STM32H7 code execution time question

[DWWelch]

Hello All,

I needed a very fast vector addition routine for the H7 so I wrote it in assembly. However, I discovered that very small changes in the code caused huge differences in execution time. Both of the following functions add a 16-bit vector to a 32-bit sum vector. The vectors are 32-bit word aligned. The sum vector is in DTCM and the other is in external SRAM. The first routine adds sequential data to the sum and the second adds every fourth point of a 4x larger vector to the same size sum. So both process the same amount of data, but the first is 3 times faster. Does anyone know what would cause such a large difference in execution time for these nearly identical functions?

Thanks

Dan

3X faster one 

loop:

LDRH r3, [r0, r2, lsl #1] // load 16bit raw data in r3

LDR r4, [r1, r2, lsl #2] // load 32bit sum in r4

ADD r4,r3 // add raw data to sum

STR r4, [r1, r2, lsl #2] // store new sum

SUBS r2,#1 // next data point

BPL loop

Slower one 

loop:

LDRH r3, [r0, r2, lsl #1] // load 16bit raw otdr data in r3

LDR r4, [r1, r2] // load 32bit sum in r4

ADD r4,r3 // add raw data to sum

STR r4, [r1, r2] // store new sum

SUBS r2,#4 // next data point

BPL loop

[waclawek.jan]

Where do you effector that loop from (memory, alignment)?

Cortex-M7 is not your friendly cycle-precise core, but an overcomplicated caching, mildly superscalar beast.

JW

[TDK]

The second uses 4x the total memory, which makes cache misses 4x more often. Memory gets loaded one cache page at a time. I don't know the size of this offhand, but it's on the order of 32 bytes. In the first loop, you have 8 data points per cache page. In the second, you only have 2.

What you're seeing is that keeping data within cache makes a huge difference in terms of speed.

[AScha.3]

Hi,

just for info : did you write it in plain C , optimizer set to -Ofast ?

How this compares to your asm ?

[DWWelch]

I am using ICache, but I'm not sure on the instruction alignment.

Dan

[DWWelch]

The larger vector is in external SRAM and DCache is enabled. So your answer makes sense, but the SRAM is running at the CPU clock speed with 1 wait state and the read is only one instruction in the loop so I don't think it would make it three times slower overall.

Thanks,

Dan

[DWWelch]

I will give this a try with the slower code and see if the compiler knows some magic.

Dan