[PATCH] D18586: Allow DeadStoreElimination to track combinations of partial later wrties

[Jake VanAdrighem via llvm-commits]

JakeVanAdrighem added a subscriber: JakeVanAdrighem.
JakeVanAdrighem added a comment.

This is great! This actually fixes a problem case I ran into some months ago in the wild, where a `memset` wasn't being wiped away despite being fully overwritten by the stores following it. Example shown below:

  %struct.foostruct = type {
  i32 (i8*, i8**, i32, i8, i8*)*,
  i32 (i8*, i8**, i32, i8, i8*)*,
  i32 (i8*, i8**, i32, i8, i8*)*,
  i32 (i8*, i8**, i32, i8, i8*)*,
  void (i8*, i32, i32)*
  }
  declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i32, i1)
  declare void @goFunc(%struct.foostruct*)
  
  define void @func()  {
    %bang = alloca %struct.foostruct, align 8
    %1 = bitcast %struct.foostruct* %bang to i8*
    call void @llvm.memset.p0i8.i64(i8* %1, i8 0, i64 40, i32 8, i1 false)  <------    This is now removed
    %2 = getelementptr inbounds %struct.foostruct, %struct.foostruct* %bang, i64 0, i32 0
    store i32 (i8*, i8**, i32, i8, i8*)* @fa, i32 (i8*, i8**, i32, i8, i8*)** %2, align 8
    %3 = getelementptr inbounds %struct.foostruct, %struct.foostruct* %bang, i64 0, i32 1
    store i32 (i8*, i8**, i32, i8, i8*)* @fa, i32 (i8*, i8**, i32, i8, i8*)** %3, align 8
    %4 = getelementptr inbounds %struct.foostruct, %struct.foostruct* %bang, i64 0, i32 2
    store i32 (i8*, i8**, i32, i8, i8*)* @fa, i32 (i8*, i8**, i32, i8, i8*)** %4, align 8
    %5 = getelementptr inbounds %struct.foostruct, %struct.foostruct* %bang, i64 0, i32 3
    store i32 (i8*, i8**, i32, i8, i8*)* @fa, i32 (i8*, i8**, i32, i8, i8*)** %5, align 8
    %6 = getelementptr inbounds %struct.foostruct, %struct.foostruct* %bang, i64 0, i32 4
    store void (i8*, i32, i32)* null, void (i8*, i32, i32)** %6, align 8
    call void @goFunc(%struct.foostruct* %bang)
    ret void
  }


http://reviews.llvm.org/D18586