STM32N657: unexpected delay in execution

[acapola]

My original aim was to prove to myself that I was able to measure execution time with cycle accuracy. In order to do that, I have 9 'test_dwt' functions, they all have same code except number of nop between 2 reads of DWT.

That gives unexpected results because M55 has dual-issue capability, as people explained here. 

Nevertheless, I am puzzled with the result of test_dwt7 (7 nops). All clocks are at 100MHz, interrupts, icache and dcache are disabled, all 'test_dwt' functions starts with isb and dsb instructions.

Here I print the result of 5 executions of each function. The execution order is 0 to 7c, repeated 5 times, saving results in an array. Then I print the results:

test_dwt0  :   1   1   1   1   1 
test_dwt1  :   1   1   1   1   1 
test_dwt2  :   1   1   1   1   1 
test_dwt3  :   2   2   2   2   2 
test_dwt4  :   2   2   2   2   2 
test_dwt5  :   3   3   3   3   3 
test_dwt6  :   3   3   3   3   3 
test_dwt7  :  27  27  27  27  27 
test_dwt8  :   5   5   5   5   5 
test_dwt7b :   4   4   4   4   4 
test_dwt7c :   4   4   4   4   4

the code of the 4 last functions:

34180824 <test_dwt7>:
34180824:	f3bf 8f6f 	isb	sy
34180828:	f3bf 8f4f 	dsb	sy
3418082c:	f8df 3018 	ldr.w	r3, [pc, #24]	@ 34180848 <.test_dwt7.dwt_base>
34180830:	6858      	ldr	r0, [r3, #4]
34180832:	bf00      	nop
34180834:	bf00      	nop
34180836:	bf00      	nop
34180838:	bf00      	nop
3418083a:	bf00      	nop
3418083c:	bf00      	nop
3418083e:	bf00      	nop
34180840:	6859      	ldr	r1, [r3, #4]
34180842:	eba1 0000 	sub.w	r0, r1, r0
34180846:	4770      	bx	lr

34180848 <.test_dwt7.dwt_base>:
34180848:	1000      	asrs	r0, r0, #32
3418084a:	e000      	b.n	3418084e <test_dwt8+0x2>

3418084c <test_dwt8>:
3418084c:	f3bf 8f6f 	isb	sy
34180850:	f3bf 8f4f 	dsb	sy
34180854:	4b06      	ldr	r3, [pc, #24]	@ (34180870 <.test_dwt8.dwt_base>)
34180856:	6858      	ldr	r0, [r3, #4]
34180858:	bf00      	nop
3418085a:	bf00      	nop
3418085c:	bf00      	nop
3418085e:	bf00      	nop
34180860:	bf00      	nop
34180862:	bf00      	nop
34180864:	bf00      	nop
34180866:	bf00      	nop
34180868:	6859      	ldr	r1, [r3, #4]
3418086a:	eba1 0000 	sub.w	r0, r1, r0
3418086e:	4770      	bx	lr

34180870 <.test_dwt8.dwt_base>:
34180870:	1000      	asrs	r0, r0, #32
34180872:	e000      	b.n	34180876 <test_dwt7b+0x2>

34180874 <test_dwt7b>:
34180874:	f3bf 8f6f 	isb	sy
34180878:	f3bf 8f4f 	dsb	sy
3418087c:	f8df 3018 	ldr.w	r3, [pc, #24]	@ 34180898 <.test_dwt7b.dwt_base>
34180880:	6858      	ldr	r0, [r3, #4]
34180882:	bf00      	nop
34180884:	bf00      	nop
34180886:	bf00      	nop
34180888:	bf00      	nop
3418088a:	bf00      	nop
3418088c:	bf00      	nop
3418088e:	bf00      	nop
34180890:	6859      	ldr	r1, [r3, #4]
34180892:	eba1 0000 	sub.w	r0, r1, r0
34180896:	4770      	bx	lr

34180898 <.test_dwt7b.dwt_base>:
34180898:	1000      	asrs	r0, r0, #32
3418089a:	e000      	b.n	3418089e <test_dwt7c+0x2>

3418089c <test_dwt7c>:
3418089c:	f3bf 8f6f 	isb	sy
341808a0:	f3bf 8f4f 	dsb	sy
341808a4:	4b06      	ldr	r3, [pc, #24]	@ (341808c0 <.test_dwt7c.dwt_base>)
341808a6:	6858      	ldr	r0, [r3, #4]
341808a8:	bf00      	nop
341808aa:	bf00      	nop
341808ac:	bf00      	nop
341808ae:	bf00      	nop
341808b0:	bf00      	nop
341808b2:	bf00      	nop
341808b4:	bf00      	nop
341808b6:	6859      	ldr	r1, [r3, #4]
341808b8:	eba1 0000 	sub.w	r0, r1, r0
341808bc:	4770      	bx	lr
	...

341808c0 <.test_dwt7c.dwt_base>:
341808c0:	1000      	asrs	r0, r0, #32
341808c2:	e000      	b.n	341808c6 <test_pmu0+0x2>

What is causing such large delay in test_dwt7 ? (I observe a similar thing with PMU)