[llvm-bugs] [Bug 43075] New: Missing return statement causes wrong branch to be taken

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43075

            Bug ID: 43075
           Summary: Missing return statement causes wrong branch to be
                    taken
           Product: clang
           Version: 7.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: doughera at lafayette.edu
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Created attachment 22409
  --> https://bugs.llvm.org/attachment.cgi?id=22409&action=edit
Program to show odd effect of missing return statement

In the following function, 

int checkit(void)
{
    int rtn = 0;
    if (rtn != 0) {
        printf("In rtn != 0 branch with rtn = %d\n", rtn);
        printf("Not Ok.\n");
        exit(1);
    }
}

the missing return statement causes clang++ -O1 to take the "Not Ok" branch,
even though 'rtn' is correctly reported to be zero.  A complete test case is
attached.  
clang++ correctly reports the error:

    warning: control may reach end of non-void function [-Wreturn-type]

and I gather that omitting the return statement in C++ leads to undefined
behavior, but taking the rtn !=0 branch is still rather surprising. 

This occurred in a C program inside Perl's Configure that a user wishes to
compile with clang++.  We have already fixed the Configure code to work
correctly.  It seems to be similar to
https://bugs.llvm.org/show_bug.cgi?id=42897, but I thought the behavior
sufficiently odd to merit reporting.  

Steps to reproduce:
Compile the attached program with
    clang++ -O1 -o try try.c && ./try; echo $?
clang++ will correctly warn about 'control may reach end of non-void function'.

Expected output:
The expected output is "Ok", with an exit status of 0.

Actual output:
     a.  With clang++-7 (clang version 7.0.1-8 (tags/RELEASE_701/final),
distributed with current Debian stable), the output is
    In rtn != 0 branch with rtn = 0
    Not Ok.
    Exit status is 1.

    b.  With a recent git checkout, clang version 10.0.0
(https://github.com/llvm/llvm-project.git
928071ae4ef5e2e6342afb126518a79fde81cf8b) the output is
    empty (nothing is printed) and the exit status is 0.    

Build date and hardware:
    2019-08-19, x86_64-pc-linux-gnu

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