MVT-based vectors: first stab

Vladimir Ivanov vladimir.x.ivanov at oracle.com
Wed Jun 28 15:20:50 UTC 2017



Best regards,
Vladimir Ivanov

On 6/28/17 6:00 PM, Maurizio Cimadamore wrote:
> 
> 
> On 28/06/17 15:36, Vladimir Ivanov wrote:
>>
>>
>> On 6/28/17 5:08 PM, Maurizio Cimadamore wrote:
>>> Great work Vlad; I have two questions before I dive in more details
>>>
>>> * why did you need to handle arrayLength() specially? Since an array 
>>> is a reference, shouldn't it work already?
>>
>> It's a workaround for C2 crashes I observed when working with 
>> arraylength method handle on Q-typed arrays. The problem for the 
>> compiler is that the implementation uses 
>> ArrayAccessor.lengthL(Object[]), so on bytecode level there's a 
>> conflict between declared & actual type (DVT[] doesn't extend Object[]).
>>
>> IMO the proper fix would be to have "erased" version for such cases 
>> (or replace all primitive array specializations with a single "erased" 
>> version):
>>
>>   static int lengthL(java.lang.Object);
>>          0: aload_0
>>          1: arraylength
>>          2: ireturn
>>
>> But it can't be expressed in Java language and requires bytecode 
>> generation.
> OK - in other words, we need to specialize array length, because of the 
> missing subtyping edge. Ugh.

Yes. Moreover, I've just checked and the verifier complains about 
non-array types on stack for arraylength instruction.

Caused by: java.lang.VerifyError: Bad type on operand stack in arraylength
Exception Details:
   Location:
     Test.length(Ljava/lang/Object;)I @1: arraylength
   Reason:
     Invalid type: 'java/lang/Object' (current frame, stack[0])
   Current Frame:
     bci: @1
     flags: { }
     locals: { 'java/lang/Object' }
     stack: { 'java/lang/Object' }

So, it seems longer term the only alternative to specialization is Unsafe.

>>> * Can you explain me exactly what you were trying to achieve with 
>>> state.push/state.pop in the intrinsic generation? Typically the state 
>>> is updated through opcodes - so, assuming you are generating 
>>> bytecodes that make sense, you should not need this - unless there 
>>> are bugs in the code builder (which is possible!)
>>
>> The following sequence usually hits the bug in the code builder:
>>   builder.goto_("l1");
>>   builder.label("l2");
>>
>> goto_() doesn't wipe the state, but the state at l2 doesn't relate to 
>> the state when jumping to l1. It usually manifests as stack depth or 
>> type mismatches when trying to merge the states in builder.label().
> I see - that's a well-known issue - the builder currently doesn't handle 
> reachability. In your case, the builder should realize that code after 
> 'goto' is dead, so it should just try to merge an empty state with the 
> state of the code that is jumping to "l2" (of course this assumes that 
> there's no other subsequent backward jump to "l2" otherwise the simple 
> stackmap analysis implemented here would fail and you'll need some fix 
> point calculation a la ASM).

Yeah, backward jumps cause issues to one-pass solutions.

