nrf52832 uart tx works only with NRF_UARTE0 read after EndTx

Question

Greetings, There is a bit of strange operation in UARTE peripheral in nrf52832 with the DK. In the simple code below, if both of the if statements with comments 'Check 1' and 'Check 2' are removed the 'hello' prints are not sent and I get some junk value. If either of the two reads to the UARTE registers is made in these if statement, then the 'hello' prints are sent out correctly. What's happening? I don't see any errata related to UARTE peripheral.

Thanks.

Toolchain: GCC 4.9.3q2 on Linux

#include <stdbool.h>
#include <stdint.h>
#include "nrf_delay.h"
#include "nrf52-gpio.h"
#include "nrf52-clock.h"
#include "board.h"
#include "nrf52-uart.h"

int main(void)
{
	nrf_gpio_cfg_output(LED1);
	nrf_gpio_cfg_output(LED2);
	nrf_gpio_cfg_output(LED3);
	nrf_gpio_cfg_output(LED4);

	/* Configure TX and RX pins from board.h */
	nrf_gpio_cfg_output(TX_PIN_NUMBER);
	nrf_gpio_cfg_input(RX_PIN_NUMBER, GPIO_PIN_CNF_PULL_Disabled);
	NRF_UARTE0->PSEL.TXD = TX_PIN_NUMBER;
	NRF_UARTE0->PSEL.RTS = RX_PIN_NUMBER;

	nrf_gpio_cfg_output(RTS_PIN_NUMBER);
	nrf_gpio_cfg_input(CTS_PIN_NUMBER, GPIO_PIN_CNF_PULL_Disabled);
	NRF_UARTE0->PSEL.RTS = RTS_PIN_NUMBER;
	NRF_UARTE0->PSEL.CTS = CTS_PIN_NUMBER;
	NRF_UARTE0->CONFIG = (UARTE_CONFIG_HWFC_Enabled << UARTE_CONFIG_HWFC_Pos);

	/* Configure other UART parameters, BAUD rate is defined in nrf52-uart.h	*/
	NRF_UARTE0->BAUDRATE = (UARTE_BAUDRATE << UARTE_BAUDRATE_BAUDRATE_Pos);
	NRF_UARTE0->ENABLE = (UARTE_ENABLE_ENABLE_Enabled << UARTE_ENABLE_ENABLE_Pos);

    // Toggle LEDs.
    while (true)
    {
    	uint32_t i;
        for (i = 0; i < 2; i++)
        {
        	if(i){
        		NRF_P0->OUTSET = (1<<LED1);
        	} else {
        		NRF_P0->OUTCLR = (1<<LED1);
        	}
        	nrf_delay_ms(500);

            uint8_t str[] = "hello\n";
            uint32_t len = 6;

            NRF_UARTE0->EVENTS_ENDTX = 0;
            NRF_UARTE0->TASKS_STOPTX = 0;

        	NRF_UARTE0->TXD.PTR = (uint32_t)((uint8_t *) str);
        	NRF_UARTE0->TXD.MAXCNT = (uint32_t) len;

        	NRF_UARTE0->TASKS_STARTTX = 1;

        	while((0 == NRF_UARTE0->EVENTS_ENDTX)&&(0 == NRF_UARTE0->EVENTS_TXSTOPPED)){}

            /*****Check 1*****/
        	if(NRF_UARTE0->EVENTS_ENDTX){
        		NRF_P0->OUTSET = (1<<LED3);
        	}

            /*****Check 2*****/    
        	if(6 == NRF_UARTE0->TXD.AMOUNT){
        		NRF_P0->OUTSET = (1<<LED2);
        	}

            NRF_UARTE0->EVENTS_ENDTX = 0;
            NRF_UARTE0->TASKS_STOPTX = 0;
        }
    }
}

RK · Accepted Answer

And the answer - which I have come around to believing, is the code falls into 'implementation dependent' and gcc is within its rights to decide that string isn't used and remove it. 
 Since the code was a piece of test code and not something anyone would be likely to write for real, initialized string on the stack going to a register and then wait for it to clock out, I don't think it's going to be a huge problem. It was however an interesting intellectual exercise.

nrf52832 uart tx works only with NRF_UARTE0 read after EndTx

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