[all-commits] [llvm/llvm-project] 14cc7a: [Clang] Allow __declspec(noalias) to access inacce...

Nikita Popov via All-commits all-commits at lists.llvm.org
Tue Aug 29 02:44:15 PDT 2023

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 14cc7a07727555102c52e711f476ec7671317d6a
  Author: Nikita Popov <npopov at redhat.com>
  Date:   2023-08-29 (Tue, 29 Aug 2023)

  Changed paths:
    M clang/lib/CodeGen/CGCall.cpp
    M clang/test/CodeGen/ms-declspecs.c

  Log Message:
  [Clang] Allow __declspec(noalias) to access inaccessible memory

MSVC defines __declspec(noalias) as follows (https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2012/k649tyc7(v=vs.110)?redirectedfrom=MSDN):

> noalias means that a function call does not modify or reference
> visible global state and only modifies the memory pointed to
> directly by pointer parameters (first-level indirections).

> If a function is annotated as noalias, the optimizer can assume
> that, in addition to the parameters themselves, only first-level
> indirections of pointer parameters are referenced or modified
> inside the function. The visible global state is the set of all
> data that is not defined or referenced outside of the compilation
> scope, and their address is not taken. The compilation scope is
> all source files (/LTCG (Link-time Code Generation) builds) or a
> single source file (non-/LTCG build).

The wording is not super clear to me, but I believe this is saying
that __declspec(noalias) functions may access inaccessible memory
(i.e. non-visible global state in their words). Indeed, the Windows
CRT applies this attribute to malloc, which does access inaccessible
memory under LLVM's memory model.

As such, change the attribute to emit
memory(argmem: readwrite, inaccessiblemem: readwrite) instead of
memory(argmem: readwrite).

Fixes https://github.com/llvm/llvm-project/issues/64827.

Differential Revision: https://reviews.llvm.org/D158984

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