Static C ++ Initialization

Question

what should be the behavior in the following case:

class C {
    boost::mutex mutex_;
    std::map<...> data_;
};

C& get() {
    static C c;
    return c;
}

int main() {
    get(); // is compiler free to optimize out the call? 
    ....
}

Does the compiler allow you to optimize the call get()?

the idea was to touch a static variable in order to initialize it before multithreading operations were required.

this is the best variant?:

C& get() {
    static C *c = new C();
    return *c;
}

Answer 1

The C and C ++ standards work according to a rather simple principle, usually known as “if-if” - basically, that the compiler can do almost anything until any relevant code can recognize the difference between what it is and what it was officially necessary.

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Answer 2

, .

:

struct C { 
    int myfrob;
    int frob();
    C(int f);
 };
C::C(int f) : myfrob(f) {}
int C::frob() { return myfrob; }

C& get() {
    static C *c = new C(5);
    return *c;
}

int main() {
    return get().frob(); // is compiler free to optimize out the call? 

}

:

; ModuleID = '/tmp/webcompile/_28088_0.bc'
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-linux-gnu"

%struct.C = type { i32 }

@guard variable for get()::c = internal global i64 0            ; <i64*> [#uses=4]

declare i32 @__cxa_guard_acquire(i64*) nounwind

declare i8* @operator new(unsigned long)(i64)

declare void @__cxa_guard_release(i64*) nounwind

declare i8* @llvm.eh.exception() nounwind readonly

declare i32 @llvm.eh.selector(i8*, i8*, ...) nounwind

declare void @__cxa_guard_abort(i64*) nounwind

declare i32 @__gxx_personality_v0(...)

declare void @_Unwind_Resume_or_Rethrow(i8*)

define i32 @main() {
entry:
  %0 = load i8* bitcast (i64* @guard variable for get()::c to i8*), align 8 ; <i8> [#uses=1]
  %1 = icmp eq i8 %0, 0                           ; <i1> [#uses=1]
  br i1 %1, label %bb.i, label %_Z3getv.exit

bb.i:                                             ; preds = %entry
  %2 = tail call i32 @__cxa_guard_acquire(i64* @guard variable for get()::c) nounwind ; <i32> [#uses=1]
  %3 = icmp eq i32 %2, 0                          ; <i1> [#uses=1]
  br i1 %3, label %_Z3getv.exit, label %bb1.i

bb1.i:                                            ; preds = %bb.i
  %4 = invoke i8* @operator new(unsigned long)(i64 4)
          to label %invcont.i unwind label %lpad.i ; <i8*> [#uses=2]

invcont.i:                                        ; preds = %bb1.i
  %5 = bitcast i8* %4 to %struct.C*               ; <%struct.C*> [#uses=1]
  %6 = bitcast i8* %4 to i32*                     ; <i32*> [#uses=1]
  store i32 5, i32* %6, align 4
  tail call void @__cxa_guard_release(i64* @guard variable for get()::c) nounwind
  br label %_Z3getv.exit

lpad.i:                                           ; preds = %bb1.i
  %eh_ptr.i = tail call i8* @llvm.eh.exception()  ; <i8*> [#uses=2]
  %eh_select12.i = tail call i32 (i8*, i8*, ...)* @llvm.eh.selector(i8* %eh_ptr.i, i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*), i8* null) ; <i32> [#uses=0]
  tail call void @__cxa_guard_abort(i64* @guard variable for get()::c) nounwind
  tail call void @_Unwind_Resume_or_Rethrow(i8* %eh_ptr.i)
  unreachable

_Z3getv.exit:                                     ; preds = %invcont.i, %bb.i, %entry
  %_ZZ3getvE1c.0 = phi %struct.C* [ null, %bb.i ], [ %5, %invcont.i ], [ null, %entry ] ; <%struct.C*> [#uses=1]
  %7 = getelementptr inbounds %struct.C* %_ZZ3getvE1c.0, i64 0, i32 0 ; <i32*> [#uses=1]
  %8 = load i32* %7, align 4                      ; <i32> [#uses=1]
  ret i32 %8
}

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Answer 3

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e.g

C& getC(){
   volatile int dummy;
   dummy++;
   // rest of the code
}