[all-commits] [llvm/llvm-project] c96623: [DataLayout] Introduce sentinel pointer value

[Shilei Tian via All-commits]

  Branch: refs/heads/users/shiltian/data-layout-sentinel-pointer
  Home:   https://github.com/llvm/llvm-project
  Commit: c966236e1ec99fbd45063e0363873c8d8abe4c50
      https://github.com/llvm/llvm-project/commit/c966236e1ec99fbd45063e0363873c8d8abe4c50
  Author: Shilei Tian <i at tianshilei.me>
  Date:   2025-03-16 (Sun, 16 Mar 2025)

  Changed paths:
    M clang/lib/Basic/Targets/AMDGPU.cpp
    M llvm/docs/LangRef.rst
    M llvm/include/llvm/IR/DataLayout.h
    M llvm/lib/IR/DataLayout.cpp
    M llvm/unittests/IR/DataLayoutTest.cpp

  Log Message:
  -----------
  [DataLayout] Introduce sentinel pointer value

The value of a null pointer is not always `0`. For example, on AMDGPU, the null
pointer in address spaces 3 and 5 is `0xffffffff`. Currently, there is no
target-independent way to get this information, making it difficult and
error-prone to handle null pointers in target-agnostic code.

We do have `ConstantPointerNull`, but it might be a little confusing and
misleading. It represents a pointer with an all-zero value rather than
necessarily a real `nullptr`.

This PR introduces the concept of a *sentinel pointer value* to `DataLayout`,
representing the actual `nullptr` value for a given address space. The changes
include:

- A new interface function:
  ```
  APInt getSentinelPointerValue(unsigned AS)
  ```
  This function returns an `APInt` representing the sentinel pointer value for
  the given address space `AS`. An `APInt` is used instead of a literal integer
  to support cases where pointers are wider than 64 bits (e.g., AMDGPU’s address
  space 8).

- An extension to the data layout string format:
  ```
  p[n]:<size>:<abi>[:<pref>[:<idx>[:<sentinel>]]]
  ```
  The new `<sentinel>` component specifies the sentinel value for the
  corresponding pointer. It currently supports two values:
    - `0` for an all-zero value
    - `f` for a full-bit set value

  These two values are the most common representations of `nullptr`. It is
  unlikely that any target would define `nullptr` as a random value.

A follow-up patch series will introduce an equivalent of `ConstantPointerNull`
that represents the actual `nullptr`, built on top of this PR.



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