[Bug c++/95226] New: Faulty aggregate initialization of vector with struct with float

[fboranek at atlas dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95226

Bug ID: 95226
   Summary: Faulty aggregate initialization of vector with struct
with float
   Product: gcc
   Version: 8.3.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: fboranek at atlas dot cz
  Target Milestone: ---

# Steps to reproduce

//prog.cc

#include 
#include 

struct T {
uint32_t a;
float b {.8};
};

int main()
{
T t = {1};
std::vector tt = {{1}, {2}};

std::cout << t.a << ", " << t.b << std::endl;
std::cout << tt[0].a << ", " << tt[0].b << std::endl;
std::cout << tt[1].a << ", " << tt[1].b << std::endl;
}

// g++ prog.cc -Wall -Wextra -std=c++17

For gcc 9.3.0, 9.2.0, 9.1.0, 8.1.0 the behavior is correct and output of the
program is:

1, 0.8
1, 0.8
1, 0.8

For gcc 8.3.0, 8.2.0 the output of the program is:

1, 0.8
1, -1.58819e-23
1072273817, 2.8026e-45

The second object in the vector is wrongly initialized.

The code can be fixed by changing of initialization of float T::b to {.8f}, {0}
or {}. It seems that only initialization by double is affected.
I used wandbox.org to test diffrent version of gcc and prevous and newer
version are OK. It sound like regression in 8.3.0 and 8.2.0, but 8.3.0 is in a
current stable Debian, which worried me.


# Used gcc from Debina Buster (current stable)
gcc (Debian 8.3.0-6) 8.3.0

[Bug rtl-optimization/80004] New: non-atomic load moved to before atomic load with std::memory_order_acquire

[fboranek at atlas dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80004

Bug ID: 80004
   Summary: non-atomic load moved to before atomic load with
std::memory_order_acquire
   Product: gcc
   Version: 6.3.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: rtl-optimization
  Assignee: unassigned at gcc dot gnu.org
  Reporter: fboranek at atlas dot cz
  Target Milestone: ---

Folowing code
(https://gist.github.com/fboranek/5c99f96a99f902f5cf9dc5d597ff8fe3) for 4
thread doesn't compute the correct result if it is compiled by gcc (Debian
6.3.0-6) 6.3.0 20170205. The code is corect compiled by gcc 4.9.2 and 5.4.1. It
is also correct compiled if the variable counter is not static bat global.

This issue can be related to bug 78778, but bug 78778 is appear in 5.1.0 while
this issue is since version 6.


static int counter {0};
static std::atomic flag {false};

void increment1(int cycles)
{
for (int i=0; i < cycles; ++i)
{
while (flag.exchange(true, std::memory_order_acquire));

++counter;

flag.store(false, std::memory_order_release);
}
}

// asm for gcc 5.4.1
004010f0 <_Z10increment1i>:
  4010f0:   31 c9   xor%ecx,%ecx
  4010f2:   85 ff   test   %edi,%edi
  4010f4:   ba 01 00 00 00  mov$0x1,%edx
  4010f9:   7e 26   jle401121 <_Z10increment1i+0x31>
  4010fb:   0f 1f 44 00 00  nopl   0x0(%rax,%rax,1)
  401100:   89 d0   mov%edx,%eax
  401102:   86 05 ec 20 20 00   xchg   %al,0x2020ec(%rip)#
6031f4 <_ZL4flag>
  401108:   84 c0   test   %al,%al
  40110a:   75 f4   jne401100 <_Z10increment1i+0x10>
  40110c:   83 c1 01add$0x1,%ecx
  40110f:   83 05 e2 20 20 00 01addl   $0x1,0x2020e2(%rip)#
6031f8 <_ZL7counter>
  401116:   39 cf   cmp%ecx,%edi
  401118:   c6 05 d5 20 20 00 00movb   $0x0,0x2020d5(%rip)#
6031f4 <_ZL4flag>
  40111f:   75 df   jne401100 <_Z10increment1i+0x10>
  401121:   f3 c3   repz retq 
  401123:   0f 1f 00nopl   (%rax)
  401126:   66 2e 0f 1f 84 00 00nopw   %cs:0x0(%rax,%rax,1)
  40112d:   00 00 00

// asm for gcc 6.3.0
1420 <_Z10increment1i>:
1420:   8b 0d d2 1d 20 00   mov0x201dd2(%rip),%ecx#
2031f8 <_ZL7counter>
1426:   85 ff   test   %edi,%edi
1428:   ba 01 00 00 00  mov$0x1,%edx
142d:   8d 34 0flea(%rdi,%rcx,1),%esi
1430:   7e 26   jle1458 <_Z10increment1i+0x38>
1432:   66 0f 1f 44 00 00   nopw   0x0(%rax,%rax,1)
1438:   89 d0   mov%edx,%eax
143a:   86 05 b4 1d 20 00   xchg   %al,0x201db4(%rip)#
2031f4 <_ZL4flag>
1440:   84 c0   test   %al,%al
1442:   75 f4   jne1438 <_Z10increment1i+0x18>
1444:   83 c1 01add$0x1,%ecx
1447:   39 ce   cmp%ecx,%esi
1449:   89 0d a9 1d 20 00   mov%ecx,0x201da9(%rip)#
2031f8 <_ZL7counter>
144f:   c6 05 9e 1d 20 00 00movb   $0x0,0x201d9e(%rip)#
2031f4 <_ZL4flag>
1456:   75 e0   jne1438 <_Z10increment1i+0x18>
1458:   f3 c3   repz retq 
145a:   66 0f 1f 44 00 00   nopw   0x0(%rax,%rax,1)