[llvm-dev] loop unrolling introduces conditional branch

[Xiangyang Guo via llvm-dev]

Hi,

I want to use loop unrolling pass, however, I find that loop unrolling will
introduces conditional branch at end of every "unrolled" part. For example,
consider the following code

*void foo( int n, int array_x[])*
*{*
*    for (int i=0; i < n; i++)*
*     array_x[i] = i; *
*}*

Then I use this command "opt-3.5 try.bc -mem2reg -loops -loop-simplify
-loop-rotate -lcssa -indvars -loop-unroll -unroll-count=3 -simplifycfg -S",
it gives me this IR:

*define void @_Z3fooiPi(i32 %n, i32* %array_x) #0 {*
*  %1 = icmp slt i32 0, %n*
*  br i1 %1, label %.lr.ph <http://lr.ph/>, label %._crit_edge*

*.lr.ph <http://lr.ph/>:                                           ; preds
= %0, %7*
*  %indvars.iv = phi i64 [ %indvars.iv.next.2, %7 ], [ 0, %0 ]*
*  %2 = getelementptr inbounds i32* %array_x, i64 %indvars.iv*
*  %3 = trunc i64 %indvars.iv to i32*
*  store i32 %3, i32* %2*
*  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1*
*  %lftr.wideiv = trunc i64 %indvars.iv.next to i32*
*  %exitcond = icmp ne i32 %lftr.wideiv, %n*
*  br i1 %exitcond, label %4, label %._crit_edge*

*._crit_edge:                                      ; preds = %.lr.ph
<http://lr.ph/>, %4, %7, %0*
*  ret void*

*; <label>:4                                       ; preds = %.lr.ph
<http://lr.ph/>*
*  %5 = getelementptr inbounds i32* %array_x, i64 %indvars.iv.next*
*  %6 = trunc i64 %indvars.iv.next to i32*
*  store i32 %6, i32* %5*
*  %indvars.iv.next.1 = add nuw nsw i64 %indvars.iv.next, 1*
*  %lftr.wideiv.1 = trunc i64 %indvars.iv.next.1 to i32*
*  %exitcond.1 = icmp ne i32 %lftr.wideiv.1, %n*
*  br i1 %exitcond.1, label %7, label %._crit_edge*

*; <label>:7                                       ; preds = %4*
*  %8 = getelementptr inbounds i32* %array_x, i64 %indvars.iv.next.1*
*  %9 = trunc i64 %indvars.iv.next.1 to i32*
*  store i32 %9, i32* %8*
*  %indvars.iv.next.2 = add nuw nsw i64 %indvars.iv.next.1, 1*
*  %lftr.wideiv.2 = trunc i64 %indvars.iv.next.2 to i32*
*  %exitcond.2 = icmp ne i32 %lftr.wideiv.2, %n*
*  br i1 %exitcond.2, label %.lr.ph <http://lr.ph/>, label %._crit_edge*
*}*

As you can see, at the end of BB <label>4 and BB<label>7 there are "add",
"icmp" and "br" instrcutions to check the boundary. I understand this is
for the correctness. However, I would expect the loop unrolling can change
my code to something like this:

*void foo( int n, int array_x[])*
*{*
*    int j = n%3;*
*    int m = n - j;*
*    for (int i=0; i < m; i+=3){*
*     array_x[i] = i;*
*     array_x[i+1] = i+1;*
*     array_x[i+2] = i+2; *
*    }*
*    for(i=m; i<n; i++)*
*     array_x[i] = i;        *
*}*

In this case, the BB<label>4 and BB<label>7 will do not have the "add",
"icmp" and "br" instructions because these BBs can be merged together.

How can I achieve this? Thanks.

Regards,

Xiangyang
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