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Star Tan

2013-Aug-15 10:32 UTC

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

Hi all,


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);
}


The basic LLVM IR code (produced by "clang -O1") is shown as:


@.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1
; Function Attrs: nounwind uwtable
define i32 @main(i32 %argc, i8** nocapture readonly %argv) {
entry:
  %cmp = icmp eq i32 %argc, 2
  br i1 %cmp, label %cond.end, label %for.cond2.preheader.lr.ph
cond.end:                                        
  %arrayidx = getelementptr inbounds i8** %argv, i64 1
  %0 = load i8** %arrayidx, align 8
  %call = tail call i32 (i8*, ...)* bitcast (i32 (...)* @atoi to i32 (i8*,
...)*)(i8* %0) #3
  %cmp117 = icmp sgt i32 %call, 0
  br i1 %cmp117, label %for.cond2.preheader.lr.ph, label %for.end8
for.cond2.preheader.lr.ph:                    
  %cond22 = phi i32 [ %call, %cond.end ], [ 46, %entry ]
  %cmp314 = icmp sgt i32 %cond22, 0
  br label %for.cond2.preheader
for.cond2.preheader:                              
  %x.019 = phi i32 [ 0, %for.cond2.preheader.lr.ph ], [ %x.1.lcssa, %for.inc6 ]
  %a.018 = phi i32 [ 0, %for.cond2.preheader.lr.ph ], [ %inc7, %for.inc6 ]
  br i1 %cmp314, label %for.body4, label %for.inc6
for.body4:                                        
  %x.116 = phi i32 [ %inc, %for.body4 ], [ %x.019, %for.cond2.preheader ]
  %b.015 = phi i32 [ %inc5, %for.body4 ], [ 0, %for.cond2.preheader ]
  %inc = add nsw i32 %x.116, 1
  %inc5 = add nsw i32 %b.015, 1
  %cmp3 = icmp slt i32 %inc5, %cond22
  br i1 %cmp3, label %for.body4, label %for.inc6
for.inc6:                                         
  %x.1.lcssa = phi i32 [ %x.019, %for.cond2.preheader ], [ %inc, %for.body4 ]
  %inc7 = add nsw i32 %a.018, 1
  %cmp1 = icmp slt i32 %inc7, %cond22
  br i1 %cmp1, label %for.cond2.preheader, label %for.end8
for.end8:                                        
  %x.0.lcssa = phi i32 [ 0, %cond.end ], [ %x.1.lcssa, %for.inc6 ]
  %call9 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([4 x
i8]* @.str, i64 0, i64 0), i32 %x.0.lcssa) #3
  ret i32 0
}
declare i32 @atoi(...)
declare i32 @printf(i8* nocapture readonly, ...)


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:


define i32 @main(i32 %argc, i8** nocapture readonly %argv) {
entry:
  %cond22.reg2mem = alloca i32
  %x.019.reg2mem = alloca i32
  %x.1.lcssa.reg2mem = alloca i32
  %x.1.lcssa.lcssa.reg2mem = alloca i32
  %x.0.lcssa.reg2mem = alloca i32
  br label %entry.split
entry.split:        
  %cmp = icmp eq i32 %argc, 2
  store i32 46, i32* %cond22.reg2mem
  br i1 %cmp, label %cond.end, label %for.cond2.preheader.lr.ph
cond.end:                            
  %arrayidx = getelementptr inbounds i8** %argv, i64 1
  %0 = load i8** %arrayidx, align 8
  %call = tail call i32 (i8*, ...)* bitcast (i32 (...)* @atoi to i32 (i8*,
...)*)(i8* %0)
  %cmp117 = icmp sgt i32 %call, 0
  store i32 0, i32* %x.0.lcssa.reg2mem
  store i32 %call, i32* %cond22.reg2mem
  br i1 %cmp117, label %for.cond2.preheader.lr.ph, label %for.end8
  ...
  
These store instructions significantly complicate the
"polly-dependence" pass, and thus leading to high compile-time
overhead.


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?


