llvm dev - May 2013 - [LLVMdev] LLVM Loop Vectorizer puzzle

Hi,
     I have the llvm loop vectorizer to complie the following sample:
//================
int test(int *a, int n) {

         for(int i = 0; i < n; i++) {

                   a[i] += i;

         }

         return 0;

}

//===============        The corresponded .ll file has a loop preheader:
//===============
for.body.lr.ph:                                   ; preds = %entry
   %cnt.cast = zext i32 %n to i64
   %0 = and i32 %n, 7
   %n.mod.vf = zext i32 %0 to i64
   %n.vec = sub i64 %cnt.cast, %n.mod.vf
   %cmp.zero = icmp eq i32 %0, %n
   br i1 %cmp.zero, label %middle.block, label %vector.body
//===============
        That is , if n <=7, the program will skip the vector.body. In 
LoopVectorize.cpp , I see the following code:
//===============
             static cl::opt<unsigned>
TinyTripCountVectorThreshold("vectorizer-min-trip-count",
cl::init(16),
... );
//===============
       The minimal loop count is 16. What is the "n<=7" meaning?  
Thanks.

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Hi,

The TinyTripCountVectorThreshold only applies to loops with a known (constant)
trip count. If a loop has a trip count below this value we don’t attempt to
vectorize the loop. The loop below has an unknown trip count.

Once we decide to vectorize a loop, we emit code to check whether we can execute
one iteration of the vectorized body. This is the code quoted below.

On May 22, 2013, at 10:23 PM, maxs <xtxwy.ustc at gmail.com> wrote:
> Hi,
>     I have the llvm loop vectorizer to complie the following sample:
> //================> int test(int *a, int n) {
> 
>         for(int i = 0; i < n; i++) {
> 
>                   a[i] += i;
> 
>         }
> 
> 
>         return 0;
> 
> } 
> 
> //===============>        The corresponded .ll file has a loop
preheader:
> //===============> 
> for.body.lr.ph:                                   ; preds = %entry
>   %cnt.cast = zext i32 %n to i64
>   %0 = and i32 %n, 7
>   %n.mod.vf = zext i32 %0 to i64
>   %n.vec = sub i64 %cnt.cast, %n.mod.vf
>   %cmp.zero = icmp eq i32 %0, %n
>   br i1 %cmp.zero, label %middle.block, label %vector.body
> //===============> 
>        That is , if n <=7, the program will skip the vector.body. In
LoopVectorize.cpp , I see the following code:
> //================ 
> 
>             static cl::opt<unsigned>
>            
TinyTripCountVectorThreshold("vectorizer-min-trip-count",
cl::init(16), ... );
> //===============> 
>       The minimal loop count is 16. What is the "n<=7"
meaning?   Thanks.
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi,

Just from personal interest, is there a canonical way in IR+metadata to
express "This small constant trip-count loop is desired to be converted
into a sequence of vector operations directly"? Ie, mapping a 4 element i32
loop into a linear sequence of <4 x i32> operations. Obviously this may
not
always be a win, but I'm just wondering if there's a way to communicate
this intent and get around the vectorizer-min-trip-count in specially
desired cases, or if I should decide to generate vectorized IR directly.
(This is in code coming from a DSL which will impliciltly insert
annotations, not manually written loops.)

Cheers,
Dave



On Thu, May 23, 2013 at 5:06 AM, Arnold Schwaighofer <
aschwaighofer at apple.com> wrote:
> Hi,
>
> The TinyTripCountVectorThreshold only applies to loops with a known
> (constant) trip count. If a loop has a trip count below this value we don’t
> attempt to vectorize the loop. The loop below has an unknown trip count.
>
> Once we decide to vectorize a loop, we emit code to check whether we can
> execute one iteration of the vectorized body. This is the code quoted
below.
>
> On May 22, 2013, at 10:23 PM, maxs <xtxwy.ustc at gmail.com> wrote:
>
> > Hi,
> >     I have the llvm loop vectorizer to complie the following sample:
> > //================> > int test(int *a, int n) {
> >
> >         for(int i = 0; i < n; i++) {
> >
> >                   a[i] += i;
> >
> >         }
> >
> >
> >         return 0;
> >
> > }
> >
> > //===============> >        The corresponded .ll file has a loop
preheader:
> > //===============> >
> > for.body.lr.ph:                                   ; preds = %entry
> >   %cnt.cast = zext i32 %n to i64
> >   %0 = and i32 %n, 7
> >   %n.mod.vf = zext i32 %0 to i64
> >   %n.vec = sub i64 %cnt.cast, %n.mod.vf
> >   %cmp.zero = icmp eq i32 %0, %n
> >   br i1 %cmp.zero, label %middle.block, label %vector.body
> > //===============> >
> >        That is , if n <=7, the program will skip the vector.body.
In
> LoopVectorize.cpp , I see the following code:
> > //===============> >
> >             static cl::opt<unsigned>
> >            
TinyTripCountVectorThreshold("vectorizer-min-trip-count",
> cl::init(16), ... );
> > //===============> >
> >       The minimal loop count is 16. What is the "n<=7"
meaning?   Thanks.
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>


-- 
cheers, dave tweed__________________________
high-performance computing and machine vision expert: david.tweed at gmail.com
"while having code so boring anyone can maintain it, use Python." --
attempted insult seen on slashdot
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