[LLVMdev] [Polly] Analysis of extra compile-time overhead for simple nested loops

Fri Aug 16 02:42:13 PDT 2013

At 2013-08-16 12:44:02,"Tobias Grosser" <tobias at grosser.es> wrote:
>Hi,
>
>I tried to reproduce your findings, but could not do so.

Sorry, I did not put all code in my previous email because the code seems a little too long and complicated.
You can refer to the detailed C code and LLVM IR code on http://llvm.org/bugs/show_bug.cgi?id=16843
There are four attachments for our C code and LLVM IR code:

nestedloop.c (http://llvm.org/bugs/attachment.cgi?id=11043): the simplified C code.
nestedloop.ll (http://llvm.org/bugs/attachment.cgi?id=11044): the basic LLVM IR code.
nestedloop.preopt.ll (http://llvm.org/bugs/attachment.cgi?id=11045): the preprocessed LLVM IR code.
nestedloop.prepare.ll (http://llvm.org/bugs/attachment.cgi?id=11046): the LLVM IR code after running the polly-prepare pass.

>> I have investigated the 6X extra compile-time overhead when Polly compiles the simple nestedloop benchmark in LLVM-testsuite. (http://188.40.87.11:8000/db_default/v4/nts/31?compare_to=28&baseline=28). Preliminary results show that such compile-time overhead is resulted by the complicated polly-dependence analysis. However, the key seems to be the polly-prepare pass, which introduces a large number of store instructions, and thus significantly complicating the polly-dependence pass.
>>
>> Let me show the results with a tiny code example:
>>
>> int main(int argc, char *argv[]) {
>>    int n = ((argc == 2) ? atoi(argv[1]) : 46);
>>    int a, b, x=0;
>>    for (a=0; a<n; a++)
>>      for (b=0; b<n; b++)
>>          x++;
>>    printf("%d\n", x);
>>    return(0);
>> }
>
>This one misses some includes. Also, it is more convenient if you attach 
>the actual C file you include.

Yes, it should includes two header file <stdio.h> and <stdlib.h>. You can refer to the source code on http://llvm.org/bugs/attachment.cgi?id=11043
Compared with the original nestedloop benchmark, I changed the accumulation operation to a single assignment as follows:
  for (a=0; a<n; a++)
    for (b=0; b<n; b++)
      for (c=0; c<n; c++)
        for (d=0; d<n; d++)
          for (e=0; e<n; e++)
            for (f=0; f<n; f++)
              x=1;  //original is x++;

>This test case misses the target data. If you would attach the original 
>file, it would be easier to reproduce.

I see, you can refer to different LLVM IR code on http://llvm.org/bugs/show_bug.cgi?id=16843.

>> Such code is very simple and there is no memory instruction at all, so the polly-dependence pass runs very fast on this code. Unfortunately, when we run "opt -load LLVMPolly.so" for this basic LLVM IR code, the polly-prepare pass would introduce a large number of store instructions like this:
>
>Which passes and commands have you actually run? The command "opt -load 
>LLVMPolly.so" does not run the -polly-prepare pass.

Oops, I actually meant "opt -load LLVMPolly.so -polly -O3" here.

>Sorry for being picky here. Unfortunately, there are too many ways you 
>could have run such passes, that I am afraid I would not guess the right 
>one. And if we are running different experiments, it is difficult to 
>discuss them.

Yes, you are right. I should explain the detailed experimental steps.

>The way I propose to run the passes is as follows:
> [...]
>$ polly-opt -basicaa -polly-prepare -polly-scops out.preopt.ll -analyze

Scop info varies in the two cases (with/without -polly-scops). 
No valid scops are detected if we run polly without -polly-prepare:
$ polly-opt -basicaa -polly-scops out.preopt.ll -analyze

However, a very large region "for.cond2.preheader => for.end31" is detected as valid region if we run Polly with -polly-prepare:
$ polly-opt -basicaa -polly-prepare -polly-scops nestedloop.preopt.ll -analyze 

Note that the region "for.cond2.preheader => for.end31" is reported as invalid as the exit BasicBlock has PHI nodes,  if we run Polly without -polly-prepare. 

