[PATCH] D99240: [ConstantFolding] Look through local aliases when simplify GEPs

[Reid Kleckner via Phabricator via llvm-commits]

rnk added a comment.

To add some specificity, we wouldn't want to go around ripping out all of our ??_7 vtable aliases and replacing them with references to the underlying private global. Consider:

  @"??_7Foo@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ({ [2 x i8*] }, { [2 x i8*] }* @0, i32 0, i32 0, i32 1)
  define dso_local nonnull %struct.Foo* @"??0Foo@@QEAA at XZ"(%struct.Foo* nonnull returned align 8 dereferenceable(8) %this) unnamed_addr #0 align 2 {
  entry:
    %0 = getelementptr inbounds %struct.Foo, %struct.Foo* %this, i64 0, i32 0
    store i32 (...)** bitcast (i8** @"??_7Foo@@6B@" to i32 (...)**), i32 (...)*** %0, align 8, !tbaa !3
    ret %struct.Foo* %this
  }

This compiles to:

  "??0Foo@@QEAA at XZ":                      # @"??0Foo@@QEAA at XZ"
  # %bb.0:                                # %entry                
          movq    %rcx, %rax
          leaq    "??_7Foo@@6B@"(%rip), %rcx
          movq    %rcx, (%rax)
          retq 

If you do the replacement here:

  define dso_local nonnull %struct.Foo* @"??0Foo@@QEAA at XZ"(%struct.Foo* nonnull returned align 8 dereferenceable(8) %this) unnamed_addr #0 align 2 {
  entry:
    %0 = getelementptr inbounds %struct.Foo, %struct.Foo* %this, i64 0, i32 0
    store i32 (...)** bitcast (i8** getelementptr inbounds ({ [2 x i8*] }, { [2 x i8*] }* @0, i32 0, i32 0, i32 1) to i32 (...)**), i32 (...)*** %0, align 8, !tbaa !3
    ret %struct.Foo* %this
  }

You get this code, which is less good:

  "??0Foo@@QEAA at XZ":                      # @"??0Foo@@QEAA at XZ"
  # %bb.0:                                # %entry
          movq    %rcx, %rax
          leaq    .L__unnamed_1+8(%rip), %rcx
          movq    %rcx, (%rax)
          retq      

That creates a relocation to the section instead of the symbol.

This alias happens to be external, so it wouldn't be stripped by this change, but even if it were internal, it's nicer to refer to the internal alias than the private symbol.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D99240/new/

https://reviews.llvm.org/D99240