[PATCH] D98287: [DSE] Queue non-memory dead instructions for later removal.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D98287#2707162 <https://reviews.llvm.org/D98287#2707162>, @ebrevnov wrote:

> I think, more robust approach would be not to depend on existing instructions but create the required ones during phi translation as needed. Phi translation already does this to some extent. In D90095 <https://reviews.llvm.org/D90095> (which prototypes PHI translation support in DSE)  pointers are simplified by GetPointerBaseWithConstantOffset during PHI translation what effectively removes dependencies on existing GEPs and BitCasts.
>
> What is your motivating example?

I think even with `GetPointerBaseWithConstantOffset`, we could still rely on existing GEPs, e.g. if one of the indices is a non-constant value, so IIUC we could still run into problems when removing such GEPs as we go along?

An example with a crash was shared in D98288 <https://reviews.llvm.org/D98288>. IIUC this could be modified to use an argument value as index in the GEPs to also cause a crash with the  `GetPointerBaseWithConstantOffset` approach.

  %"struct.type" = type <{ i8 }>
  
  ; Function Attrs: noreturn
  define void @f() {
  entry:
    br label %while.cond
  
  while.cond:                                       ; preds = %exit, %entry
    br i1 undef, label %if.end.i, label %if.then.i
  
  if.then.i:                                        ; preds = %while.cond
    br label %exit
  
  if.end.i:                                         ; preds = %while.cond
    store i8* undef, i8** inttoptr (i64 16 to i8**), align 16
    %.cast1.i = bitcast i8* undef to %"struct.type"*
    %IsMemberPointer.i.i = getelementptr inbounds %"struct.type", %"struct.type"* %.cast1.i, i64 0, i32 0
    store i8 0, i8* %IsMemberPointer.i.i, align 4
    br label %exit
  
  exit: ; preds = %if.end.i, %if.then.i
    %retval.0.i = phi %"struct.type"* [ undef, %if.then.i ], [ %.cast1.i, %if.end.i ]
    %IsMemberPointer = getelementptr inbounds %"struct.type", %"struct.type"* %retval.0.i, i64 0, i32 0
    store i8 1, i8* %IsMemberPointer, align 4
    br label %while.cond
  }


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  https://reviews.llvm.org/D98287/new/

https://reviews.llvm.org/D98287