RFR: 8020282: Generated code quality: redundant LEAs in the chained dereferences [v4]

[Manuel Hässig]

On Thu, 12 Jun 2025 14:46:25 GMT, Manuel Hässig <mhaessig at openjdk.org> wrote:

>> ## Summary
>> 
>> On x86, chained dereferences of narrow oops at a constant offset from the base oop can use a `lea` instruction to perform the address computation in one go using the `leaP8Narrow`, `leaP32Narrow`, and `leaPCompressedOopOffset` matching rules. However, the generated code contains an additional `lea` with an unused result:
>> 
>> ; OptoAssembly
>> 03d     decode_heap_oop_not_null R8,R10
>> 041     leaq    R10, [R12 + R10 << 3 + #12] (compressed oop addressing) ; ptr compressedoopoff32
>> 
>> ; x86
>> 0x00007f1f210625bd:   lea    (%r12,%r10,8),%r8        ; result is unused
>> 0x00007f1f210625c1:   lea    0xc(%r12,%r10,8),%r10    ; the same computation as decode, but with offset
>> 
>> 
>> This PR adds a peephole optimization to remove such redundant `lea`s.
>> 
>> ## The Issue in Detail
>> 
>> The ideal subgraph producing redundant `lea`s, or rather redundant `decodeHeapOop_not_null`s, is `LoadN -> DecodeN -> AddP`, where both the address and base edge of the `AddP` originate from the `DecodeN`. After matching, this becomes
>> 
>> LoadN -> decodeHeapOop_not_null -> leaP*
>>     ______________________________Î
>> 
>> where `leaP*` is either of `leaP8Narrow`, `leaP32Narrow`, or `leaPCompressedOopOffset` (depending on the heap location and size). Here, the base input of `leaP*` comes from the decode. Looking at the matching code path, we find that the `leaP*` rules match both the `AddP` and the `DecodeN`, since x86 can fold this, but the following code adds the decode back as the base input to `leaP*`:
>> 
>> https://github.com/openjdk/jdk/blob/c29537740efb04e061732a700582d43b1956cff4/src/hotspot/share/opto/matcher.cpp#L1894-L1897
>> 
>> On its face, this is completely unnecessary if we matched a `leaP*`, since it already computes the result of the decode,  so adding the `LoadN` node as base seems like the logical choice. However, if the derived oop computed by the `leaP*` gets added to an oop map, this `DecodeN` is needed as the base for the derived oop. Because as of now, derived oops in oop maps cannot have narrow base pointers.
>> 
>> This leaves us with a handful of possible solutions:
>>  1. implement narrow bases for derived oops in oop maps,
>>  2. perform some dead code elimination after we know which oops are part of oop maps,
>>  3. add a peephole optimization to simply remove unused `lea`s.
>> 
>> Option 1 would have been ideal in the sense, that it is the earliest possible point to remove the decode, which would simplify the graph and reduce pressure on the regi...
>
> Manuel Hässig has updated the pull request incrementally with four additional commits since the last revision:
> 
>  - Factor out address nodes for simplification
>  - Add assert to codepath only reachable with stressing.
>  - Rename for clarity
>    
>    Confused myself....
>  - Revert change to unrelated lines
>    
>    This reverts commit d1c6a653770bfe578b1982ac726b258fa08d57b8.

> Is this scenario exercised by any of the new tests? If not, would it be possible to construct an additional test where we verify that the peephole is not applied in this case?

It is not. I was only able to find such a case once with all VM intrinsics disabled some time ago, but was not able to reproduce one since. I'll have another try to find one.

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PR Comment: https://git.openjdk.org/jdk/pull/25471#issuecomment-2967229328