[llvm-bugs] [Bug 28036] New: LoopStrengthReduce does not choose the optimum IV

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28036

            Bug ID: 28036
           Summary: LoopStrengthReduce does not choose the optimum IV
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: arnaud.degrandmaison at arm.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16484
  --> https://llvm.org/bugs/attachment.cgi?id=16484&action=edit
C testcase

The following trivial testcase:

void foo(int * restrict a, int * restrict b, int * restrict c, unsigned LEN) {
  for (unsigned i = 0; i < LEN; i++)
    c[i] = a[i] + b[i];
}

Compile with : clang -target aarch64-linux-gnu -O2 -Wall -S -o - test2.c

 We get:
 .LBB0_3:
         and     x8, x9, #0x1fffffff8
         cbz     x8, .LBB0_7
         add     x10, x0, #16
         add     x11, x1, #16
         add     x12, x2, #16
         mov      x13, x8
 .LBB0_5:
         ldp     q0, q1, [x10, #-16]
         ldp     q2, q3, [x11, #-16]
         sub     x13, x13, #8 
         add     x10, x10, #32
         add     x11, x11, #32
         add     v0.4s, v2.4s, v0.4s
         add     v1.4s, v3.4s, v1.4s
         stp     q0, q1, [x12, #-16]
         add     x12, x12, #32
         cbnz    x13, .LBB0_5

 But I would have expected something like:
 .LBB0_3:
         and     x8, x9, #0x1fffffff8
         cbz     x8, .LBB0_7
         mov      x13, x8
 .LBB0_5:
         ldp     q0, q1, [x0], #32
         ldp     q2, q3, [x1], #32
         sub     x13, x13, #8
         add     v0.4s, v2.4s, v0.4s
         add     v1.4s, v3.4s, v1.4s
         stp     q0, q1, [x2], #32
         cbnz    x13, .LBB0_5

LoopStrengthReduce is transforming the code to a state which prevents using
writeback at all.

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