[PATCH] D139295: [Coroutines] Don't mark the parameter attribute of resume function as noalias

[Chuanqi Xu via Phabricator via llvm-commits]

ChuanqiXu reclaimed this revision.
ChuanqiXu added a reviewer: bcl5980.
ChuanqiXu added a comment.

Although there were some misunderstandings, now this one looks like the correct fix still. See https://github.com/llvm/llvm-project/issues/59221 for the details. Long story short, for this C++ program (https://compiler-explorer.com/z/6qGcozG93), the optimized frame will be something like:

  struct test_frame {
      void (*__resume_)(), // a function pointer points to the `test.resume` function, which can be imaged as the test() function in the example.
      ....
      struct a_frame {
            ...
            void **caller; // may points to test_frame at runtime.
      };
  };

And the function a and function test looks just like:

  define i32 @a(ptr noalias %alloc_8) {
    %alloc_8_16 = getelementptr ptr, ptr %alloc_8, i64 16
    store i32 42, ptr %alloc_8_16, align 8
    %alloc_8_8 = getelementptr ptr, ptr %alloc_8, i64 8
    %alloc = load ptr, ptr %alloc_8_8, align 8
    %p = load ptr, ptr %alloc, align 8
    %r = call i32 %p(ptr %alloc)
    ret i32 %r
  }
  
  define i32 @b(ptr %p) {
  entry:
    %alloc = alloca [128 x i8], align 8
    %alloc_8 = getelementptr ptr, ptr %alloc, i64 8
    %alloc_8_8 = getelementptr ptr, ptr %alloc_8, i64 8
    store ptr %alloc, ptr %alloc_8_8, align 8
    store ptr %p, ptr %alloc, align 8
    %r = call i32 @a(ptr nonnull %alloc_8)
    ret i32 %r
  }

Here inside the function `a`,  we can access the parameter `%alloc_8` by `%alloc` and we pass `%alloc` to an unknown function. So it breaks the assumption of `noalias` parameter.

Note that although only CoroElide optimization can put a frame inside another frame directly, the following case is not valid too:

  struct test_frame {
      ....
     void **a_frame; // may points to a_frame at runtime.
  };
  
   struct a_frame {
      void **caller; // may points to test_frame at runtime.
  };

Since the C++ language allows the programmer to get the address of coroutine frames, we can't assume the above case wouldn't happen in the source codes. So we can't set the parameter as noalias no matter if CoroElide applies or not. And for other languages, it may be safe if they don't allow the programmers to get the address of coroutine frames.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D139295/new/

https://reviews.llvm.org/D139295