Perhaps it would help if I explained my motivation. I'm trying to evaluate
the overhead of conditional deoptimization checks. One way is by running
workloads that have the checks in their normal configuration and measuring
the runtime. Then removing checks that were never triggered and rerunning
the workload and comparing the runtime. Obviously I understand this is not
safe in the general case.
For some workloads I was able to remove the call to DeoptimizeIf in
LCodeGen::DoCheckMaps and the benchmark still ran correctly. But if I
removed the call to CompareMap(reg,map) I would get an error about
unreachable code similar to what I posted earlier when I remove the
CompareMaps hydrogen instructions.
Aside from that I think that I want to be able to choose when to remove
checks at the hydrogen instruction level because later I will want to pick
which functions to remove the checks from. I would profile a benchmark
first and see which functions have conditional deopts that are never
triggered and then remove the deopts from those functions.
Again this is all part of a performance evaluation study, not something to
be used for production code. I hope this makes sense, but if you think
there's something I'm overlooking for why this won't work I'd be interested
to know why. From looking at the assembly code sequences that are
generated I think this should be okay, but there's also obviously something
I'm missing that is leading to the unreachable code error that I've seen.
Thanks,
-Gabriel
On Wednesday, November 5, 2014 5:42:20 AM UTC-8, Jakob Kummerow wrote:
>
> Removing check instructions is so utterly wrong and dangerous that I can't
> bring myself to even try to help you. Just don't do it!
>
>
> On Wed, Nov 5, 2014 at 8:19 AM, Gabriel Southern <south...@gmail.com
> <javascript:>> wrote:
>
>> 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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>
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