[llvm-dev] [MachineCopyPropagation] Issue with register forwarding/allocation/verifier in out-of-tree target

[Geoff Berry via llvm-dev]

Hi all,

Mikael reported a machine verification failure in his out-of-tree target 
with the MachineCopyPropagation changes to forward registers (which is 
currently reverted).  The verification in question is:

*** Bad machine code: Multiple connected components in live interval ***
- function:    utils_la_suite_matmul_ref
- interval:    %vreg77 
[192r,208B:0)[208B,260r:1)[312r,364r:2)[380r,464B:3)  0 at 192r 1 at 208B-phi 
2 at 312r 3 at 380r
0: valnos 0 1 3
1: valnos 2

In this particular case, I believe that it is the greedy allocator that 
is creating the multiple components in the %vreg77 live interval.  If 
you look at the attached debug dump file, just after the greedy 
allocator runs, the segment of %vreg77 from the def at 312B to the use 
at 380B seems to be separable from the other segments.  The reason the 
above verification failure is not hit at that point seems to be related 
to the FIXME in the following snippet from 
ConnectedVNInfoEqClasses::Classify():

       // Normal value defined by an instruction. Check for two-addr redef.
       // FIXME: This could be coincidental. Should we really check for 
a tied
       // operand constraint?
       // Note that VNI->def may be a use slot for an early clobber def.
       if (const VNInfo *UVNI = LR.getVNInfoBefore(VNI->def))
         EqClass.join(VNI->id, UVNI->id);

Just after the greedy allocator runs, the instruction at 380B also 
defines %vreg77, so the verification check treats it as a two-addr 
redefinition (even though it is not) and allows it. 
MachineCopyForwarding renames the use of %vreg77 at 380B so the segment 
in question no longer ends at an instruction that is also a def, so the 
verification check fires.

It appears to me that this check is too loose, and if so that means 
there is something going wrong either in the allocator itself or in its 
interaction with this particular target in this case.

-- 
Geoff Berry
Employee of Qualcomm Datacenter Technologies, Inc.
  Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm 
Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the Code 
Aurora Forum, a Linux Foundation Collaborative Project.
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