[PATCH] D145854: [GlobalDCE] Don't add dependency via icmp against inbouds pointer

[Ayke via Phabricator via llvm-commits]

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This patch optimizes the following case:

  static int x;
  bool foo(int *n) {
      return &x == n;
  }

To remove the static global that isn't otherwise used:

  bool foo(int *n) {
      return false;
  }

This is not a valid optimization in C. However, there are cases where
this can be valid in other language if the pointer parameter n is known
to point into a valid object (either via the dereferenceable_or_null
attribute or an inbouds gep). The logic here is the same as for
https://reviews.llvm.org/D60047.

The motivator for this is runtime type information in TinyGo. There are
two ways type information globals are referenced: either by storing it
in an interface value, or by comparing the interface type against the
type information global. It is this second use (the comparison) that
leads to a significant code size increase of double digit percents in
some cases.

We currently work around this using a hack, but I'd like this to be
supported natively in LLVM so we can simplify the compiler and make use
of a normal LTO pipeline instead of our homegrown LTO variant.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D145854

Files:
  llvm/include/llvm/Analysis/TypeMetadataUtils.h
  llvm/lib/Analysis/TypeMetadataUtils.cpp
  llvm/lib/Transforms/IPO/GlobalDCE.cpp
  llvm/test/Transforms/GlobalDCE/icmp.ll

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