[LLVMdev] Missing optimization - constant parameter

[Jakob Stoklund Olesen]

On Oct 10, 2013, at 7:59 AM, Maurice Marks <maurice.marks at gmail.com> wrote:

> I went back to this problem after looking at some other things. Turning on debugging I noticed that the register coalescer is trying to do the "right thing" and merge vreg0 (the previous load of the constant) with the first parameter of the call (%rdi), which is exactly what gcc does in this case - eliminating a second constant load, but its refusing to do the merge:
> 
> Here's the debug output for that part of the compilation:
> 
> ********** SIMPLE REGISTER COALESCING **********
> ********** Function: caller
> ********** JOINING INTERVALS ***********
> entry:
> 64B     %RDI<def> = COPY %vreg0; GR64:%vreg0
>         Considering merging %vreg0 with %RDI
>         Can only merge into reserved registers.
> Remat: %RDI<def> = MOV64ri 12345123400
> Shrink: [32r,64r:0)  0 at 32r
> Shrunk: [32r,48r:0)  0 at 32r
> Trying to inflate 0 regs.
> ********** INTERVALS **********
> %vreg0 = [32r,48r:0)  0 at 32r
> RegMasks: 80r
> 
> Jakob, what does "can only merge into reserved registers" mean in this instance. I don't see any reason for it not to do the merge.

The coalescer won’t merge virtual and allocatable physical registers because that will extend the live range of the physical registers, constraining register allocation.

Instead, the register allocator will attempt to choose %rdi for%vreg0 so the copy can be eliminated after register allocation.

/jakob