Compile primitiveAtPut - Problem with Null s

[PalumboN]

• I was going to compile this primitive, fixing many bugs in the meta-compiler and bah.... BUT I arrived at a problem that is better to see in another issue.

• Now the primitive "works" (store the expected value in the expected object) but it is returning a trash value:

• The problem is a phi function created during tail duplication:

  • since we duplicate a block, then new merge points are created, and the phi should be propagated. See DRBasicBlock >> mergeInstructionsWithDuplicatedBlock: duplicatedBlock withSuccessors: successors usingUsers: getUsersBlock
  • this algorithm creates phis of phis based on the dominator tree (ok), and it leaves Null values if any value dominates the phi (bad).
  • in this case, the problem is on splitting the exitBlock, which has many predecessors.
  • this is a "real" problem because, in the end, a copy to Null is saved as result:
    
  • but the CodeGenerator is ignoring the copies to Null:
    

    druid/Druid/DRCogitCodeGenerator.class.st

    Lines 851 to 855 in 6f8878a

    	DRCogitCodeGenerator >> visitCopy: aDRMove [
    	| moveOperands |
    	"Do not generate moves of null values"
    	aDRMove operand1 isNullValue ifTrue: [ ^ self ].

  • So at the end, the Cogit code is copying trash to the ReceiverResultReg 👎

[PalumboN]

Some work around the commented bug was done in https://github.com/Alamvic/druid/pull/132/files

But it needs more work, this primitive was not fixed.

[PalumboN]

Also check if we can rollback this dangerous change: #132 (comment)