[LLVMbugs] [Bug 23599] New: Clang miscompiles assignments to packed struct members

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23599

            Bug ID: 23599
           Summary: Clang miscompiles assignments to packed struct members
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: vonosmas at gmail.com
                CC: llvmbugs at cs.uiuc.edu, nlewycky at google.com
    Classification: Unclassified

$ cat tmp/a.cc
#include <stdio.h>
#include <stdint.h>

#define ETH_ALEN 6

struct ether_addr
{
  uint8_t ether_addr_octet[ETH_ALEN];
} __attribute__ ((__packed__));

struct ether_header
{
  uint8_t ether_dhost[ETH_ALEN];
  uint8_t ether_shost[ETH_ALEN];
} __attribute__ ((__packed__));

void Print(const ether_header &eth) {
  for (int i = 0; i < 6; i++)
    printf("%d ", eth.ether_dhost[i]);
  printf("\n");
  for (int i = 0; i < 6; i++)
    printf("%d ", eth.ether_shost[i]);
  printf("\n");
}

void FooBarBaz(const ether_addr& ether_src, const ether_addr& ether_dst) {
  ether_header eth{};
  *((ether_addr*)eth.ether_shost) = ether_src;
  *((ether_addr*)eth.ether_dhost) = ether_dst;
  Print(eth);
}

int main() {
  FooBarBaz({{1,2,3,4,5,6}}, {{7,8,9,10,11,12}});
  return 0;
}

$ ./bin/clang++ -O2 tmp/a.cc -std=c++11 && ./a.out
7 8 9 10 11 12 
0 0 0 0 0 0

$ ./bin/clang++ -O0 tmp/a.cc -std=c++11 && ./a.out
7 8 9 10 11 12 
1 2 3 4 5 6

I haven't yet figured out which optimization pass is responsible for that. Note
that if I swap the assignments of ether_src and ether_dst, the program works as
expected.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20150520/9ab83d45/attachment.html>