external irq called twice depending on optimization level

Controller: STM32F103
Compiler: ARM-MDK 4.14

I have a flow-meter connected to a portpin. I like to count on rising and falling slope. So I initialized this ext. irq to meet these requirements. The software works since a couple of weeks without any problem.

This week I recognized that the optimiziation level is accidentally set to 0. Ok, no problem, I set it to maximum level 3.

After this action the code shrunk significantly - nice!

But my flow-meter gave me values twice as high as usual.

I put some LED-switchings into the isr and could detect that every slope of the flow meter triggers the isr exactly twice.

This is my isr for EXTI Line 14

  void IHW_DfmImpulseIsr(void) {
    COUNT_iCount(COUNT_kDFMBRUEHWASSER);  //count on each slope
    EXTI_ClearITPendingBit(EXTI_Line14);
  } // IHW_DfmImpulseIsr()

I found an other thread, that the reason might be that the isr-pending-bit is not cleared in time. But where is the casual connetion to optimization level.

And the crucial question, HOW to clear pending-bit correctly? Unfortunately I could not find any commendation neither from KEIL/ARM nor from ST.