ARM Cortex-M compiler differences

Question

I'm about to develop some firmwares for Cortex-M cores on STM32 processors using C for my projects, and searching on the web I've found a lot of different compilers: Keil, IAR, Linaro, Yagarto and GNU Tools for ARM Embedded Processors.

I was wondering, what functional differences are there between these compilers that might influence my choice? For example as an enthusiast I don't need support or assistance from the vendor, and a limitation on the code size is OK for the moment. Also the ease of use is not a main concern since I like to learn (and for the moment I have both Keil Lite and Eclipse with GNU ARM configured and working).

Is the generated code so different in terms of size/speed between these compilers? Are there any comparison table? (I've found only stale infos on the web)

Answer 1

Performance should not be your first concern unless when it becomes so in a production environment. The reason is that first, most ARM compilers are plenty good enough, and really you are down to GCC based, Keil, and IAR. Second, most ARM MCU are "blazingly fast" and have "so much memory" (these are comparing to 8-bit MCU like AVR/PIC but also to older PC). A decent Cortex-M4 MCU runs up to 100MHz and has 256K of flash. Again, to put it in perspective, that's more memory and 10x faster clock rate than the original Macintosh etc. We went to the Moon with much less ;-)

Now the performance of the tools itself, in particular, the IDE and the debuggers, differ greatly. For example, the popular Eclipse is written in Java, might be a bit sluggish to slower or memory-starved PCs. The best thing to do is to install GCC+Eclipse, and the vendors' demos and see for yourself.

Answer 2

benchmarking is an artform in and of itself, usually easy to manipulate the results to show whatever you want. I would not expect the compilers to generate the same results except for very small test cases, and sometimes in those small test cases their results are either identical or sometimes vastly different as your test has exposed an optimization that one compiler knows/uses and one the other doesnt.

I used to keep track of such things (compiler performance numbers) with dhrystone for example, but in the case of known benchmarks (not that dhrystone means much anymore, but others) you may find that some compilers are tuning themselves to look good under benchmarks perhaps at the expense of something else.

There is no right answer, there is no universal "best", it is all in the eye of the beholder, you. Which tool is easier for you to use, which do you like better be it for the gui or pretty colors or sound card sounds or whatever. And go from there.

The gnu compiler generally for applications I have tested does not produce code as "fast" which is my benchmark, compared to the others, but there are way more people using the free gnu tools so the support for it is considerably wider due to the number of web pages and forums and examples. gnu wont have a size restriction either, but it may require more learning or whatever to get up and running...

The cortex-ms are split into the armv6m and armv7m families, the v6m (cortex-m0) only have a small number of thumb2 extensions, the armv7m have about 150 thumbv2 extensions to thumb, so you need to know what your tools support and not use the wrong stuff on the wrong chip. Then the compilers if they know all of this may and will produce different instruction mixes from the same source code. Further within the same compiler or family using different command line options you can/will get vastly different code. And then beyond that with a cortex-m4 with cache on if you have one with such a thing, depending on how the code lies in the cache lines you may get vastly different performance, so benchmarking is a research project in itself for each blob of C code you want to benchmark. The performance range within a single compiler may shadow another compiler or the overlap may be enough to not matter.

If you have access to the tools you add value to yourself professionally by learning to use the competing tools and being able to walk into a job and or within your job choose what you see as the right tool for the job or walk into a Kiel house and be able to work right away or a gnu house and work right away. Where you might lose a job if you are gnu only and the job is for a Kiel house.