[LLVMdev] Missing optimization - constant parameter

[Maurice Marks]

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.




On Mon, Aug 5, 2013 at 11:19 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:

>
> On Aug 5, 2013, at 8:34 AM, Maurice Marks <maurice.marks at gmail.com> wrote:
>
> > Are you sure that's it? I commented that block out, rebuilt llvm 3.3,
>  and it still duplicates the constant.
> > My concern is that long constant loads increase code size and if they
> can be avoided by better targeting it would be a win. My project's
> application of llvm tends to use a lot of long constants so this can be a
> significant optimization.
> > I'll do some more debugging now that you have pointed me in the right
> direction.
>
> It is also possible that the coalescer is duplicating the instruction like
> Rafael suggested. It will do that if it can't eliminate a copy.
>
> /jakob
>
>
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