[Bug c/104884] New: functions miss their 'ret' instruction (and fall through) in certain cases with '-O3' under x86-84

[zamfofex at twdb dot moe]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104884

            Bug ID: 104884
           Summary: functions miss their 'ret' instruction (and fall
                    through) in certain cases with '-O3' under x86-84
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zamfofex at twdb dot moe
  Target Milestone: ---

Created attachment 52613
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52613&action=edit
code in C function is wrongly executed

It seems as if certain loops that rely on undefined behavior cause generated
functions to be allowed to fall through (i.e. they don’t include an appropriate
‘ret’ instruction as necessary) with ‘-O3’, at least under x86‐64.

Even though they do make use of undefined behavior, letting the code execution
fall through to subsequent code in the executable seems potentially really
dangerous to me, unless I’m missing something (which is not impossible).

In the attached ‘main.c’ file, the code in the function ‘bar’ should never be
executed, yet it somehow is.

If the loop from the function ‘foo’ is placed within an exported function, the
generated function body is empty, and upon calling, the execution falls through
and likely causes a segfault.