Loop reroller not able to reroll when the array base address is a derivative of the loop induction variable

[llvmbot]

Bugzilla Link	18646
Version	trunk
OS	Linux
Attachments	test.c, test.ll
Reporter	LLVM Bugzilla Contributor
CC	@hfinkel

Extended Description

The loop reroller is not able to reroll loops of the form where the array base address is a derivative of the loop induction variable.

$ cat test.c

void foo(int *Arr, int n) {
  for (int i = 0; i < n; i += 3) {
     Arr[0]=0;
     Arr[1]=0;
     Arr[2]=0;
     Arr+=3;
   }
}

$ clang -target -arm-none-linux-gnueabi reroll.cpp -S  -O1 -emit-llvm -o test.ll
$ opt -loop-reroll  -S test.ll -debug-only=loop-reroll -stats

This is probably because as the array accesses are not a function of the loop induction variable, which is the loop body form the reroller pass requires.

$ cat test.ll

..
..
for.body:                                         ; preds = %entry, %for.body
  %i.09 = phi i32 [ %add, %for.body ], [ 0, %entry ]
  %Arr.addr.08 = phi i32* [ %add.ptr, %for.body ], [ %Arr, %entry ]
  store i32 0, i32* %Arr.addr.08, align 4, !tbaa !​1
..
..

The following pattern is successfully rerolled as it satisfies the requirement,

void foo(int *Arr, int n) {
  for (int i = 0; i < n; i += 3) {
     Arr[i]=0;
     Arr[i+1]=0;
     Arr[i+2]=0;
   }
}

[nikic]

The LoopReroll pass has been removed by #80972.