[Bug tree-optimization/86839] New: O3 create wrong code (segmentation fault) on extern non-aligned buffer

[galileaman.bugzilla at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86839

Bug ID: 86839
   Summary: O3 create wrong code (segmentation fault) on extern
non-aligned buffer
   Product: gcc
   Version: 5.4.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: tree-optimization
  Assignee: unassigned at gcc dot gnu.org
  Reporter: galileaman.bugzilla at gmail dot com
  Target Milestone: ---

gcc-5.4 miscompiles the following code on x86_64-linux-gnu at -O3 in 64-bit
mode. The resulting binary cause segmentation fault.

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
5.4.0-6ubuntu1~16.04.10' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.10)

$ g++ -Wall -c simple_struct.cpp
$ g++ -Wall -O2 simple.cpp simple_struct.o
$ ./a.out 
After alignInt, address 0x7fffc21fb274
After nonAlignInt, address 0x7fffc21fb211
After alignExt, address 0x6011a4
After nonAlignExt, address 0x601201
$ g++ -Wall -O3 simple.cpp simple_struct.o
$ ./a.out 
After alignInt, address 0x7ffce7dd6fa4
After nonAlignInt, address 0x7ffce7dd6f41
After alignExt, address 0x6021a4
Segmentation fault (core dumped)



simple_struct.h:
#ifndef _TEST_H
#define _TEST_H

#pragma pack(push)
#pragma pack(1)

#define BUF_LEN (20)
struct BufNonAligned {
  char buf[1];
  float vals[BUF_LEN];
};
#pragma pack(pop)

#pragma pack(push)
#pragma pack(1)
struct BufAligned {
  char buf[4];
  float vals[BUF_LEN];
};
#pragma pack(pop)

#endif




simple_struct.cpp:
#include "simple_struct.h"

struct BufAlignedalignExt;
struct BufNonAligned nonAlignExt;


---
simple.cpp:

#include 
#include "simple_struct.h"

extern BufAlignedalignExt;
extern BufNonAligned nonAlignExt;

int main(void) {
  struct BufAlignedalignInt;
  struct BufNonAligned nonAlignInt;
  float val = 0.5;


  for (float *p = alignInt.vals; p != alignInt.vals + BUF_LEN ; p++) {
*p = val;
  }
  std::cout << "\tAfter alignInt, address " << alignInt.vals << std::endl;

  for (float *p = nonAlignInt.vals; p != nonAlignInt.vals + BUF_LEN ; p++) {
*p = val;
  }
  std::cout << "\tAfter nonAlignInt, address " << nonAlignInt.vals <

[Bug tree-optimization/86839] O3 create wrong code (segmentation fault) on extern non-aligned buffer

[galileaman.bugzilla at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86839

--- Comment #3 from galileaman bugzilla  
---
OK, always thought that non-aligned vars only hit performance.
I see know it is a requirement in the standard.
Sorry, and thanks.