB#2:
>
> The first ldr instruction is to load the address from constant pool. It
also should be outside the loop.
>
> I'm not sure if this is because of the optimisations are not enough in
the common SelectionDAG optimisation phase, or  should this kind of optimisation
be implemented by the SelectionDAG instruction lowering phase for each target?

SelectionDAG doesn't do LICM. Are you running machine-licm pass?

Evan