[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 14:39:45 PDT 2017
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
// Normal value defined by an instruction. Check for two-addr redef.
// FIXME: This could be coincidental. Should we really check for
// operand constraint?
// Note that VNI->def may be a use slot for an early clobber def.
if (const VNInfo *UVNI = LR.getVNInfoBefore(VNI->def))
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.
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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