Informing transformation passes of non-concurrent memory accesses

Hi!

The example in the docs1 states that the LLVM concurrency model does not allow the hoisting of loads and introduction of local variables due to another thread potentially altering the value whilst running the function. Given that I have analysis which shows that the memory location of variable x can not be concurrently accessed – how would I inform the LLVM transformation passes of this information?

I tried injecting this information in to the AliasAnalysis under getModRefInfo() by returning NoModref for the memory locations I knew not to be accessed concurrently. However, it seemed that the usages of the variables was not respected in that case, such that we could reorder variables and change the semantics of the program.

Thanks in advance,

Andreas

You’ll need to modify the LICM code to use this information (look for calls to isSimple()). Can you describe what your analysis does? -Hal

Hi Andreas,

I don’t think we have the extension point you’re asking for today. We have something really close though: pointerMayBeCaptured and pointerMayBeCapturedBefore. These two try to establish the property that a given memory location is known to be thread local because it hasn’t yet escaped. What you’re asking for is a bit different because the memory location might have escaped, but there’s some external mechanism (e.g. locking) preventing another thread from accessing this location at this point in time. We don’t have anything which models that additional power or optimizes based on it. Are you sure you need the power you asked for? Or is there a weaker predicate which works?