[Bug tree-optimization/85610] New: Unable to optimize away mov followed by compare into a cmpb in case of atomic_load

[hiraditya at msn dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85610

            Bug ID: 85610
           Summary: Unable to optimize away mov followed by compare into a
                    cmpb in case of atomic_load
           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<bool> flag_atomic{false};

extern void f1();
extern void f2();

void foo() {
    bool b = flag_atomic.load(std::memory_order_relaxed);
    if (b == false) {
        f1();
    } else {
        f2();
    }   
}


$ g++-7 -O3 -S -o - test.cpp -std=c++14

__Z3foov:
LFB342:
  movzbl  _flag_atomic(%rip), %eax
  testb %al, %al 
  je  L4  
  jmp __Z2f2v
  .align 4,0x90
L4:
  jmp __Z2f1v


We could just use `cmpb $0, _flag_atomic(%rip)` and avoid a register in this
case. When _flag_atomic is a scalar boolean global variable, that's what
happens.