Best regards,
Vladimir Ivanov

>>> On 28/06/17 14:17, Vladimir Ivanov wrote:
>>>> Hi,
>>>>
>>>> I started playing with MVT-based vectors recently and want to share 
>>>> my experience so far.
>>>>
>>>> For the first step, I decided to avoid specific treatment of 
>>>> super-longs (Long2/4/8) in the JVM we have in Panama and just mark 
>>>> them as VCCs for now:
>>>>   @DeriveValueType
>>>>   final class Long2 { public final long lo, hi; }
>>>>
>>>> As a test case I chose sum of array elements:
>>>>
>>>>   long sum(QLong2[] va) {
>>>>     QLong2 v = QLong2.default; // (0,0)
>>>>     for (int i = 0; i < va.length; i++) {
>>>>       QLong2 vi = va[i];
>>>>       v = QLong2(v.lo + vi.lo, v.hi + vi.hi);
>>>>     }
>>>>     return v.lo + v.hi;
>>>>   }
>>>>
>>>> Here are the changes:
>>>>  http://cr.openjdk.java.net/~vlivanov/valhalla/vectors.mvt/webrev.00/
>>>>
>>>> SUMMARY & RESULTS
>>>>
>>>> I hit some bugs along the way and had to extend the code for Q-types 
>>>> in some places to get decent code shape generated. (I'll shepherd 
>>>> the fixes in the repo separately.) I ended up with the following 
>>>> generated code:
>>>>
>>>> http://cr.openjdk.java.net/~vlivanov/valhalla/vectors.mvt/sumArrayL2.log 
>>>>
>>>>
>>>> Unfortunately, there are still 2 allocations left in the generated 
>>>> code (out of 5 initially). C2 can't scalarize the accumulator (v) 
>>>> inside the loop and has to put updated components (lo & hi) in 
>>>> allocated heap buffer on every iteration over the array (generated 
>>>> code [1], C2 EA output [2]).
>>>>
>>>>
>>>> GORY DETAILS
>>>>
>>>>   hotspot/src/share/vm/opto/type.cpp
>>>>
>>>>     * Type::get_typeflow_type() doesn't distinguish between 
>>>> TypeValueTypePtr & TypeValueType
>>>>
>>>>
>>>> jdk/src/java.base/share/classes/java/lang/invoke/LambdaFormBuilder.java
>>>>
>>>>     * introduced some LF intrinsics, e.g. LOOP and SELECT_ALTERNATIVE;
>>>>
>>>>     * had to workaround control flow-related bugs in the bytecode 
>>>> library (jdk.experimental.bytecode): broken JVM state after goto_() 
>>>> (see FIXME marks in the code);
>>>>
>>>>     * fixed 2-slot types support: wrong local indexes were used
>>>>
>>>>
>>>>
>>>> jdk/src/java.base/share/classes/java/lang/invoke/MethodHandleImpl.java
>>>>
>>>>     * MHI.unboxResultHandle misses unboxing step for Q-types
>>>>
>>>>
>>>> jdk/src/java.base/share/classes/java/lang/invoke/MethodHandles.java
>>>> jdk/src/java.base/share/classes/jdk/experimental/value/ValueType.java
>>>>
>>>>     * had to add a specialization for arraylength on Q-typed array, 
>>>> because C2 can't handle arraylength Object[] when Q[] instance is on 
>>>> stack;
>>>>
>>>>
>>>> jdk/src/java.base/share/classes/jdk/experimental/bytecode/TypedCodeBuilder.java: 
>>>>
>>>>
>>>>     * changes required to workaround goto_() bug
>>>>
>>>>     * fixed CP patching support
>>>>
>>>>
>>>> jdk/src/java.base/share/classes/jdk/experimental/value/MethodHandleBuilder.java 
>>>>
>>>>
>>>>     * fixed CP patching support
>>>>
>>>>
>>>>   hotspot/src/share/vm/memory/metachunk.hpp
>>>>   hotspot/src/share/vm/oops/method.hpp
>>>>     Fix optimized JVM build.
>>>>
>>>> Best regards,
>>>> Vladimir Ivanov
>>>>
