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

Matthias Braun via llvm-dev llvm-dev at lists.llvm.org
Tue Sep 26 15:47:42 PDT 2017

> On Sep 26, 2017, at 3:33 PM, Geoff Berry <gberry at codeaurora.org> wrote:
> 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.
The copy propagation seemed to be working on vregs. This was extra confusing as D30751 seems to be currently reverted from trunk so I couldn't find references to that code.

>> 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.
I would expect you know the code in D30751 and can take a look into why only 1 of the instructions is rewritten?
From all I've seen so far the verification code seems to behave as expected.

- Matthias

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