[LLVMdev] MachineOperand: Subreg defines and the Undef flag

[Jakob Stoklund Olesen]

On Jul 5, 2012, at 6:01 PM, "Pranav Bhandarkar" <pranavb at codeaurora.org> wrote:

> Hi Jakob,
> 
>> New_MI_1:: Vreg1 = 0                                    ; Vreg1 and Vreg2
>> are 32 bit virt. regs.
>> New_MI_2:: Vreg2 = COPY C:lo_sub_reg.
>> New_MI_3:: B= REG_SEQUENCE<Vreg1, hi_sub_reg, Vreg2, lo_sub_reg>  ; B
>> is a
>> 64 bit virt reg.
> 
> I used this approach and it worked find until I hit, what I believe is, a
> bug in the register coalescer. 
> When the register coalescer cannot trivially coalesce a copy, say C,, it
> calls AdjustCopiesBackFrom. In this function, we try to see if have this
> situation.
> 
> A3 = B0
>   .....
>   .....
> B1 = A3   <--The copy C
> 
> And if so, we check if we can merge the two ranges of B into a single range.
> However, this is not safe if A3 is a subreg define while A3 is not a subreg
> use.
> For instance, consider this code (part of a single block loop).
> 
> MI1::  %vreg7:subreg_loreg<def,undef>, %vreg30<def> = POST_LDriuh %vreg30,
> 2,  // Post Inc. Load. Vreg7 is a 64bit reg.
> MI2::  %vreg7:subreg_hireg<def> = COPY %vreg32:subreg_hireg<kill>
> // This is the A3 = B0 above.
> MI3::  %vreg31<def> = ADD_rr %vreg31<kill>, %vreg32:subreg_loreg<kill>
> // Use the lo subreg that was setup in MI1:
> ....
> ....
> MI4::  %vreg32<def> = COPY %vreg7;                           //Not trivial
> because 7 is not killed.  This is the Copy C i.e. B1=A3.
> ....
> MI5:: Conditional jump back to start of the block.
> 
> The coalesce coalesces the copy withouth realizing that the instruction that
> defined the source of the copy was a copy instruction that only copied a
> subreg not the whole register.
> In the event, we lose the lower subreg.
> 
> Let me know If I am missing something.

That sounds like a bug, probably adjustCopiesBackFrom needs to check ACopyMI->isFullCopy().

Do you have a test case for this?

/jakob