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

Geoff Berry via llvm-dev llvm-dev at lists.llvm.org
Tue Sep 26 15:33:32 PDT 2017

On 9/26/2017 6:11 PM, Matthias Braun wrote:
>> On Sep 26, 2017, at 2:39 PM, Geoff Berry via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 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():
> That dump seems to be well before greedy runs, isn't it?

I'm not sure what you mean.  The attached log contains -print-before-all 
-print-after-all and -debug output starting with the coalescer pass. 
The verification failure is right after the first pass of 
MachineCopyPropagation which runs after the greedy allocator.

> At a first glance the odd thing there is that the operand of fladd_a32_a32_a32 is rewritten from vreg77 to vreg76, but the vreg77 operand of the BUNDLE is not. Maybe you can find out why that is?

Sorry, I should have pointed this out before: because the loop over 
instructions in MachineCopyPropagation is only visiting the BUNDLE 
instructions themselves (i.e. it does not visit the instructions inside 
the BUNDLE) and we don't forward to implicit uses (which all of the 
BUNDLE operands are marked as), we won't currently forward a use to a 
bundled instruction.  I believe handling bundles more aggressively can 
be added as a follow-on enhancement unless we think not doing has an 
inherent problem.

> - Matthias

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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