>>>> [1] 
>>>> http://cr.openjdk.java.net/~vlivanov/valhalla/vectors.mvt/sumArrayL2.log 
>>>>
>>>>
>>>> ;; B7: # B8 <- B9  top-of-loop
>>>>
>>>> mov    %r9,0x10(%rsp)
>>>> mov    0x8(%rsp),%r11d
>>>>
>>>> ;; B8: # B16 B9 <- B6 B7   Loop: B8-B7 inner
>>>>
>>>> mov    %r11d,0x8(%rsp)
>>>> movabs $0x7c0060420,%rsi  ; {metadata('valhalla/vector/Long2$Value')}
>>>> ...
>>>> callq  0x0000000117579820 ;   {runtime_call _new_instance_Java}
>>>>
>>>> ;; B9: # B7 B10 <- B8
>>>>
>>>> mov    %rax,%r9
>>>> mov    0x10(%rsp),%r11
>>>> mov    0x10(%r11),%r10
>>>> mov    0x18(%r11),%r11
>>>> movslq %ebp,%r8
>>>> shl    $0x4,%r8
>>>> mov    (%rsp),%rcx
>>>> mov    0x18(%rcx,%r8,1),%rax
>>>> mov    0x10(%rcx,%r8,1),%r8  ;*vaload
>>>> add    %r11,%rax          ;*ladd
>>>> mov    %rax,0x18(%r9)
>>>> add    %r10,%r8           ;*ladd
>>>> mov    %r8,0x10(%r9)
>>>> inc    %ebp               ;*iadd
>>>> cmp    0x8(%rsp),%ebp
>>>> jl     <B7>               ;*if_icmpge
>>>>
>>>> [2] Pruned EA output (full output in the log)
>>>> ======== Connection graph for valhalla.vector.VectorTest::sumArrayL2
>>>> JavaObject NoEscape(NoEscape) NSR [ 1267F 1265F 756F 754F 1581F 
>>>> 1579F [ 1259 1264 532 1525 ]]   1247    Allocate
>>>> LocalVar [ 1247P [ 1264 ]]   1259    Proj ... #5
>>>> LocalVar [ 1259 1247P [ 1267b 1265b 532 1525 ]]   1264 CheckCastPP 
>>>> ... #valuetype* valhalla/vector/Long2$Value:NotNull
>>>> LocalVar [ 324 1264 1247P 307P [ 756b 754b ]]   532    Phi ... 
>>>> #valuetype* valhalla/vector/Long2$Value:NotNull
>>>> LocalVar [ 1264 324 1247P 307P [ 1581b 1579b ]]   1525    Phi ... 
>>>> #valuetype* valhalla/vector/Long2$Value:NotNull
>>>>
>>>> JavaObject NoEscape(NoEscape) [ 1154F [ 1148 1153 ]]   1136 Allocate
>>>> LocalVar [ 1136P [ 1153 ]]   1148    Proj ... #5
>>>> LocalVar [ 1148 1136P [ 1154b ]]   1153    CheckCastPP ... 
>>>> #valuetype* valhalla/vector/Long2$Value:NotNull
>>>>
>>>> JavaObject NoEscape(NoEscape) [ [ 1053 ]]   1041    Allocate
>>>> LocalVar [ 1041P [ ]]   1053    Proj ... #5
>>>>
>>>> JavaObject NoEscape(NoEscape) [ 653F 651F [ 645 650 ]]   633 Allocate
>>>> LocalVar [ 633P [ 650 ]]   645    Proj ... #5
>>>> LocalVar [ 645 633P [ 653b 651b ]]   650    CheckCastPP ... 
>>>> #valuetype* valhalla/vector/Long2$Value:NotNull
>>>>
>>>> JavaObject NoEscape(NoEscape) NSR [ 756F 754F 1581F 1579F [ 319 324 
>>>> 532 1525 ]]   307    Allocate
>>>> LocalVar [ 307P [ 324 ]]   319    Proj ... #5
>>>> LocalVar [ 319 307P [ 532 1525 ]]   324    CheckCastPP  ... 
>>>> #valuetype* valhalla/vector/Long2$Value:NotNull
>>>> LocalVar [ 324 1264 1247P 307P [ 756b 754b ]]   532    Phi ... 
>>>> #valuetype* valhalla/vector/Long2$Value:NotNull
>>>> LocalVar [ 1264 324 1247P 307P [ 1581b 1579b ]]   1525    Phi ... 
>>>> #valuetype* valhalla/vector/Long2$Value:NotNull
>>>>
>>>> Scalar  1153    CheckCastPP ... #valuetype* 
>>>> valhalla/vector/Long2$Value:NotNull
>>>> ++++ Eliminated: 1136 Allocate
>>>> Scalar  1041    Allocate ...
>>>> ++++ Eliminated: 1041 Allocate
>>>> Scalar  650    CheckCastPP ... #valuetype* 
>>>> valhalla/vector/Long2$Value:NotNull
>>>> ++++ Eliminated: 633 Allocate
>>>
> 


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