Thanks,
Star Tan
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Sebastian Pop

2013-Aug-15 13:12 UTC

head link

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

Codeprepare and independent blocks are introducing these loads and stores.
These are prepasses that polly runs prior to building the dependence graph
to transform scalar dependences into data dependences.
Ether was working on eliminating the rewrite of scalar dependences.

On Thu, Aug 15, 2013 at 5:32 AM, Star Tan <tanmx_star at yeah.net>
wrote:> Hi all,
>
> 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);
> }
>
> The basic LLVM IR code (produced by "clang -O1") is shown as:
>
> @.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align
1
> ; Function Attrs: nounwind uwtable
> define i32 @main(i32 %argc, i8** nocapture readonly %argv) {
> entry:
>   %cmp = icmp eq i32 %argc, 2
>   br i1 %cmp, label %cond.end, label %for.cond2.preheader.lr.ph
> cond.end:
>   %arrayidx = getelementptr inbounds i8** %argv, i64 1
>   %0 = load i8** %arrayidx, align 8
>   %call = tail call i32 (i8*, ...)* bitcast (i32 (...)* @atoi to i32 (i8*,
> ...)*)(i8* %0) #3
>   %cmp117 = icmp sgt i32 %call, 0
>   br i1 %cmp117, label %for.cond2.preheader.lr.ph, label %for.end8
> for.cond2.preheader.lr.ph:
>   %cond22 = phi i32 [ %call, %cond.end ], [ 46, %entry ]
>   %cmp314 = icmp sgt i32 %cond22, 0
>   br label %for.cond2.preheader
> for.cond2.preheader:
>   %x.019 = phi i32 [ 0, %for.cond2.preheader.lr.ph ], [ %x.1.lcssa,
> %for.inc6 ]
>   %a.018 = phi i32 [ 0, %for.cond2.preheader.lr.ph ], [ %inc7, %for.inc6 ]
>   br i1 %cmp314, label %for.body4, label %for.inc6
> for.body4:
>   %x.116 = phi i32 [ %inc, %for.body4 ], [ %x.019, %for.cond2.preheader ]
>   %b.015 = phi i32 [ %inc5, %for.body4 ], [ 0, %for.cond2.preheader ]
>   %inc = add nsw i32 %x.116, 1
>   %inc5 = add nsw i32 %b.015, 1
>   %cmp3 = icmp slt i32 %inc5, %cond22
>   br i1 %cmp3, label %for.body4, label %for.inc6
> for.inc6:
>   %x.1.lcssa = phi i32 [ %x.019, %for.cond2.preheader ], [ %inc, %for.body4
> ]
>   %inc7 = add nsw i32 %a.018, 1
>   %cmp1 = icmp slt i32 %inc7, %cond22
>   br i1 %cmp1, label %for.cond2.preheader, label %for.end8
> for.end8:
>   %x.0.lcssa = phi i32 [ 0, %cond.end ], [ %x.1.lcssa, %for.inc6 ]
>   %call9 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([4
> x i8]* @.str, i64 0, i64 0), i32 %x.0.lcssa) #3
>   ret i32 0
> }
> declare i32 @atoi(...)
> declare i32 @printf(i8* nocapture readonly, ...)
>
> 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:
>
> define i32 @main(i32 %argc, i8** nocapture readonly %argv) {
> entry:
>   %cond22.reg2mem = alloca i32
>   %x.019.reg2mem = alloca i32
>   %x.1.lcssa.reg2mem = alloca i32
>   %x.1.lcssa.lcssa.reg2mem = alloca i32
>   %x.0.lcssa.reg2mem = alloca i32
>   br label %entry.split
> entry.split:
>   %cmp = icmp eq i32 %argc, 2
>   store i32 46, i32* %cond22.reg2mem
>   br i1 %cmp, label %cond.end, label %for.cond2.preheader.lr.ph
> cond.end:
>   %arrayidx = getelementptr inbounds i8** %argv, i64 1
>   %0 = load i8** %arrayidx, align 8
>   %call = tail call i32 (i8*, ...)* bitcast (i32 (...)* @atoi to i32 (i8*,
> ...)*)(i8* %0)
>   %cmp117 = icmp sgt i32 %call, 0
>   store i32 0, i32* %x.0.lcssa.reg2mem
>   store i32 %call, i32* %cond22.reg2mem
>   br i1 %cmp117, label %for.cond2.preheader.lr.ph, label %for.end8
>   ...
>
> These store instructions significantly complicate the
"polly-dependence"
> pass, and thus leading to high compile-time overhead.
>
> 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?
>
> Thanks,
> Star Tan
>
> --
> You received this message because you are subscribed to the Google Groups
> "Polly Development" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to polly-dev+unsubscribe at googlegroups.com.
> For more options, visit https://groups.google.com/groups/opt_out.

