llvm dev - Feb 2020 - Have the debugger show an away with a dynamic size?

Hi. I searched and the closest thing I could find was this
http://lists.llvm.org/pipermail/llvm-dev/2018-February/121348.html

Currently a known sized array looks and debugs as expected. I use
llvm.dbg.declare with DICompositeType tag: DW_TAG_array_type and the size
field. In my language arrays are always passed around with a pointer and
size pair. I'd like debugging to show up as nicely instead of a pointer
addr with no information about the elements. How would I do this? I don't
use the C API, I output llvm-ir directly. I was hoping I can call
llvm.dbg.declare/addr/value to specify the pointer, name and size of the
variable but I really have no idea how to pass the size to the debugger.
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If you use the Clang options `-emit-llvm -S` it will produce a .ll file
containing the textual IR produced by Clang.
Use this to compile a simple C example using a VLA and it should give you a
model of the IR and metadata you would need to produce.  I used a source
something like this:
    int foo(int s) {
      int a[s];
      int i;
      for (i = 0; i < s; ++i)
        a[i] = s;
      return 0;
    }

What I see is a DILocalVariable for “a” that has a type that is a
DICompositeType with DW_TAG_array_type, and the `elements` array of the
DICompositeType points to a DISubrange whose `count` points to a DILocalVariable
that is the upper bound.

The resulting DWARF looks okay to me, although I admit I haven’t actually tried
running gdb on it to make sure.

I haven’t looked up the DIBuilder APIs, if that’s what you are looking for, but
based on what you said, it sounds like you would be able to work that out
yourself.
HTH,
--paulr

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Levo
DeLellis via llvm-dev
Sent: Thursday, February 13, 2020 1:03 PM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Have the debugger show an away with a dynamic size?

Hi. I searched and the closest thing I could find was this
http://lists.llvm.org/pipermail/llvm-dev/2018-February/121348.html

Currently a known sized array looks and debugs as expected. I use
llvm.dbg.declare with DICompositeType tag: DW_TAG_array_type and the size field.
In my language arrays are always passed around with a pointer and size pair.
I'd like debugging to show up as nicely instead of a pointer addr with no
information about the elements. How would I do this? I don't use the C API,
I output llvm-ir directly. I was hoping I can call llvm.dbg.declare/addr/value
to specify the pointer, name and size of the variable but I really have no idea
how to pass the size to the debugger.

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Take a look at the IR clang produces for C99 variable-length arrays.

-- adrian
> On Feb 13, 2020, at 10:03 AM, Levo DeLellis via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi. I searched and the closest thing I could find was this
http://lists.llvm.org/pipermail/llvm-dev/2018-February/121348.html
<http://lists.llvm.org/pipermail/llvm-dev/2018-February/121348.html>
> 
> Currently a known sized array looks and debugs as expected. I use
llvm.dbg.declare with DICompositeType tag: DW_TAG_array_type and the size field.
In my language arrays are always passed around with a pointer and size pair.
I'd like debugging to show up as nicely instead of a pointer addr with no
information about the elements. How would I do this? I don't use the C API,
I output llvm-ir directly. I was hoping I can call llvm.dbg.declare/addr/value
to specify the pointer, name and size of the variable but I really have no idea
how to pass the size to the debugger.
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Thanks for the suggestions but it doesn't appear to be working correctly
for me. I tried building the below after seeing the results with "clang -g
-std=c99 test.c" and got the same result

LLDB thinks MyArray is 81 elements long even though 81 and 80 doesn't show
up anywhere in the llvm-ir (I tried again using an llvm ir file made by
clang -g -std=c99 test.c -S -emit-llvm and clang -g test.ll)

$ cat test.c
int foo(int s) {
    int MyArray[s];
    int i;
    for (i = 0; i < s; ++i)
        MyArray[i] = s;
    return 0;
}

int main(){
    foo(5);
    return 0;
}
$ clang -g test.c
$ lldb ./a.out
(lldb) target create "./a.out"
Current executable set to './a.out' (x86_64).
(lldb) break set -f test.c -l 6
Breakpoint 1: where = a.out`foo + 101 at test.c:7, address 0x0000000000400505
(lldb) r
Process 3205 launched: './a.out' (x86_64)
Process 3205 stopped
* thread #1, name = 'a.out', stop reason = breakpoint 1.1
    frame #0: 0x0000000000400505 a.out`foo(s=5) at test.c:7
   4      for (i = 0; i < s; ++i)
   5          MyArray[i] = s;
   6      return 0;
-> 7   }
   8
   9   int main(){
   10      foo(5);
(lldb) frame variable
(int) s = 5
(unsigned long) __vla_expr0 = 5
(int) i = 5
(int [81]) MyArray = {
  [0] = 5
  [1] = 5
  [2] = 5
  [3] = 5
  [4] = 5
  [5] = 0
  [6] = -136481184
  [7] = 32767
  [8] = -8408
  [9] = 32767
  [10] = -8544
  [11] = 32767
  [12] = 1
  [13] = 5
  [14] = 5
  [15] = 0
  [16] = -8512
  [17] = 32767
  [18] = 0
  [19] = 5
  [20] = -8432
  [21] = 32767
  [22] = 4195641
  [23] = 0
  [24] = -8208
  [25] = 32767
  [26] = 0
  [27] = 0
  [28] = 4195664
  [29] = 0
  [30] = -140485737
  [31] = 32767
  [32] = 0
  [33] = 32
  [34] = -8200
  [35] = 32767
  [36] = 0
  [37] = 1
  [38] = 4195616
  [39] = 0
  [40] = 0
  [41] = 0
  [42] = -1953144313
  [43] = 1284291557
  [44] = 4195248
  [45] = 0
  [46] = -8208
  [47] = 32767
  [48] = 0
  [49] = 0
  [50] = 0
  [51] = 0
  [52] = 1064657415
  [53] = -1284291430
  [54] = 933978631
  [55] = -1284287451
  [56] = 0
  [57] = 32767
  [58] = 0
  [59] = 0
  [60] = 0
  [61] = 0
  [62] = -136423629
  [63] = 32767
  [64] = -136530376
  [65] = 32767
  [66] = 386784
  [67] = 0
  [68] = 0
  [69] = 0
  [70] = 0
  [71] = 0
  [72] = 0
  [73] = 0
  [74] = 4195248
  [75] = 0
  [76] = -8208
  [77] = 32767
  [78] = 4195290
  [79] = 0
  [80] = -8216
}


On Thu, Feb 13, 2020 at 3:53 PM Adrian Prantl <aprantl at apple.com>
wrote:
> Take a look at the IR clang produces for C99 variable-length arrays.
>
> -- adrian
>
> On Feb 13, 2020, at 10:03 AM, Levo DeLellis via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi. I searched and the closest thing I could find was this
> http://lists.llvm.org/pipermail/llvm-dev/2018-February/121348.html
>
> Currently a known sized array looks and debugs as expected. I use
> llvm.dbg.declare with DICompositeType tag: DW_TAG_array_type and the size
> field. In my language arrays are always passed around with a pointer and
> size pair. I'd like debugging to show up as nicely instead of a pointer
> addr with no information about the elements. How would I do this? I
don't
> use the C API, I output llvm-ir directly. I was hoping I can call
> llvm.dbg.declare/addr/value to specify the pointer, name and size of the
> variable but I really have no idea how to pass the size to the debugger.
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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