[llvm-bugs] [Bug 49316] New: program with #pragma pack(1) and __int128 behaves differents under O1 and higher optimization

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49316

            Bug ID: 49316
           Summary: program with #pragma pack(1) and __int128 behaves
                    differents under O1 and higher optimization
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zhan3299 at purdue.edu
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Following code has different behaviors under different optimization level. I am
not sure whether it is caused by asan or optimization, tbh.

$ cat test.c
#pragma pack(1)

struct {
  char a;
  __int128 b;
} c;

__int128 *d = &c.b;

int main() { *d = 0; }



In short, when compiling with '-O0', the exit code is 0. But when compiling
with '-O1', the exit code is 139 (segment fault).

O0 (program returns 0): https://godbolt.org/z/x3d68s
O1 (program returns 139): https://godbolt.org/z/bef6T9
O2 (program returns 139): https://godbolt.org/z/brvz77
O3 (program returns 139): https://godbolt.org/z/K8sEaa
Os (program returns 129): https://godbolt.org/z/PxToY5

I further use asan to report more details:

O0 (normal): https://godbolt.org/z/5G9bbv
O1 (segfault): https://godbolt.org/z/3ob34b
AddressSanitizer:DEADLYSIGNAL
=================================================================
==1==ERROR: AddressSanitizer: SEGV on unknown address (pc 0x0000004f3ddc bp
0x000000000000 sp 0x7ffead06c7b0 T0)
==1==The signal is caused by a READ memory access.
==1==Hint: this fault was caused by a dereference of a high value address (see
register values below).  Disassemble the provided pc to learn which register
was used.
    #0 0x4f3ddc  (/app/output.s+0x4f3ddc)
    #1 0x7f4b493950b2  (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #2 0x41adad  (/app/output.s+0x41adad)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/app/output.s+0x4f3ddc) 
==1==ABORTING

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