Star Tan

2013-Aug-16 00:28 UTC

head link

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

Hi Sebpop,

Thanks for your explanation.  
I noticed that Polly would finally run the SROA pass to transform these
load/store instructions into scalar operations.  Is it possible to run such a
pass before polly-dependence analysis?
Star Tan
At 2013-08-15 21:12:53,"Sebastian Pop" <sebpop at gmail.com>
wrote:>Codeprepare and independent blocks are introducing these loads and stores.
>These are prepasses that polly runs prior to building the dependence graph
>to transform scalar dependences into data dependences.
>Ether was working on eliminating the rewrite of scalar dependences.
>
>On Thu, Aug 15, 2013 at 5:32 AM, Star Tan <tanmx_star at yeah.net>
wrote:
>> Hi all,
>>
>> 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);
>> }
>>
>> The basic LLVM IR code (produced by "clang -O1") is shown as:
>>
>> @.str = private unnamed_addr constant [4 x i8] c"%d\0A\00",
align 1
>> ; Function Attrs: nounwind uwtable
>> define i32 @main(i32 %argc, i8** nocapture readonly %argv) {
>> entry:
>>   %cmp = icmp eq i32 %argc, 2
>>   br i1 %cmp, label %cond.end, label %for.cond2.preheader.lr.ph
>> cond.end:
>>   %arrayidx = getelementptr inbounds i8** %argv, i64 1
>>   %0 = load i8** %arrayidx, align 8
>>   %call = tail call i32 (i8*, ...)* bitcast (i32 (...)* @atoi to i32
(i8*,
>> ...)*)(i8* %0) #3
>>   %cmp117 = icmp sgt i32 %call, 0
>>   br i1 %cmp117, label %for.cond2.preheader.lr.ph, label %for.end8
>> for.cond2.preheader.lr.ph:
>>   %cond22 = phi i32 [ %call, %cond.end ], [ 46, %entry ]
>>   %cmp314 = icmp sgt i32 %cond22, 0
>>   br label %for.cond2.preheader
>> for.cond2.preheader:
>>   %x.019 = phi i32 [ 0, %for.cond2.preheader.lr.ph ], [ %x.1.lcssa,
>> %for.inc6 ]
>>   %a.018 = phi i32 [ 0, %for.cond2.preheader.lr.ph ], [ %inc7,
%for.inc6 ]
>>   br i1 %cmp314, label %for.body4, label %for.inc6
>> for.body4:
>>   %x.116 = phi i32 [ %inc, %for.body4 ], [ %x.019, %for.cond2.preheader
]
>>   %b.015 = phi i32 [ %inc5, %for.body4 ], [ 0, %for.cond2.preheader ]
>>   %inc = add nsw i32 %x.116, 1
>>   %inc5 = add nsw i32 %b.015, 1
>>   %cmp3 = icmp slt i32 %inc5, %cond22
>>   br i1 %cmp3, label %for.body4, label %for.inc6
>> for.inc6:
>>   %x.1.lcssa = phi i32 [ %x.019, %for.cond2.preheader ], [ %inc,
%for.body4
>> ]
>>   %inc7 = add nsw i32 %a.018, 1
>>   %cmp1 = icmp slt i32 %inc7, %cond22
>>   br i1 %cmp1, label %for.cond2.preheader, label %for.end8
>> for.end8:
>>   %x.0.lcssa = phi i32 [ 0, %cond.end ], [ %x.1.lcssa, %for.inc6 ]
>>   %call9 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds
([4
>> x i8]* @.str, i64 0, i64 0), i32 %x.0.lcssa) #3
>>   ret i32 0
>> }
>> declare i32 @atoi(...)
>> declare i32 @printf(i8* nocapture readonly, ...)
>>
>> 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:
>>
>> define i32 @main(i32 %argc, i8** nocapture readonly %argv) {
>> entry:
>>   %cond22.reg2mem = alloca i32
>>   %x.019.reg2mem = alloca i32
>>   %x.1.lcssa.reg2mem = alloca i32
>>   %x.1.lcssa.lcssa.reg2mem = alloca i32
>>   %x.0.lcssa.reg2mem = alloca i32
>>   br label %entry.split
>> entry.split:
>>   %cmp = icmp eq i32 %argc, 2
>>   store i32 46, i32* %cond22.reg2mem
>>   br i1 %cmp, label %cond.end, label %for.cond2.preheader.lr.ph
>> cond.end:
>>   %arrayidx = getelementptr inbounds i8** %argv, i64 1
>>   %0 = load i8** %arrayidx, align 8
>>   %call = tail call i32 (i8*, ...)* bitcast (i32 (...)* @atoi to i32
(i8*,
>> ...)*)(i8* %0)
>>   %cmp117 = icmp sgt i32 %call, 0
>>   store i32 0, i32* %x.0.lcssa.reg2mem
>>   store i32 %call, i32* %cond22.reg2mem
>>   br i1 %cmp117, label %for.cond2.preheader.lr.ph, label %for.end8
>>   ...
>>
>> These store instructions significantly complicate the
"polly-dependence"
>> pass, and thus leading to high compile-time overhead.
>>
>> 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?
>>
>> Thanks,
>> Star Tan
>>
>> --
>> You received this message because you are subscribed to the Google
Groups
>> "Polly Development" group.
>> To unsubscribe from this group and stop receiving emails from it, send
an
>> email to polly-dev+unsubscribe at googlegroups.com.
>> For more options, visit https://groups.google.com/groups/opt_out.-------------- next part --------------
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Tobias Grosser

