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Bastian_B_
New Contributor I
78 Views

Intel Compiler 2019 Update 1 bug: Too aggressive optimization at -O2

Dear all,

the following code:

#include <iostream>

#include <unistd.h>
#include <pthread.h>

class Test {
public:
  Test() : fDone(false) { }
  ~Test() {}

  bool IsDone() const { return fDone; }
  void SetDone() { fDone = true; }

private:
  bool fDone;

};

void* run(void* data) {

  // input data
  Test* t = static_cast<Test*>(data);

  // sleep for 1 second
  usleep(1000*1000);

  // thread is done
  t->SetDone();
  std::cout << t->IsDone() << std::endl;
  std::cout << "Thread finished" << std::endl;

  return 0;
}

int main(int argc, char** argv) {

  Test t;
  pthread_t thread;
  pthread_create(&thread, NULL, &run, &t);

  while (!t.IsDone()) {
    usleep(100*1000);
  }
  std::cout << "never reached" << std::endl;

  void* status;
  pthread_join(thread, &status);

  return 0;
}

Does not run as expected with Intel Compiler 2019, Update 1 on Linux x86_64. The loop in the main function never finishes, even though the auxiliary thread sets 't.IsDone()' to true after 1 second.

The code is compiled using:

icpc -O2 -g -fPIC -pthread -o bug bug.cc
 

The bug does not occur if compiled with -00 or -O1. It also doesn't occur when the loop is modified as follows:

while (!t.IsDone()) { 
  std::cout << "..." << std::endl;
  usleep(100*1000); 
}

The loop in question is actually translated by the compiler into this piece of assembly:

Dump of assembler code for function main:
   0x0000000000401280 <+0>:     push   %rbp
   0x0000000000401281 <+1>:     mov    %rsp,%rbp
   0x0000000000401284 <+4>:     and    $0xffffffffffffff80,%rsp
   0x0000000000401288 <+8>:     sub    $0x80,%rsp
   0x000000000040128f <+15>:    xor    %esi,%esi
   0x0000000000401291 <+17>:    mov    $0x3,%edi
   0x0000000000401296 <+22>:    callq  0x4013d0 <__intel_new_feature_proc_init>
   0x000000000040129b <+27>:    stmxcsr 0x8(%rsp)
   0x00000000004012a0 <+32>:    xor    %esi,%esi
   0x00000000004012a2 <+34>:    lea    (%rsp),%rdi
   0x00000000004012a6 <+38>:    orl    $0x8040,0x8(%rdi)
   0x00000000004012ad <+45>:    lea    0x10(%rsp),%rcx
   0x00000000004012b2 <+50>:    lea    0x57(%rip),%rdx        # 0x401310 <run(void*)>
   0x00000000004012b9 <+57>:    ldmxcsr -0x8(%rcx)
   0x00000000004012bd <+61>:    movb   $0x0,(%rcx)
   0x00000000004012c0 <+64>:    callq  0x4010a0 <pthread_create@plt>
   0x00000000004012c5 <+69>:    cmpb   $0x0,0x10(%rsp)
   0x00000000004012ca <+74>:    jne    0x4012d8 <main+88>
   0x00000000004012cc <+76>:    mov    $0x186a0,%edi
   0x00000000004012d1 <+81>:    callq  0x401130 <usleep@plt>
=> 0x00000000004012d6 <+86>:    jmp    0x4012cc <main+76>
   0x00000000004012d8 <+88>:    lea    0x1d25(%rip),%rsi        # 0x403004
   0x00000000004012df <+95>:    mov    0x4cd2(%rip),%rdi        # 0x405fb8
   0x00000000004012e6 <+102>:   callq  0x4010c0 <_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@plt>
   0x00000000004012eb <+107>:   mov    %rax,%rdi
   0x00000000004012ee <+110>:   mov    0x4cbb(%rip),%rsi        # 0x405fb0
   0x00000000004012f5 <+117>:   callq  0x4010d0 <_ZNSolsEPFRSoS_E@plt>
   0x00000000004012fa <+122>:   mov    (%rsp),%rdi
   0x00000000004012fe <+126>:   lea    0x8(%rsp),%rsi
   0x0000000000401303 <+131>:   callq  0x401070 <pthread_join@plt>
   0x0000000000401308 <+136>:   xor    %eax,%eax
   0x000000000040130a <+138>:   mov    %rbp,%rsp
   0x000000000040130d <+141>:   pop    %rbp
   0x000000000040130e <+142>:   retq   
   0x000000000040130f <+143>:   nop

In particular, note the infinite loop marked in bold, which never checks the status of IsDone() at all and just runs usleep forever.

This bug was not present in older Intel Compiler releases, I think 2019 is the first release that introduced this issue, but I am not 100% sure.

0 Kudos
3 Replies
Viet_H_Intel
Moderator
78 Views

Thank for report this bug. I have submitted to our developer. Bug number is CMPLRIL0-30961

jimdempseyatthecove
Black Belt
78 Views

I do not think this is a bug. It is a side effect of an optimization (loop invariant code lifting).

In class Test use

  atomic<bool> fDone

or, if you prefer:

  volatile bool fDone;

Jim Dempsey

Bastian_B_
New Contributor I
78 Views

Jim, thanks for poiting that out - I believe you are correct.

In my actual code the thread that changes the equivalent of fDone is in another translation unit, so there is no way for the compiler to know that another thread can potentially change it. I was unaware of this subtlety, thanks again.