[Bug c/114923] New: gcc ignores escaping pointer and applies invalid optimization

["nfxjfg at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114923

            Bug ID: 114923
           Summary: gcc ignores escaping pointer and applies invalid
                    optimization
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nfxjfg at googlemail dot com
  Target Milestone: ---

Created attachment 58088
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58088&action=edit
Test case

In this test case, the pointer to a stack-allocated variable is passed to a
volatile variable. That means the pointer definitely escapes. But the compiler
seems to assume that the memory can't change, and returns the initialization
value of the buf variable.

I tried multiple variations of this. Sometimes it generates the correct code,
even though the situation is exactly the same. A call to an external function
often fixes it, but sometimes not (could not produce a reduced test case of the
latter).

This is reduced from some real-word code. The first register access passes the
memory pointer to the hardware, the second access starts the DMA. In the
real-world code there was some more stuff such as waiting for the DMA to
finish, but it doesn't matter for code generation. The code started to fail
after a DMA buffer was moved from a global variable to the stack.

Originally experienced on 12.2.0 / riscv32-unknown-elf, but on godlbolt I can
reproduce it with trunk and some other architectures.