2013-Aug-16 04:44 UTC

head link

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

On 08/15/2013 03:32 AM, Star Tan wrote:> Hi all,
Hi,

I tried to reproduce your findings, but could not do so.
> 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.
> The basic LLVM IR code (produced by "clang -O1") is shown as:
> @.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align
1
> ; Function Attrs: nounwind uwtable
> define i32 @main(i32 %argc, i8** nocapture readonly %argv) {
> entry:
>    %cmp = icmp eq i32 %argc, 2
>    br i1 %cmp, label %cond.end, label %for.cond2.preheader.lr.ph
> cond.end:
>    %arrayidx = getelementptr inbounds i8** %argv, i64 1
>    %0 = load i8** %arrayidx, align 8
>    %call = tail call i32 (i8*, ...)* bitcast (i32 (...)* @atoi to i32 (i8*,
...)*)(i8* %0) #3
>    %cmp117 = icmp sgt i32 %call, 0
>    br i1 %cmp117, label %for.cond2.preheader.lr.ph, label %for.end8
> for.cond2.preheader.lr.ph:
>    %cond22 = phi i32 [ %call, %cond.end ], [ 46, %entry ]
>    %cmp314 = icmp sgt i32 %cond22, 0
>    br label %for.cond2.preheader
> for.cond2.preheader:
>    %x.019 = phi i32 [ 0, %for.cond2.preheader.lr.ph ], [ %x.1.lcssa,
%for.inc6 ]
>    %a.018 = phi i32 [ 0, %for.cond2.preheader.lr.ph ], [ %inc7, %for.inc6 ]
>    br i1 %cmp314, label %for.body4, label %for.inc6
> for.body4:
>    %x.116 = phi i32 [ %inc, %for.body4 ], [ %x.019, %for.cond2.preheader ]
>    %b.015 = phi i32 [ %inc5, %for.body4 ], [ 0, %for.cond2.preheader ]
>    %inc = add nsw i32 %x.116, 1
>    %inc5 = add nsw i32 %b.015, 1
>    %cmp3 = icmp slt i32 %inc5, %cond22
>    br i1 %cmp3, label %for.body4, label %for.inc6
> for.inc6:
>    %x.1.lcssa = phi i32 [ %x.019, %for.cond2.preheader ], [ %inc,
%for.body4 ]
>    %inc7 = add nsw i32 %a.018, 1
>    %cmp1 = icmp slt i32 %inc7, %cond22
>    br i1 %cmp1, label %for.cond2.preheader, label %for.end8
> for.end8:
>    %x.0.lcssa = phi i32 [ 0, %cond.end ], [ %x.1.lcssa, %for.inc6 ]
>    %call9 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds
([4 x i8]* @.str, i64 0, i64 0), i32 %x.0.lcssa) #3
>    ret i32 0
> }
> declare i32 @atoi(...)
> declare i32 @printf(i8* nocapture readonly, ...)
This test case misses the target data. If you would attach the original 
file, it would be easier to reproduce.
> 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.

