In the current compiler pipeline, PushArgument instructions can
end up outliving the calls to which they are an argument in some
circumstances. For example, the PushArgument is left in the graph
if it also appears in the environments of interleaving instructions.
When the dead code eliminator calculates liveness of a definition,
it ignores appearances in PushArgument instructions. Instead, it
looks for uses of the definition either directly or via
PushArgument in the arguments of calls and environments of other
instructions in the graph. An separate pass run before DCE
removes environments from instructions, so a definition used only
in PushArguments that have outlived their calls is considered dead.
The current DCE removes dead definitions but leaves PushArgument
instructions be. For orphaned PushArguments of dead definitions,
this means after DCE they now reference definitions no longer in
the graph.
This change instead checks the definitions referenced by
PushArgument instructions for liveness. If a PushArgument
references a dead definition, then the DCE removes the PushArgument
instruction, as its call must have already been removed (else
the definition would still be live), and environments, and thus
environmental uses, have already been removed from the graph.
Bug: https://github.com/dart-lang/sdk/issues/39767
Change-Id: I74c8e92a0d93f2aae42f8ff6f06ee70899b2f794
Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-linux-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/128403
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Teagan Strickland <sstrickl@google.com>
Teagan Strickland
217e265b02