[PATCH] D29014: [SelDag] Implement FREEZE node

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added inline comments.


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Comment at: lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp:2238
+  // MachineInstr.
+  // LLVM translates an UNDEF node into multiple IMPLICIT_DEF
+  // instructions (in MachineInstr) if the UNDEF has multiple uses.
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mkuper wrote:
> I'm a bit confused about this. The function above selects UNDEF to IMPLICIT_DEF.
> Where do we do the duplication? If we duplicate UNDEFs on the DAG level, then everything's already fine, and you can just select a FREEZE directly to IMPLICIT_DEF.
> If we're duplicating IMPLICIT_DEFs, then this is a problem regardless of what you do here, if the freeze ends up being an IMPLICIT_DEF. Or are you saying we're duplicating IMPLICIT_DEFs in the DAG, but not in MI? I'd find that surprising.
> 
> (Note that the documentation for IMPLICIT_DEF just says that "this is the MachineInstr-level equivalent of undef." If we're not allowed to duplicate IMPLICIT_DEFs and they have a single defined value, that should be corrected.)
If you have e.g.:
 mul undef, undef

this eventually may get translated into:
  %v1 = IMPLICIT_DEF
  %v2 = IMPLICIT_DEF
  %v3 = mul %v1, %v2

Even if it doesn't get translated to this explicitly, the semantics of UNDEF/IMPLICIT_DEF are that each read may see a different value.  For example, IMPLICIT_DEF can be sank into a loop and each iteration may see a different value, and known-bits analyses will also take advantage of the fact that each bit can take any value it wants.
Therefore we use this "hack" of making a copy of the vreg where IMPLICIT_DEF is assigned to. This seems to guarantee that all uses see the same value, since there isn't a pass propagating IMPLICIT_DEF to copies.


https://reviews.llvm.org/D29014