llvm dev - Aug 2017 - BranchProbability/BlockFrequency for a chain of ifs

I'm look at some code that does something roughly like this

if (a >= 10 && a < 20) {
  // do calculation
} else if (a >= 30 && a < 40) {
  // do calculation
} else if (a >= 50 && a < 60) {
  // do something
} else if (a >= 70 && a < 80) {
  // do something
}

// some other code that got split based on whether we entered any of the
'if' bodies. So each if body merges to a slightly different spot than
the
not taken side of the final if.

In my specific case there are really 8 specific range checks.

Without profile data it seems branch probability assumes the branch of each
'if' has 50% probability. Due to the way the ifs are cascaded this
causes
block frequency to think the final 'if' body can only execute some
really
small percentage of the time. Similarly it thinks that less than %1 of the
time will we enter the code path that occurs if none of the ifs are taken.

In reality in my specific case we end up executing none of the ifs most
often. But I think i'd settle for all bodies and the no ifs taken path
having equal weighting

The bad block frequency and the fact that we have a split path for merging
the ifs versus not going into any ifs at all seems to be causing block
placement to create a layout that pessimizes the real common case.

Since all the compares are on the same variable, the originally code is
kinda like a switch and as such we should maybe give each possible path
equal weighting.

Is there somewhere we can apply a heuristic to the branch probability so
that it can provide a better answer here?

~Craig
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While it is possible that all the value ranges are equally likely, the
opposite situation may be true too.   It is reasonable to assume that if
the developer knows the program behavior, he/she will choose to test  the
most likely range first .This can be used by the compiler as a heuristic to
make it more likely (not that LLVM is doing it today).  The 50% BP is a
sign that there is no known heuristic applied.

For better performance, using PGO or sample PGO is the best possible way.
Another direction is to use source annotation. Unfortunately,
builtin_expect is too strong for many cases. One enhancement that can be
done is to teach this builtin to take an additional likelihood parameter.
Restructuring the code is also possible, e.g.  by compressing the range and
using a switch ..

David


On Fri, Aug 4, 2017 at 4:10 PM, Craig Topper via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I'm look at some code that does something roughly like this
>
> if (a >= 10 && a < 20) {
>   // do calculation
> } else if (a >= 30 && a < 40) {
>   // do calculation
> } else if (a >= 50 && a < 60) {
>   // do something
> } else if (a >= 70 && a < 80) {
>   // do something
> }
>
> // some other code that got split based on whether we entered any of the
> 'if' bodies. So each if body merges to a slightly different spot
than the
> not taken side of the final if.
>
> In my specific case there are really 8 specific range checks.
>
> Without profile data it seems branch probability assumes the branch of
> each 'if' has 50% probability. Due to the way the ifs are cascaded
this
> causes block frequency to think the final 'if' body can only
execute some
> really small percentage of the time. Similarly it thinks that less than %1
> of the time will we enter the code path that occurs if none of the ifs are
> taken.
>
> In reality in my specific case we end up executing none of the ifs most
> often. But I think i'd settle for all bodies and the no ifs taken path
> having equal weighting
>
> The bad block frequency and the fact that we have a split path for merging
> the ifs versus not going into any ifs at all seems to be causing block
> placement to create a layout that pessimizes the real common case.
>
> Since all the compares are on the same variable, the originally code is
> kinda like a switch and as such we should maybe give each possible path
> equal weighting.
>
> Is there somewhere we can apply a heuristic to the branch probability so
> that it can provide a better answer here?
>
> ~Craig
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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This came from a benchmark so I can't change the code. The original source
code doesn't even have the 'else's. The jump threading pass is
determining
the ranges are mutex.

~Craig

On Fri, Aug 4, 2017 at 7:14 PM, Xinliang David Li <xinliangli at
gmail.com>
wrote:
> While it is possible that all the value ranges are equally likely, the
> opposite situation may be true too.   It is reasonable to assume that if
> the developer knows the program behavior, he/she will choose to test  the
> most likely range first .This can be used by the compiler as a heuristic to
> make it more likely (not that LLVM is doing it today).  The 50% BP is a
> sign that there is no known heuristic applied.
>
> For better performance, using PGO or sample PGO is the best possible way.
> Another direction is to use source annotation. Unfortunately,
> builtin_expect is too strong for many cases. One enhancement that can be
> done is to teach this builtin to take an additional likelihood parameter.
> Restructuring the code is also possible, e.g.  by compressing the range and
> using a switch ..
>
> David
>
>
> On Fri, Aug 4, 2017 at 4:10 PM, Craig Topper via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I'm look at some code that does something roughly like this
>>
>> if (a >= 10 && a < 20) {
>>   // do calculation
>> } else if (a >= 30 && a < 40) {
>>   // do calculation
>> } else if (a >= 50 && a < 60) {
>>   // do something
>> } else if (a >= 70 && a < 80) {
>>   // do something
>> }
>>
>> // some other code that got split based on whether we entered any of
the
>> 'if' bodies. So each if body merges to a slightly different
spot than the
>> not taken side of the final if.
>>
>> In my specific case there are really 8 specific range checks.
>>
>> Without profile data it seems branch probability assumes the branch of
>> each 'if' has 50% probability. Due to the way the ifs are
cascaded this
>> causes block frequency to think the final 'if' body can only
execute some
>> really small percentage of the time. Similarly it thinks that less than
%1
>> of the time will we enter the code path that occurs if none of the ifs
are
>> taken.
>>
>> In reality in my specific case we end up executing none of the ifs most
>> often. But I think i'd settle for all bodies and the no ifs taken
path
>> having equal weighting
>>
>> The bad block frequency and the fact that we have a split path for
>> merging the ifs versus not going into any ifs at all seems to be
causing
>> block placement to create a layout that pessimizes the real common
case.
>>
>> Since all the compares are on the same variable, the originally code is
>> kinda like a switch and as such we should maybe give each possible path
>> equal weighting.
>>
>> Is there somewhere we can apply a heuristic to the branch probability
so
>> that it can provide a better answer here?
>>
>> ~Craig
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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Slightly OT for the original question, but do we manage to factor out 
the common factor here and actually form the switch?

