[PATCH] D64866: [PredicateInfo] Replace pointer comparisons with deterministic compares.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: davide, efriedma.
Herald added subscribers: dmgreen, mgrang, hiraditya.
Herald added a project: LLVM.

Currently there are a few pointer comparisons in ValueDFS_Compare, which
can cause non-deterministic ordering when materializing values. There
are 2 cases this patch fixes:

1. Order defs before uses used to compare pointers, which guarantees defs before uses, but causes non-deterministic ordering between 2 uses or 2 defs, depending on the allocation order. By converting the pointers to booleans, we can circumvent that problem.

2. comparePHIRelated was comparing the basic block pointers of edges, which also results in a non-deterministic order and is also not really meaningful for ordering. By ordering by their destination DFS numbers we guarantee a deterministic order.

For the example below, we can end up with 2 different uselist orderings,
when running `opt -mem2reg -ipsccp` hundreds of times. Because the
non-determinism is caused by allocation ordering, we cannot reproduce it
with ipsccp alone.

  declare i32 @hoge() local_unnamed_addr #0
  
  define dso_local i32 @ham(i8* %arg, i8* %arg1) #0 {
  bb:
    %tmp = alloca i32
    %tmp2 = alloca i32, align 4
    br label %bb19
  
  bb4:                                              ; preds = %bb20
    br label %bb6
  
  bb6:                                              ; preds = %bb4
    %tmp7 = call i32 @hoge()
    store i32 %tmp7, i32* %tmp
    %tmp8 = load i32, i32* %tmp
    %tmp9 = icmp eq i32 %tmp8, 912730082
    %tmp10 = load i32, i32* %tmp
    br i1 %tmp9, label %bb11, label %bb16
  
  bb11:                                             ; preds = %bb6
    unreachable
  
  bb13:                                             ; preds = %bb20
    br label %bb14
  
  bb14:                                             ; preds = %bb13
    %tmp15 = load i32, i32* %tmp
    br label %bb16
  
  bb16:                                             ; preds = %bb14, %bb6
    %tmp17 = phi i32 [ %tmp10, %bb6 ], [ 0, %bb14 ]
    br label %bb19
  
  bb18:                                             ; preds = %bb20
    unreachable
  
  bb19:                                             ; preds = %bb16, %bb
    br label %bb20
  
  bb20:                                             ; preds = %bb19
    indirectbr i8* null, [label %bb4, label %bb13, label %bb18]
  }


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D64866

Files:
  llvm/lib/Transforms/Utils/PredicateInfo.cpp
  llvm/test/Transforms/SCCP/ipsccp-predinfo-uselists.ll

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