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.

The way I propose to run the passes is as follows:

$ clang -O0 -S -emit-llvm -o out.ll

# Get the polly passes to that are run
$ polly-opt test.ll  -O3 -polly -debug-passes=Structure

Pass Arguments:  -datalayout -notti -basictti -x86tti -targetlibinfo 
-no-aa -tbaa -basicaa -preverify -domtree -verify -mem2reg -instcombine 
-simplifycfg -tailcallelim -simplifycfg -reassociate -domtree -loops 
-loop-simplify -lcssa -loop-rotate -instcombine -scalar-evolution 
-loop-simplify -lcssa -iv-users -polly-indvars -polly-prepare 
-postdomtree -domfrontier -regions -scalar-evolution -polly-detect 
-polly-independent -polly-analyze-ir -polly-scops -polly-dependences 
-polly-opt-isl -polly-cloog -polly-codegen -simplifycfg -domtree -sroa 
-early-cse -lower-expect

[...]

# Run everything up to the offending pass
$ polly-opt test.ll -targetlibinfo -no-aa -tbaa -basicaa -preverify 
-domtree -verify -mem2reg -instcombine -simplifycfg -tailcallelim 
-simplifycfg -reassociate -domtree -loops -loop-simplify -lcssa 
-loop-rotate -instcombine -scalar-evolution -loop-simplify -lcssa 
-iv-users -polly-indvars -o out.preopt.ll -S

# Show scops with and without preopt

~$ polly-opt -basicaa -view-scops out.preopt.ll -disable-output
Writing '/tmp/scops-f3a2fa.dot'...  done.
Running 'xdot.py' program...  done.
~$ polly-opt -basicaa -polly-prepare -view-scops out.preopt.ll 
-disable-output
Writing '/tmp/scops-37f35f.dot'...  done.
Running 'xdot.py' program...  done.

# Print the scop info:
$ polly-opt -basicaa -polly-prepare -polly-scops out.preopt.ll -analyze

# Time with and without preoptimization
~$ time polly-opt -basicaa -polly-prepare -polly-dependences 
out.preopt.ll -disable-output

real	0m0.040s
user	0m0.032s
sys	0m0.004s

~$ time polly-opt -basicaa -polly-dependences out.preopt.ll -disable-output

real	0m0.011s
user	0m0.004s
sys	0m0.004s

Looking at the timings, this does in fact not look too bad. Run within -O3

~$ time polly-opt -basicaa -O3 out.ll -disable-output

real	0m0.015s
user	0m0.004s
sys	0m0.008s

~$ time polly-opt -basicaa -O3 out.ll -disable-output -polly

real	0m0.029s
user	0m0.024s
sys	0m0.004s

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.

[..]
> 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.

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.
> 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 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.

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?


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] = ...

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.

