[LLVMdev] LLVM Loop Vectorizer puzzle

[Arnold Schwaighofer]

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.
> 
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