># Time with and without preoptimization
>~$ time polly-opt -basicaa -polly-prepare -polly-dependences 
>out.preopt.ll -disable-output
>[..]
>
>Within -O3 the overhead is still far away from the 60 slowdown we have 
>seen on the other test case. A 2x slowdown for a kernel that we fully 
>analyse and code generate is not optimal, but probably not an issue. The 
>question is if the increasing depth of the loop nest can yield this 60x 
>slowdown.

This is because the depth of the loop nest is only two. If we increase the depth of the loop nest, then the compile-time would significantly increase. I have posted results for the original code which contains 6 nested loops on http://llvm.org/bugs/show_bug.cgi?id=16843#c5

In fact, the compile time is exponentially growing as the depth of loop nest increases:
2-nested: ~2X
4-nested: ~10X
6-nested: ~50X
8-nested: ~200X
10-nested: ~700X
12-nested: ~2000X

>> These store instructions significantly complicate the "polly-dependence" pass, and thus leading to high compile-time overhead.
>
>What do you mean by "complicated the 'polly-dependence' pass"? Without 
>-polly-prepare the scop that is detected is a lot smaller. Hence, the 
>two scops are not really comparable.

Yes, no valid scop is detected at all without -polly-prepare.

>The point of the -polly-prepare pass is to transform some scalar 
>dependences into memory dependences. Introducing explicit load/store 
>instructions simplifies the later passes as scalar dependences do not
>be handled specially. However, introducing store instructions does (or 
>should) not make the actual dependence analysis problem more 
>complicated. If Polly would be extended to detect SCoPs with scalar 
>dependences those scalar dependences should be the very same once that
>are now created for the memory dependences introduced by the 
>-polly-prepare pass.

At first, I thought -polly-dependence mainly do dependence analysis for memory instructions, so I dump all intermediate LLVM IR code for each opt passes using "-print-after-all". Then I find all store instructions are introduced by the -polly-prepare pass. That is why I asked about the -polly-prepare pass.

Now I see, -polly-prepare just transforms some scalar dependences into memory dependences and it should not introduce new dependences. So, -polly-prepare may be not the key problem ~

>> I have noticed that such memory instructions are finally simplified to scalar operations by the SROA pass, so one possible way to reduce such compile-time overhead is to move the SROA pass ahead of polly-dependence analysis.
>>
>> Can anyone give me some hints that why the polly-prepare pass introduces such memory instructions? Is it possible to move the SROA pass ahead of polly-dependence analysis?
>
>Moving the SROA pass is not the solution. It would just prevent Polly 
>from detecting some SCoPs.

I see. We must find other ways.

>I think there are two angles we can have a look at:
>
>1) Does -polly-prepare introduce unneeded dependences?
>
>This means, could we model the scalar dependences more efficiently than 
>how they are modeled by the array accesses introduced by -polly-prepare.

This is really necessary. In fact, there should be no dependence in our code example as shown in our previous example. There is only a single and constant assignment "a=1" in the nested loop. Unfortunately, Polly still reports very complex data dependences.

>2) What triggers the 60x slowdown in dependence analysis
>
>Is this due to the increasing loop nest depth? Or the increasing number 
>of allocations introduced by -polly-prepare?
>
I have found that the compile time is exponentially growing as the depth of loop nest increases. However, increasing the loop nest depth would also increases the number of allocations and store instructions.

>An interesting experiment would be to see if the dependence analysis 
>runs faster on a loop that uses a single element array to implement the 
>reduction:
>
>for (i
>   for (j
>      ...
>        X[0] = ...

Yes, I have changed the original code to the form you suggested:
  for (i
      for (j
          ...
               x=1
There is no data dependence at all. However,  it seems that -polly-prepare still introduces a lot of allocs and store instructions.

>Meaning one alloc instruction and a single load and store in the 
>innermost loop, without any non-induction variable PHI nodes.
>
>When implementing this in C, LLVM may introduce again PHI nodes. So it 
>may possibly be necessary to write this directly in LLVM-IR.
>
>This became a long mail. Hope it gives you some ideas how to proceed.
>

Thank you so much, Tobias!
I sincerely appreciate your helpful suggestions!

Cheers,
Star Tan
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