[LLVMdev] interesting minor llvm optimizer flaw

[Reed Kotler]

sorry;;;  original pattern was

def: Mips16Pat
   <(setge CPU16Regs:$lhs, immSExt16:$rhs),
    (XorRxRxRy16 (SltiCCRxImmX16 CPU16Regs:$lhs, immSExt16:$rhs), 
(LiRxImmX16 1))>;

(cut and pasted  immZExt16:$rhs into example accidentally)

On 10/18/2012 08:13 PM, reed kotler wrote:
> if I write:
>
> int z, x;
> ...
> z = (x >= k);  -- where k is a constant
>
> The compiler always wants to translate this into:
>
> z = (x > (k-1));
>
> In general this can often lead to better code (and it does for Mips 16
> for sure), except at
> the boundary condition where k==-32768
>
> Then it creates the literal -32769 which cannot be placed in a simple
> immediate field.
> That creates a lot of extra code for Mips 16.
>
> I had originally written a pattern for setge when the right operand is a
> constant.
>
> def: Mips16Pat
>    <(setge CPU16Regs:$lhs, immZExt16:$rhs),
>     (XorRxRxRy16 (SltiCCRxImmX16 CPU16Regs:$lhs, immZExt16:$rhs),
>     (LiRxImmX16 1))>;
>
> I was able to work around this boundary case by doing the reverse
> transformation at the boundary.
>
> i..e.
>
> z = (x > (k-1)) =>  z = (x >= k)
>
> def: Mips16Pat
>    <(setgt CPU16Regs:$lhs, -32769),
>     (XorRxRxRy16 (SltiCCRxImmX16 CPU16Regs:$lhs, -32768),
>     (LiRxImmX16 1))>;