[PATCH] D145293: [GlobalOpt] Don't replace the aliasee if it has other references.

[DianQK via Phabricator via llvm-commits]

DianQK updated this revision to Diff 503342.
DianQK added a comment.

In D145293#4176631 <https://reviews.llvm.org/D145293#4176631>, @MaskRay wrote:

> How does rustc use `llvm.compiler.used` or `llvm.uses`?

I have found at least two cases where rustc is used.

The first is the `sym` keyword used to keep the referenced symbol in the `global_asm` macro, which in this case would be marked as `llvm.compiler.used`.

  extern "Rust" {
      pub fn foo1();
      pub fn foo2();
  }
  
  fn bar1() {}
  fn bar2() {}
  
  core::arch::global_asm!(".global foo1
  foo1: jmp {}
  .global foo2
  foo2: jmp {}
  ", sym bar1, sym bar2);

The second is to used declare that the symbol is reserved.

  #[used]
  fn foo() {}

If we focus only on the first case, https://rust.godbolt.org/z/Ph419hqWv is a minimal example.
Due to the LTO pipeline of `rustc`, this optimized IR will be optimized again, which triggers the current problem, where `bar2` replaces `bar1` and removes `bar1`.

> If you can write an inline asm expression or module-level asm to demonstrate the issue, change the test to reflect the case.

Added.

> Do we have a use case with an internal alias and an aliasee which are not in `llvm.used`/`llvm.compiler.used`? If no, we should add such a test.

I think the `@fa2` in the previous `llvm/test/Transforms/GlobalOpt/alias-used.ll` use case corresponds to this use case.

It looks like a similar problem occurs in MergeFunctions, see Rust #99059 <https://github.com/rust-lang/rust/pull/99059> and D127751 <https://reviews.llvm.org/D127751>.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D145293/new/

https://reviews.llvm.org/D145293

Files:
  llvm/lib/Transforms/IPO/GlobalOpt.cpp
  llvm/test/Transforms/GlobalOpt/X86/alias-used-with-asm.ll
  llvm/test/Transforms/GlobalOpt/alias-used-address-space.ll
  llvm/test/Transforms/GlobalOpt/alias-used.ll

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