[PATCH] D116532: [LangRef] Add noreadonunwind attribute

[Nikita Popov via Phabricator via llvm-commits]

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This introduces a `noreadonunwind` attribute, which is intended to allow additional optimizations in DSE and MemCpyOpt. The simplest example is:

  declare void @may_unwind()
  define void @test(i32* noalias noreadonunwind sret(i32) %out) {
      store i32 0, i32* %out
      call void @may_unwind()
      store i32 1, i32* %out
      ret void
  }

Without the `noreadonunwind` attribute, we can not eliminate the first store, because the call might unwind, and the caller may observe the state after the first store.

The attribute is currently defined in a way that does not make a read after unwind immediate UB and just makes it return poison instead. I'm open to making it immediate UB though, I don't think there's a lot of practical difference either way.

Frontends will likely want to annotate `sret` parameters with `noreadonunwind`.

See https://lists.llvm.org/pipermail/llvm-dev/2021-December/154164.html for preliminary discussion on this.


https://reviews.llvm.org/D116532

Files:
  llvm/docs/LangRef.rst


Index: llvm/docs/LangRef.rst
===================================================================
--- llvm/docs/LangRef.rst
+++ llvm/docs/LangRef.rst
@@ -1267,6 +1267,15 @@
     This indicates that callee does not free the pointer argument. This is not
     a valid attribute for return values.
 
+``noreadonunwind``
+    This indicates that the underlying object of a pointer argument is not read
+    on the unwind path in the following sense: If a call with ``noreadonunwind``
+    argument unwinds, then the underlying object associated with the pointer
+    argument is overwritten with a :ref:`poison value <poisonvalue>`. As such,
+    reads from this object do not result in immediate undefined behavior, but
+    program behavior cannot depend on the object containing any specific value.
+    This attribute cannot be applied to return values.
+
 .. _nest:
 
 ``nest``


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