llvm dev - May 2018 - [SROA][DebugInfo][GSoC] Testing SROA on amalgamated sqlite source

Introduction
===========
  `SROA' is an early stage pass running at the very beginning of the
  pipeline in `-O{1,2,3}'. Greg Bedwell's report from his DExTer tool
  shows SROA on function as one of the major culprits of Debug Info
  loss.

  With debugify-each partially done I tried testing this on the
  amalgamated sqlite source.

  The steps are as follows:
  ,----
  | # generate IR file without debug info and optnone
  | clang -O0 -Xclang -disable-O0-optnone -S -emit-llvm sqlite3.c -o sqlite
  | 
  | # run opt
  | opt -sroa -debugify-each sqlite -disable-output 2> results
  `----


Results
======
  [Here] is the full results file.

  The results were gather by `grep -c`'ing the results file according to
  the error's message.

   SROA runs | FAIL | PASS | empty DebugLoc | Missing line warnings 
  -----------|------|------|----------------|-----------------------
        1978 | 1100 |  878 |           5214 |                 75166 

  What's worth noticing is that the only error that SROA produces is
  that of empty debug locations. It does not report any missing
  DILocalVariables or llvm.dbg.value intrinsics. Thus all of the 1100
  SROA runs that failed are due to empty debug locations.

  Also, while skimming through the results file I noticed that the
  instructions with missing DL were `phi' instructions. To confirm this
  I did:

    $ grep "ERROR: Instruction" results | grep phi -c

  witch gives us a result of `5214' and this means that all the
  instructions with missing Debug Loc are `phi' instructions.


[Here] https://gramanas.github.io/sroa-results.csv


Conclusion
=========
  This implies SROA does a pretty good job of preserving debug values.

  The DExTer results are not in line with what I've found though. This
  might be for a number of reasons. E.g. it could score so low due to
  the DebugLoc loss. Also debugify doesn't test the backend so it's
  going to detect a narrower set of issues.

  Do you have any ideas as to why this happens? Can you confirm the
  results?

  @Adrian, @Greg any thoughts would be appreciated!

> On May 30, 2018, at 8:37 AM, Anast Gramm <anastasis.gramm2 at
gmail.com> wrote:
> 
> Introduction
> ===========> 
>  `SROA' is an early stage pass running at the very beginning of the
>  pipeline in `-O{1,2,3}'. Greg Bedwell's report from his DExTer
tool
>  shows SROA on function as one of the major culprits of Debug Info
>  loss.
> 
>  With debugify-each partially done I tried testing this on the
>  amalgamated sqlite source.
> 
>  The steps are as follows:
>  ,----
>  | # generate IR file without debug info and optnone
>  | clang -O0 -Xclang -disable-O0-optnone -S -emit-llvm sqlite3.c -o sqlite
>  | 
>  | # run opt
>  | opt -sroa -debugify-each sqlite -disable-output 2> results
>  `----
> 
> 
> Results
> ======> 
>  [Here] is the full results file.
> 
>  The results were gather by `grep -c`'ing the results file according to
>  the error's message.
> 
>   SROA runs | FAIL | PASS | empty DebugLoc | Missing line warnings 
>  -----------|------|------|----------------|-----------------------
>        1978 | 1100 |  878 |           5214 |                 75166 
> 
>  What's worth noticing is that the only error that SROA produces is
>  that of empty debug locations. It does not report any missing
>  DILocalVariables or llvm.dbg.value intrinsics. Thus all of the 1100
>  SROA runs that failed are due to empty debug locations.
> 
>  Also, while skimming through the results file I noticed that the
>  instructions with missing DL were `phi' instructions. To confirm this
>  I did:
> 
>    $ grep "ERROR: Instruction" results | grep phi -c
> 
>  witch gives us a result of `5214' and this means that all the
>  instructions with missing Debug Loc are `phi' instructions.
> 
> 
> [Here] https://gramanas.github.io/sroa-results.csv
> 
> 
> Conclusion
> =========> 
>  This implies SROA does a pretty good job of preserving debug values.
> 
That is good to hear / expected, since we went though great lengths to ensure
that SROA preserves variable debug info.

Have you looked at the location-less phi instructions? Does empty mean line 0 or
do you mean they have no debug location whatsoever? Are they empty for good
reasons or does it look more like an oversight in the implementation?

-- adrian
>  The DExTer results are not in line with what I've found though. This
>  might be for a number of reasons. E.g. it could score so low due to
>  the DebugLoc loss. Also debugify doesn't test the backend so it's
>  going to detect a narrower set of issues.
> 
>  Do you have any ideas as to why this happens? Can you confirm the
>  results?
> 
>  @Adrian, @Greg any thoughts would be appreciated!

