[llvm-dev] Potentially unsafe loop optimization

[Richard Kenner via llvm-dev]

> Long story short, from what I can see there is no miscompilation
> or change in semantics for that matter.

So why does the .s file not contain the loop exit test?

        .text
        .file   "c26006a.adb"
        .globl  _ada_c26006a                    # -- Begin function _ada_c26006a
        .p2align        4, 0x90
        .type   _ada_c26006a, at function
_ada_c26006a:                           # @_ada_c26006a
        .cfi_startproc
# %bb.0:                                # %entry
        pushq   %rbx
        .cfi_def_cfa_offset 16
        subq    $32, %rsp
        .cfi_def_cfa_offset 48
        .cfi_offset %rbx, -16
        movw    $8257, 16(%rsp)                 # imm = 0x2041
        movb    $49, 18(%rsp)
        movw    $8257, (%rsp)                   # imm = 0x2041
        movb    $50, 2(%rsp)
        xorl    %ebx, %ebx
        jmp     .LBB0_1
        .p2align        4, 0x90
.LBB0_3:                                # %loop.cond.iter
                                        #   in Loop: Header=BB0_1 Depth=1
        incb    %bl
.LBB0_1:                                # %loop.cond
                                        # =>This Inner Loop Header: Depth=1
        movb    %bl, 17(%rsp)
        movb    %bl, 1(%rsp)
        movzwl  (%rsp), %eax
        xorw    16(%rsp), %ax
        movzbl  2(%rsp), %ecx
        xorb    18(%rsp), %cl
        movzbl  %cl, %ecx
        orw     %ax, %cx
        jne     .LBB0_3
# %bb.2:                                #   in Loop: Header=BB0_1 Depth=1
        callq   abort
        jmp     .LBB0_3
.Lfunc_end0:
        .size   _ada_c26006a, .Lfunc_end0-_ada_c26006a
        .cfi_endproc
                                        # -- End function
        .section        ".note.GNU-stack","", at progbits