[llvm-bugs] [Bug 51185] New: Sub-optimal placement of loop exit blocks

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51185

            Bug ID: 51185
           Summary: Sub-optimal placement of loop exit blocks
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: momchil.velikov at arm.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 25057
  --> https://bugs.llvm.org/attachment.cgi?id=25057&action=edit
test case

In the attached test case there's is a (fully unrolled) loop roughly of the
following form

    for ... {
       if (cond) {
          stuff();
          break;
       }
       more_stuff();
    }

when compiled for AArch64 with `llc store.ll`, the output looks like:

// iteration 0
        cmp     w11, w12
        b.ne    .LBB0_2

        mov     x11, xzr
        b       .LBB0_14
.LBB0_2:
        ...

// iteration 1
        cmp     w17, w12
        b.ne    .LBB0_6

        add     w10, w10, #1
        mov     w11, #1    
        b       .LBB0_14
.LBB0_6:
        ...

// iteration 2
        cmp     w15, w12
        b.ne    .LBB0_10

        add     w10, w10, #2
        mov     w11, #2
        b       .LBB0_14
.LBB0_10:
        ...

// iteration 3
        cmp     w16, w12
        b.ne    .LBB0_15

        add     w10, w10, #3
        mov     w11, #3
        // fallthrough
.LBB0_14:
        // iterations 0, 1, 2, 3 early exit
        ...

i.e. the exit blocks are interspersed with "proper" body blocks and the 
execution constantly jumps over them. In contrast, GCC's output looks like:

// iteration 0
        ...
        cmp     w6, w0
        beq     .L19
        ...

// iteration 1
        ...
        cmp     w2, w6
        beq     .L20
        ...  

// iteration 2
        ...
        cmp     w6, w4
        beq     .L22
        ...
// iteration 3
        ...
        cmp     w2, w6
        beq     .L23


i.e. exit blocks an entirely stashed away and loop iterations are executed
essentially like straight line code.

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