incorrect line number reported for index out of range error

[mlubin]

If you run:

function foo()
    x = ones(10)
    print("first access\n")
    x[1]
    print("second access\n")
    x[20]
end
foo()

the result is

first access
second access
in foo: arrayref: index out of range
 in foo at none:4

Note the line number is incorrect. As in #900, this isn't helpful when trying to debug :)

[JeffBezanson]

Looking in to this. So far it seems to happen on x86/64 but not 32-bit. Disabling LLVM's CFG simplification pass also makes the problem go away (at the cost of some performance). Not sure if that is an LLVM issue or has to do with how I'm using it.

[timholy]

I think it's Christmas early this year. This will help a lot.

[JeffBezanson]

@vtjnash The BoundsError improvement seems to have broken this.

[vtjnash]

it seems this can be a pretty tough optimization to avoid, since llvm seems to detect the common sub-sequence and remove it (I don't know what pass is doing this, however):

oob1:                                             ; preds = %idxend
  %25 = alloca i64, align 8
  store i64 20, i64* %25, align 8
  call void @jl_bounds_error_ints(%jl_value_t* %20, i64* %25, i64 1)
  unreachable
oob:                                              ; preds = %top
  %17 = alloca i64, align 8
  store i64 1, i64* %17, align 8
  call void @jl_bounds_error_ints(%jl_value_t* %12, i64* %17, i64 1)
  unreachable

Source line: 4
L319:   movq    %rsp, %rax
    leaq    -16(%rax), %rsi
    movq    %rsi, %rsp
    movq    $1, -16(%rax)
    jmpq    L360
Source line: 6
L342:   movq    %rsp, %rax
    leaq    -16(%rax), %rsi
    movq    %rsi, %rsp
    movq    $20, -16(%rax)
Source line: 4
L360:   movabsq $jl_bounds_error_ints, %rax
    movl    $1, %edx
    callq   *%rax

[vtjnash]

this behavior in llvm can be "corrected" at it's source by turning off this optimization in the FPM http://llvm.org/docs/doxygen/html/CodeGen_2Passes_8h_source.html#l00160

(or with a command line flag: JULIA_LLVM_ARGS=-enable-tail-merge=0 ../usr/bin/julia-debug)