[llvm-bugs] [Bug 52361] New: wrong code at -O2 code on MacOSX with clang 13.0.0

via llvm-bugs llvm-bugs at lists.llvm.org
Sat Oct 30 06:12:59 PDT 2021


            Bug ID: 52361
           Summary: wrong code at -O2 code on MacOSX with clang 13.0.0
           Product: clang
           Version: 13.0
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: kharris at mathworks.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Created attachment 25405
  --> https://bugs.llvm.org/attachment.cgi?id=25405&action=edit
source code and run script to demonstate this problem.

See code below,
At -O2 optimization, it appears that clang boils away the "else" path in the
"bool test(const Name& name)" routine below - based only on a slight variation
in the anatomy of the structure  (i.e. adding the "long t" field).

Initially, we discovered and reproduced this with Apple's Xcode 13. We then
obtained clang 13.0.0 directly from llvm.org and reproduced identical results.

The attached reproduction contains the source and script for three test

The first demonstrates expected behavior (no optimization).
The second demonstrates expected behavior with -O2 optimization.
The third demonstrates the unexpected behavior triggered by the addition of
"long t;" to the structure.

Thank you very much for looking into this!


#include <stdio.h>
#include <stdint.h>

struct Name
        char j[32];
        long t;

bool test(const Name& name)
    if (0 == ((uint64_t) &name & 2)) {
        return 0;
    } else {
        return 1;

int main()
        Name myname;
        bool r = test(myname);
        printf("NORMAL CASE :%s\n", r ? "!!!WRONG!!!" : "CORRECT");

        const Name *px = (const Name *) 0x1e2;
        r = test(*px);
        printf("SPECIAL CASE:%s\n", r ? "CORRECT" : "!!!WRONG!!!");

        return 0;

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