[LLVMdev] Register allocation in two passes

Borja Ferrer borja.ferav at gmail.com
Thu Dec 8 12:15:35 PST 2011

Jakob I've just noticed that I'm getting false positives about spills when
there are actually none.
What is happening is that although execution reaches to the line
spiller().spill(LRE); inside RAGreedy::selectOrSplit() the insertion of the
spill is avoided because the register gets rematted. This is the debug
output I'm getting to show what I mean:

Inline spilling DLDREGS:%vreg25,1.436782e-03 = [344r,640r:0)  0 at 344r
>From original %vreg8,1.838235e-03 = [224r,640r:0)  0 at 224r
Value %vreg25:0 at 344r may remat from %vreg25<def> = LDIWRdK 2;
    remat:  632r    %vreg28<def> = LDIWRdK 2; DLDREGS:%vreg28
            640e    %R15R14<def> = COPY %vreg28<kill>; DLDREGS:%vreg28
    interval: %vreg28,inf = [632r,640r:0)  0 at 632r
All defs dead: %vreg25<def,dead> = LDIWRdK 2; DLDREGS:%vreg25
Remat created 1 dead defs.
Deleting dead def 344r    %vreg25<def,dead> = LDIWRdK 2; DLDREGS:%vreg25
0 registers to spill after remat. <----- NO SPILL

My question is how could I detect this behaviour so i don't get any false
positives? I've thought of checking if the variable NewVRegs increases in
size after the call to the spill() method meaning that a remat happened,
but I don't know if this is always correct in all situation and another
important thing, are there any more cases where a spill could be avoided
getting more false positives?

Thanks for your help.
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