[llvm-bugs] [Bug 37716] New: Poor codegen with atomics

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37716

            Bug ID: 37716
           Summary: Poor codegen with atomics
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: redbeard0531 at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

https://godbolt.org/g/QcLkce

#include <atomic>

std::atomic<int> progress{-1};

void combine_writes1() {
    // These should be reduced to a single store(0,release),
    // At least as long as release-sequence includes same-thread
    // relaxed stores. If that is removed, this should just be
    // a single relaxed store.
    progress.store(0, std::memory_order_relaxed);
    progress.store(0, std::memory_order_relaxed);
    progress.store(0, std::memory_order_release);
    progress.store(0, std::memory_order_release);
    progress.store(0, std::memory_order_relaxed);
    progress.store(0, std::memory_order_relaxed);
}

void combine_writes2() {
    // Ditto above, but should store 5.
    progress.store(0, std::memory_order_relaxed);
    progress.store(1, std::memory_order_relaxed);
    progress.store(2, std::memory_order_release);
    progress.store(3, std::memory_order_release);
    progress.store(4, std::memory_order_relaxed);
    progress.store(5, std::memory_order_relaxed);
}

void combine_loads() {
    // These should be reduced to either a single acquire-load
    // or an acquire fence. 
    progress.load(std::memory_order_relaxed);
    progress.load(std::memory_order_relaxed);
    progress.load(std::memory_order_acquire);
    progress.load(std::memory_order_acquire);
    progress.load(std::memory_order_relaxed);
    progress.load(std::memory_order_relaxed);
}

combine_writes1(): # @combine_writes1()
  mov dword ptr [rip + progress], 0
  mov dword ptr [rip + progress], 0
  mov dword ptr [rip + progress], 0
  mov dword ptr [rip + progress], 0
  mov dword ptr [rip + progress], 0
  mov dword ptr [rip + progress], 0
  ret
combine_writes2(): # @combine_writes2()
  mov dword ptr [rip + progress], 0
  mov dword ptr [rip + progress], 1
  mov dword ptr [rip + progress], 2
  mov dword ptr [rip + progress], 3
  mov dword ptr [rip + progress], 4
  mov dword ptr [rip + progress], 5
  ret
combine_loads(): # @combine_loads()
  mov eax, dword ptr [rip + progress]
  mov eax, dword ptr [rip + progress]
  mov eax, dword ptr [rip + progress]
  mov eax, dword ptr [rip + progress]
  mov eax, dword ptr [rip + progress]
  mov eax, dword ptr [rip + progress]
  ret
progress:
  .long 4294967295 # 0xffffffff

Possibly related to https://bugs.llvm.org/show_bug.cgi?id=37690

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