[Bug tree-optimization/85611] New: Suboptimal code generation for (potentially) redundant atomic loads

[hiraditya at msn dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85611

            Bug ID: 85611
           Summary: Suboptimal code generation for (potentially) redundant
                    atomic loads
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hiraditya at msn dot com
  Target Milestone: ---

$ cat test.cpp
#include <atomic>

std::atomic<int> atomic_var{100};
int somevar;
bool cond;

void run1() {
    auto a = atomic_var.load(std::memory_order_relaxed);
    auto b = atomic_var.load(std::memory_order_relaxed);
   // Some code using a and b;
}

void run2() {
    if (atomic_var.load(std::memory_order_relaxed) == 2 && cond) {
         if (atomic_var.load(std::memory_order_relaxed) * somevar > 3) {
               /*...*/
         }
    }
}


$ g++-7 -O3 -std=c++17 -S -o - test.cpp -fno-exceptions

        .text
        .align 4,0x90
        .globl __Z4run1v
__Z4run1v:
LFB339:
        movl    _atomic_var(%rip), %eax
        movl    _atomic_var(%rip), %eax
        ret
LFE339:
        .align 4,0x90
        .globl __Z4run2v
__Z4run2v:
LFB340:
        movl    _atomic_var(%rip), %eax
        cmpl    $2, %eax
        je      L5
L3:
        ret
        .align 4,0x90
L5:
        cmpb    $0, _cond(%rip)
        je      L3
        movl    _atomic_var(%rip), %eax
        ret