[LLVMdev] interesting minor llvm optimizer flaw

[reed kotler]

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))>;