Cheers,
Tobi

P.S: Sebastian, also thank you for your comments!
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; ModuleID = '/tmp/test.c'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@b = common global [2048 x i32] zeroinitializer, align 16
@a = common global [2048 x i32] zeroinitializer, align 16

; Function Attrs: noinline nounwind uwtable
define void @example7(i32 %x) #0 {
entry:
  br label %entry.split

entry.split:                                      ; preds = %entry
  %0 = zext i32 %x to i64
  br label %for.body

for.body:                                         ; preds = %entry.split,
%for.body
  %indvar = phi i64 [ 0, %entry.split ], [ %indvar.next, %for.body ]
  %1 = add i64 %0, %indvar
  %add = trunc i64 %1 to i32
  %arrayidx2 = getelementptr [2048 x i32]* @a, i64 0, i64 %indvar
  %idxprom = sext i32 %add to i64
  %arrayidx = getelementptr inbounds [2048 x i32]* @b, i64 0, i64 %1
  %2 = load i32* %arrayidx, align 4
  store i32 %2, i32* %arrayidx2, align 4
  %indvar.next = add i64 %indvar, 1
  %exitcond = icmp ne i64 %indvar.next, 1024
  br i1 %exitcond, label %for.body, label %for.end

for.end:                                          ; preds = %for.body
  ret void
}

attributes #0 = { noinline nounwind uwtable
"less-precise-fpmad"="false"
"no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf"="true"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"stack-protector-buffer-size"="8"
"unsafe-fp-math"="false"
"use-soft-float"="false" }

-------------- next part --------------
; ModuleID = 'test.ll'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"

@.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1

; Function Attrs: nounwind uwtable
define i32 @main(i32 %argc, i8** %argv) #0 {
  %1 = icmp eq i32 %argc, 2
  br i1 %1, label %2, label %6

; <label>:2                                       ; preds = %0
  %3 = getelementptr inbounds i8** %argv, i64 1
  %4 = load i8** %3, align 8
  %5 = tail call i32 (i8*, ...)* bitcast (i32 (...)* @atoi to i32 (i8*,
...)*)(i8* %4) #2
  br label %6

; <label>:6                                       ; preds = %0, %2
  %7 = phi i32 [ %5, %2 ], [ 46, %0 ]
  %8 = icmp sgt i32 %7, 0
  br i1 %8, label %.preheader.lr.ph, label %15

.preheader.lr.ph:                                 ; preds = %6
  br label %.preheader

.preheader:                                       ; preds = %.preheader.lr.ph,
%13
  %x.04 = phi i32 [ 0, %.preheader.lr.ph ], [ %x.1.lcssa, %13 ]
  %a.03 = phi i32 [ 0, %.preheader.lr.ph ], [ %14, %13 ]
  %9 = icmp sgt i32 %7, 0
  br i1 %9, label %.lr.ph, label %13

.lr.ph:                                           ; preds = %.preheader
  br label %10

; <label>:10                                      ; preds = %.lr.ph, %10
  %b.01 = phi i32 [ 0, %.lr.ph ], [ %11, %10 ]
  %11 = add nsw i32 %b.01, 1
  %exitcond = icmp ne i32 %11, %7
  br i1 %exitcond, label %10, label %._crit_edge

._crit_edge:                                      ; preds = %10
  %12 = add i32 %7, %x.04
  br label %13

; <label>:13                                      ; preds = %._crit_edge,
%.preheader
  %x.1.lcssa = phi i32 [ %12, %._crit_edge ], [ %x.04, %.preheader ]
  %14 = add nsw i32 %a.03, 1
  %exitcond7 = icmp ne i32 %14, %7
  br i1 %exitcond7, label %.preheader, label %._crit_edge5

._crit_edge5:                                     ; preds = %13
  %x.1.lcssa.lcssa = phi i32 [ %x.1.lcssa, %13 ]
  br label %15

; <label>:15                                      ; preds = %._crit_edge5,
%6
  %x.0.lcssa = phi i32 [ %x.1.lcssa.lcssa, %._crit_edge5 ], [ 0, %6 ]
  %16 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([4 x i8]*
@.str, i64 0, i64 0), i32 %x.0.lcssa) #2
  ret i32 0
}

declare i32 @atoi(...) #1

declare i32 @printf(i8*, ...) #1

attributes #0 = { nounwind uwtable
"less-precise-fpmad"="false"
"no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf"="true"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"stack-protector-buffer-size"="8"
"unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #1 = { "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf"="true"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"stack-protector-buffer-size"="8"
"unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #2 = { nounwind }

Star Tan

2013-Aug-16 09:42 UTC

head link

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

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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