This could be rewritten as:

int tmp = a/10;
switch(tmp) {
case 1: ...
case 3: ...
case 5: ...
case 7: ...
}

Do we do so?

Philip

On 08/04/2017 04:10 PM, Craig Topper via llvm-dev wrote:
> I'm look at some code that does something roughly like this
>
> if (a >= 10 && a < 20) {
>   // do calculation
> } else if (a >= 30 && a < 40) {
>   // do calculation
> } else if (a >= 50 && a < 60) {
>   // do something
> } else if (a >= 70 && a < 80) {
>   // do something
> }
>
> // some other code that got split based on whether we entered any of 
> the 'if' bodies. So each if body merges to a slightly different
spot
> than the not taken side of the final if.
>
> In my specific case there are really 8 specific range checks.
>
> Without profile data it seems branch probability assumes the branch of 
> each 'if' has 50% probability. Due to the way the ifs are cascaded 
> this causes block frequency to think the final 'if' body can only 
> execute some really small percentage of the time. Similarly it thinks 
> that less than %1 of the time will we enter the code path that occurs 
> if none of the ifs are taken.
>
> In reality in my specific case we end up executing none of the ifs 
> most often. But I think i'd settle for all bodies and the no ifs taken 
> path having equal weighting
>
> The bad block frequency and the fact that we have a split path for 
> merging the ifs versus not going into any ifs at all seems to be 
> causing block placement to create a layout that pessimizes the real 
> common case.
>
> Since all the compares are on the same variable, the originally code 
> is kinda like a switch and as such we should maybe give each possible 
> path equal weighting.
>
> Is there somewhere we can apply a heuristic to the branch probability 
> so that it can provide a better answer here?
>
> ~Craig
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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In my original case we we definitely didn't. I sort of made up the numbers
in the example.

I think in the original code the first range was from 390-450. The second
range was from 840-900. Then 1290-1350, etc. There is a multiply by 60
proceeding the compares. But as you can see not all of the ranges start at
a multiple of 60. Though they are 60 entries wide.

~Craig

On Fri, Aug 11, 2017 at 11:31 AM, Philip Reames <listmail at
philipreames.com>
wrote:
> Slightly OT for the original question, but do we manage to factor out the
> common factor here and actually form the switch?
>
> This could be rewritten as:
> int tmp = a/10;
> switch(tmp) {
> case 1: ...
> case 3: ...
> case 5: ...
> case 7: ...
> }
>
> Do we do so?
>
> Philip
>
> On 08/04/2017 04:10 PM, Craig Topper via llvm-dev wrote:
>
> I'm look at some code that does something roughly like this
>
> if (a >= 10 && a < 20) {
>   // do calculation
> } else if (a >= 30 && a < 40) {
>   // do calculation
> } else if (a >= 50 && a < 60) {
>   // do something
> } else if (a >= 70 && a < 80) {
>   // do something
> }
>
> // some other code that got split based on whether we entered any of the
> 'if' bodies. So each if body merges to a slightly different spot
than the
> not taken side of the final if.
>
> In my specific case there are really 8 specific range checks.
>
> Without profile data it seems branch probability assumes the branch of
> each 'if' has 50% probability. Due to the way the ifs are cascaded
this
> causes block frequency to think the final 'if' body can only
execute some
> really small percentage of the time. Similarly it thinks that less than %1
> of the time will we enter the code path that occurs if none of the ifs are
> taken.
>
> In reality in my specific case we end up executing none of the ifs most
> often. But I think i'd settle for all bodies and the no ifs taken path
> having equal weighting
>
> The bad block frequency and the fact that we have a split path for merging
> the ifs versus not going into any ifs at all seems to be causing block
> placement to create a layout that pessimizes the real common case.
>
> Since all the compares are on the same variable, the originally code is
> kinda like a switch and as such we should maybe give each possible path
> equal weighting.
>
> Is there somewhere we can apply a heuristic to the branch probability so
> that it can provide a better answer here?
>
> ~Craig
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at
lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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