I'm experimenting with removing deoptimization checks and I have a question
about how to remove hydrogen instructions.
I'm looking at a benchmark where the CheckMaps deoptimization checks are
never triggered and I'm trying to remove them. I know this is not safe in
the general case, but when I traced the deoptimizations for this benchmark
there were not any that were triggered because of CheckMaps.
I've tried to follow the HDeadCodeEliminationPhase as a guide because what
I want to do is delete instructions that match a certain criteria, so I
thought that pass might be a good example. The main loop in my pass is:
for (int i = 0; i < graph()->blocks()->length(); ++i) {
HBasicBlock* block = graph()->blocks()->at(i);
for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
HInstruction* instr = it.Current();
if (instr->opcode() == HValue::kCheckMaps) {
instr->DeleteAndReplaceWith(NULL);
}
}
}
When I run this and just print the list of instructions that will be
removed the list looks okay. However if I actually delete the instruction
I get a runtime error as follows:
#
# Fatal error in ../src/objects.cc, line 10380
# unreachable code
#
==== C stack trace ===============================
1: V8_Fatal
2:
v8::internal::Code::FindAndReplace(v8::internal::Code::FindAndReplacePattern
const&)
3:
v8::internal::CodeStub::GetCodeCopy(v8::internal::Code::FindAndReplacePattern
const&)
4:
v8::internal::PropertyICCompiler::ComputeCompareNil(v8::internal::Handle<v8::internal::Map>,
v8::internal::CompareNilICStub*)
5:
v8::internal::CompareNilIC::CompareNil(v8::internal::Handle<v8::internal::Object>)
6: ??
7: v8::internal::CompareNilIC_Miss(int, v8::internal::Object**,
v8::internal::Isolate*)
8: ??
Segmentation fault (core dumped)
I'm wondering if anyone has suggestions for what I can look at it
understand what's going on and debug the problem. Obviously the specific
thing I'm trying to do of removing CheckMaps is not something that should
work in general. But I think it should be possible to remove a hydrogen
instruction during the optimization phase. I've tried to pattern my
attempt off of the existing code, but obviously I'm missing something. If
anyone has suggestions about what I should try that is appreciated.
Thanks,
Gabriel
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