>
>
>   `SROA' is an early stage pass running at the very beginning of the
>   pipeline in `-O{1,2,3}'. Greg Bedwell's report from his DExTer
tool
>   shows SROA on function as one of the major culprits of Debug Info
>   loss.
>
>
The methodology I used is with the opt-bisect-limit option on clang, so
it's not strictly the case that the results presented show that the loss in
"debugability" occurs during the SROA pass itself directly but rather
that
the effect of running SROA is some loss in debugability to the final
program.  I didn't make that point clear enough during my lightning talk
session!

Looking at some of the results in more detail, I think the reason that the
reason our results differ is because what I'm measuring is not actually
variable info loss directly, but as a consequence of the impact on
stepping.  My standard disclaimer with all results from this tool is that a
non-perfect debugging experience score is not necessarily indicative of
something wrong, but should be looked at in conjunction with all the other
factors such as what the pass is trying to achieve optimization-wise, etc
etc.  That is to say, it's not necessarily a bug, but it can be (especially
when observed as a regression between compiler revisions).

Using the standard Fibonacci example with clang and lldb from this
afternoon:

 1 #ifdef _MSC_VER
 2 # define DEX_NOINLINE __declspec(noinline)
 3 #else
 4 # define DEX_NOINLINE __attribute__((__noinline__))
 5 #endif
 6
 7 DEX_NOINLINE
 8 void Fibonacci(int terms, int& total)
 9 {
10   int first = 0;
11   int second = 1;
12
13   for (int i = 0; i < terms; ++i)
14   {
15     int next = first + second; // DexWatch('i', 'first',
'second',
'total')
16     total += first;            // DexWatch('i', 'first',
'second',
'total', 'next')
17     first = second;            // DexWatch('i', 'first',
'second',
'total', 'next')
18     second = next;             // DexWatch('i', 'first',
'second',
'total', 'next')
19   }
20 }
21
22 int main()
23 {
24   int total = 0;
25   Fibonacci(5, total);
26   return total;
27 }

not running  "SROA on function (_Z9FibonacciiRi)" the DExTer trace
output
is:

## BEGIN ##
[1, "main", "tests/fibonacci/test.cpp", 24, 6,
"BREAKPOINT", "FUNC", {}]
[2, "main", "tests/fibonacci/test.cpp", 25, 2,
"BREAKPOINT", "FORWARD", {}]
.   [3, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 10, 6,
"BREAKPOINT", "FUNC", {}]
.   [4, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 11, 6,
"BREAKPOINT", "FORWARD", {}]
.   [5, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 11,
"BREAKPOINT", "FORWARD", {}]
.   [6, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 14,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0"}]
.   [7, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 12,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0", "next": "1"}]
.   [8, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 17, 11,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0", "next": "1"}]
.   [9, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 18, 12,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"1", "next": "1"}]
.   [10, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [11, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 14,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "0",
"first":
"1"}]
.   [12, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 12,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "0",
"first":
"1", "next": "2"}]
.   [13, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 17, 11,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "1",
"first":
"1", "next": "2"}]
.   [14, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 18, 12,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "1",
"first":
"1", "next": "2"}]
.   [15, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [16, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 14,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "1",
"first":
"1"}]
.   [17, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 12,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "1",
"first":
"1", "next": "3"}]
.   [18, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 17, 11,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "2",
"first":
"1", "next": "3"}]
.   [19, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 18, 12,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "2",
"first":
"2", "next": "3"}]
.   [20, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [21, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 14,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "2",
"first":
"2"}]
.   [22, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 12,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "2",
"first":
"2", "next": "5"}]
.   [23, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 17, 11,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "4",
"first":
"2", "next": "5"}]
.   [24, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 18, 12,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "4",
"first":
"3", "next": "5"}]
.   [25, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [26, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 14,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "4",
"first":
"3"}]
.   [27, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 12,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "4",
"first":
"3", "next": "8"}]
.   [28, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 17, 11,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "7",
"first":
"3", "next": "8"}]
.   [29, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 18, 12,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "7",
"first":
"5", "next": "8"}]
.   [30, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [31, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 20, 1,
"BREAKPOINT", "FORWARD", {}]
[32, "main", "tests/fibonacci/test.cpp", 26, 9,
"BREAKPOINT", "FUNC", {}]
## END (32 steps) ##

So we do a couple of steps in main, before stepping into Fibonacci(int,
int&) at step 3.  It's fairly easy to see that we iterate the loop 5
times,
stepping on lines 13, 15, 16, 17 and 18 in order each time.

