Common Code Elimination
cortex-m3 uVision 5.13
I recent had to up the optimization level on my project to level 2 because my code size was too large for my target memory. In doing this I found that it seems to be doing common code elimination where valid routines are not being compiled/linked into the image.
This has caused some routines to be practically wiped out because another is very similar but has some differences. I guess they were common enough for the compiler to decide to get rid of one of them.
I have been Googling and searching everywhere for a way to turn off just the common code elimination but cannot find anything that I can use with uVision.
There is a C51 pragma to change the optimization level to stop this on a file by file basis but the uVision compile causes a error.
There must be a way to turn this off. I understand it is trying to save me space but it is also changing or in some cases eliminating functionality.
Does anyone know of a pragma that will work or something I can do?
Thanks
P.S. of course this happens 2 days before a major delivery on a weekend!!
I understand it is trying to save me space but it is also changing or in some cases eliminating functionality.
You've yet to actually convince anyone that it did eliminate functionality. So far you've only talked about it eliminating code. You don't seem to have bothered checking the difference between those two.
I have to say I would not have posted a question if there was not a problem.
I have two routines. One sets up a actuator and the other a sensor. The set is is basically the same EXECPT I have print statements in this "common looking" code that are different. They would be text strings.
And why I say it is not working is because when I call the routine to set up the sensor it executes for 3 lines which are not unique to the other routine and THEN it exits doing nothing. It does not excute what should now be common code. It does nothing.
I used the ulink debugger which given the optimization is not as reliable as it normally is, to step into the routine and step through those first 3 lines, the remaining ones were not marked as executable (grey box to the left of each line) and it stepped to the return and exited. It did do anything.
I have gotten round this because I have run out of time by changing the sensor routine enough so tha it no longer looks enough like the other one for it to be optimized out.
I see that there are dozens and dozens of other areas that have been optimized as common code but this was a very large block of code compared to those which were only a few lines.
Obviously my system would not run at all of the common block optimization did not work at all. Those smaller optimzed lines of code most be working or the system would fall apart.
It has just been this large block of code that seems to be the problem. It was 20 or 30 lines of code it removed. And like I mentioned they were not exactly the same. There were some text strings that were different that my print statement would output. It obviously did not look that the strings themselves and notice the difference. If it had it would not have optimized it.
I am telling you the truth about what happened. The sensor code which was to send a messge out and print some text strings did not execute at all. It was not until I forced it not to optimized did I get my functionality back.
Its obviously a one off case that I have uncovered. I was/am just afraid of it doing that to some other part of my software and I just haven't stumbled over it yet. That is why I wanted to pay it safe and turn that part of optimization off.
I guess I should post the two routines to technical support so they can research why the optimizer thought these were the same when there were some differences. Not in code executed but in the text strings that the code would output. Smnall but significant differences.
So I'll ask my question again, is there a pragma or switch that I can use to prevent common block optimization?
thanks
Your request to the manual lookup service has found this:
www.keil.com/.../armcc_chr1359124215328.htm - NOTHING APPROPRIATE FOUND
www.keil.com/.../armcc_chr1359124194749.htm - NOTHING APPROPRIATE FOUND
If you think really found a problem, I suggest you attempt to be a tad more precise when passing information to support.
Around the specific routines you do not want optimized
#pragma push // save current pragma state #pragma O0 Your routine that you do not want optimized #pragma pop // restore previous optimization level
It does seem reasonable for the compiler to optimize out redundant code if only the strings are different (even seems like it SHOULD do this at higher optimization levels). It is easy to pass in different strings to the same code. It does seem like the issue may be larger than this but I don't believe the proper fix (on Keil's part) would be to not optimize out the code, but to just make sure the proper strings are used in each case. As people have said it is the functionality that needs to be the same, not the exact code generated.
Also is next to impossible to debug the source code at Optimization level 2 and above (level 1 is not horrible, save switch statements). I find that there is often no way to set break points based on the high level code. Going into the disassembly and setting break points and stepping through at that level can be very helpful. I am not sure if this is the case, but my bet is that when you say that "nothing happens" in the code, if you actually step through the disassembly you will find there is a lot more than nothing going on.
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