[llvm] Add 'initialized' attribute langref (PR #84803)

[Haopeng Liu via llvm-commits]

https://github.com/haopliu created https://github.com/llvm/llvm-project/pull/84803

We propose adding a new LLVM attribute, `initialized((Lo1,Hi1),(Lo2,Hi2),...)`, which expresses the notion of memory space (i.e., intervals, in bytes) that the argument pointing to is initialized in the function.

Will commit the attribute inferring in the follow-up PRs.

https://discourse.llvm.org/t/rfc-llvm-new-initialized-parameter-attribute-for-improved-interprocedural-dse/77337

>From 2554c821dfa3893213f934253ecbca5d9d43ceab Mon Sep 17 00:00:00 2001
From: Haopeng Liu <haopliu at google.com>
Date: Mon, 11 Mar 2024 17:47:08 +0000
Subject: [PATCH] Add 'initialized' attribute langref

---
 llvm/docs/LangRef.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index b70220dec92615..39a555edfa80d6 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1621,6 +1621,28 @@ Currently, only the following parameter attributes are defined:
     ``readonly`` or a ``memory`` attribute that does not contain
     ``argmem: write``.
 
+``initialized((Lo1,Hi1),...)``
+    This attribute is a list of const ranges in ascending order with no
+    overlapping or continuous. It indicates that the function initializes the
+    memory through the pointer argument, [%p+LoN, %p+HiN): there are no reads,
+    and no special accesses (such as volatile access or untrackable capture)
+    before the initialization in the function. LoN/HiN are 64-bit ints;
+    negative values are allowed in case a pointer to partway through the
+    allocation is passed to.
+
+    This attribute implies that the function initializes and does not read
+    before initialization through this pointer argument, even though it may
+    read the memory before initialization that the pointer points to, such
+    as through other arguments.
+
+    The ``writable`` or ``dereferenceable`` attribute does not imply
+    ``initialized`` attribute, and ``initialized`` does not imply ``writeonly``
+    since cases that read from the pointer after write, can be ``initialized``
+    but not ``writeonly``.
+
+    Note that this attribute does not apply to the unwind edge: the memory may
+    not actually be written to when unwinding happens.
+
 ``dead_on_unwind``
     At a high level, this attribute indicates that the pointer argument is dead
     if the call unwinds, in the sense that the caller will not depend on the