[LLVMdev] Counting load and stores of variables

mats petersson mats at planetcatfish.com
Mon Mar 9 05:47:00 PDT 2015

Further on that last comment:

If you are interested in counting accesses to `x`, and have the following code:

    a = x + x + x + x;

and the compiler translates that to:

    a= x * 4;

should that count as 1 or 4 accesses?

What if you have code like this:

     x = 1;
     if (a) x = 4;
     if (b) x = 5;
     if (c) x = 7;

if a, b and c are true, and the compiler decided to re-arrange the
code like this:

     reg_temp = 1;
     if (a) reg_temp = 4;
     if (b) reg_temp = 5;
     if (c) reg_temp = 7;
     x = reg_temp;

how many accesses to `x` is that? 1 or 4?  Do you care about accesses
in the original source or accesses to the memory location where it
stores the value?


On 9 March 2015 at 12:08, mats petersson <mats at planetcatfish.com> wrote:
> Seems tricky.
> The complexity of the solution depends on
> int x;    //  our variable of interest
> int func(int &a)
> {
>     a++;       // a++ actually updates `x` because it's passed in below.
> }
> int main()
> {
>     func(x);
> }
> We can of course make MUCH more complex variants of the above, but
> there is no way the compiler, whilst compiling `func` can know if the
> variable is `x` or not - until it optimizes and inlines `func`, at
> least.
> The simple C++ solution would be to replace the simple variable with
> an object that has a "counter" applied to all accesses of the content
> - but it will affect code-gen and it will affect the "compatibility"
> of the variable (it's no longer a regular `int` or `double`). Aliasing
> gets challenging here.
> Using the compiler's profiling options will tell you which blocks are
> hit how many times, and if you analyse which blocks contain your
> variable [and how many times the variable is updated] - this assumes
> your variable isn't aliased.
> Another thought that comes to my mind is to use a virtual machine, and
> place those variables in a special section of memory, that can then be
> attributed with "read_only" or "no access" and have the VM catch the
> access, update the value and then continue. But that's not an easy
> solution.
> If the number of variables is small, you could use debug functionality
> for memory acesss breakpoints (register a "break on read" and "break
> on write" for a certain variable - in the handler for this, count up
> the relevant counter and continue). This is probably doable even if
> the number of available hardware breakpoints is smaller than the
> number of variables you want to monitor, by running the program
> multiple times (assuming your program is repeatable with the same
> result every time - if it's some sort of random number generation with
> new seed every time you run it, or if it is based on some real-time
> events (stock market, traffic, weather, live video etc), this won't
> work).
> Hardware solutions (either virtual machine or memory access
> breakpoints) are the only reasonable ways to solve the aliasing
> situation.
> If you have plenty of time, you could also use a processor emulator,
> for example qemu - and let it catch writes to certain addresses and
> count how many times. Not practical for most applications tho', since
> the processor emulators run much slower than real hardware (100-1000x
> slower in a good case)
> It gets even more interesting if we consider the actual machine
> instructions. The `a++` above could in say x86 be implemented as `inc
> a` or `mov a, reg; inc reg; move reg, a` - both will indeed read,
> modify the value and write it back - but one is explicitly
> loading/storing the value, the other is not. How do you account for
> that? Is it one or two accesses?
> On 9 March 2015 at 11:24, MKT <taram.mohammad at gmail.com> wrote:
>> Hi all,
>> I’m working on my thesis in which I need to count the number of memory
>> access (i.e. load and stores) of each variable of program. how can I do
>> that? these are some approaches came to my mind :
>> I need to know during the execution of the program how many load and store
>> instructions are executed on a specific variable, hence simply counting
>> static load and store instructions doesn’t help.
>> I also cannot write an IR pass to inject some instructions to count memory
>> accesses of variables (instrumenting) , because in IR optimization I’m not
>> aware of what will happen in register allocation.
>> And if I write a machine pass to inject machine instructions, I guess maybe
>> I loose some informations( e.g. variable names) during the optimizations.
>> Thanks,
>> MohammadKazem
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