[LLVMdev] MachineOperand: Subreg defines and the Undef flag

[Pranav Bhandarkar]

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.

Pranav

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