[llvm-bugs] [Bug 43835] New: Differences between `base+width*i` and `base+=width`

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43835

            Bug ID: 43835
           Summary: Differences between `base+width*i` and `base+=width`
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

https://godbolt.org/z/TR-s8A

#include <array>
#include <algorithm>

void sink(int* row);

void v0(int* data, int len, int width) {
    for(int i = 0; i < len; i++) {
        int *row = data + width*i;
        sink(row);
    }
}
void v1(int* row, int len, int width) {
    for(int i = 0; i < len; i++, row+=width) {
        sink(row);
    }
}

In first case we get

9:                                                ; preds = %9, %5
  %10 = phi i64 [ 0, %5 ], [ %13, %9 ]
  %11 = mul nsw i64 %10, %6
  %12 = getelementptr inbounds i32, i32* %0, i64 %11
  tail call void @_Z4sinkPi(i32* %12)
  %13 = add nuw nsw i64 %10, 1

but in second:

  %9 = phi i32 [ 0, %5 ], [ %11, %8 ]
  %10 = phi i32* [ %0, %5 ], [ %12, %8 ]
  tail call void @_Z4sinkPi(i32* %10)
  %11 = add nuw nsw i32 %9, 1
  %12 = getelementptr inbounds i32, i32* %10, i64 %6


I'd guess the second variant is better?
The difference is interesting.

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