After "SROA on function (_Z9FibonacciiRi)" we now get:

## BEGIN ##
[1, "main", "tests/fibonacci/test.cpp", 24, 6,
"BREAKPOINT", "FUNC", {}]
[2, "main", "tests/fibonacci/test.cpp", 25, 2,
"BREAKPOINT", "FORWARD", {}]
.   [3, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 20,
"BREAKPOINT", "FUNC", {}]
.   [4, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0"}]
.   [5, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0", "next": "1"}]
.   [6, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29, "STEP",
"BACKWARD", {}]
.   [7, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 20,
"BREAKPOINT", "BACKWARD", {}]
.   [8, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "0",
"first":
"1"}]
.   [9, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "0",
"first":
"1", "next": "2"}]
.   [10, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [11, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 20,
"BREAKPOINT", "BACKWARD", {}]
.   [12, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "1",
"first":
"1"}]
.   [13, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "1",
"first":
"1", "next": "3"}]
.   [14, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [15, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 20,
"BREAKPOINT", "BACKWARD", {}]
.   [16, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "2",
"first":
"2"}]
.   [17, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "2",
"first":
"2", "next": "5"}]
.   [18, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [19, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 20,
"BREAKPOINT", "BACKWARD", {}]
.   [20, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "4",
"first":
"3"}]
.   [21, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "4",
"first":
"3", "next": "8"}]
.   [22, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [23, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 20,
"BREAKPOINT", "BACKWARD", {}]
.   [24, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 20, 1,
"BREAKPOINT", "FORWARD", {}]
[25, "main", "tests/fibonacci/test.cpp", 26, 9,
"BREAKPOINT", "FUNC", {}]
## END (25 steps) ##

So we've now gained some extra steps at line 13, but at different columns
(the "++i" and "i < terms" are now treated as separate
steps), but we're
now also no longer ever stepping onto lines 17 or 18 at all.  Whether these
are the expected effects of SROA are not something DExTer is in the
business of deciding, but it does score it as a difference because it now,
at no point, sees the expression "first" evaluate to a value of 5, or
"total" to a value of 8 which it did previously.  From the
source-level
debugging experience, the variables now just get updated between iterations.

So in short, it seems SROA is having some effect on debugging but it seems
that it's not in the form of failing to preserve variable info.
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>
> is in the business of deciding, but it does score it as a difference
> because it now, at no point, sees the expression "first" evaluate
to a
> value of 5, or "total" to a value of 8 which it did previously. 
From the
> source-level debugging experience, the variables now just get updated
> between iterations.
>
Obviously should've been 7 for that value of "total".

To try and be a bit more helpful there's definitely something dodgy
happening to variable visibility on the fibonacci example in LICM.

With "BISECT: NOT running pass (32) Loop Invariant Code Motion on
loop":

## BEGIN ##
[1, "main", "tests/fibonacci/test.cpp", 24, 6,
"BREAKPOINT", "FUNC", {}]
[2, "main", "tests/fibonacci/test.cpp", 25, 2,
"BREAKPOINT", "FORWARD", {}]
.   [3, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 20,
"BREAKPOINT", "FUNC", {}]
.   [4, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0"}]
.   [5, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0", "next": "1"}]
.   [6, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29, "STEP",
"BACKWARD", {}]
.   [7, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "0",
"first":
"1"}]
.   [8, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "0",
"first":
"1", "next": "2"}]
.   [9, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29, "STEP",
"BACKWARD", {}]
.   [10, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "1",
"first":
"1"}]
.   [11, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "1",
"first":
"1", "next": "3"}]
.   [12, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [13, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "2",
"first":
"2"}]
.   [14, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "2",
"first":
"2", "next": "5"}]
.   [15, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [16, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "4",
"first":
"3"}]
.   [17, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "4",
"first":
"3", "next": "8"}]
.   [18, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [19, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 20, 1,
"BREAKPOINT", "FORWARD", {}]
[20, "main", "tests/fibonacci/test.cpp", 26, 9,
"BREAKPOINT", "FUNC", {}]
## END (20 steps) ##

But with: "BISECT: running pass (32) Loop Invariant Code Motion on
loop":

## BEGIN ##
[1, "main", "tests/fibonacci/test.cpp", 24, 6,
"BREAKPOINT", "FUNC", {}]
[2, "main", "tests/fibonacci/test.cpp", 25, 2,
"BREAKPOINT", "FORWARD", {}]
.   [3, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 20,
"BREAKPOINT", "FUNC", {}]
.   [4, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0", "next": null}]
.   [5, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 2, "STEP",
"BACKWARD", {}]
.   [6, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "0",
"second": "1", "total": "0",
"first":
"0"}]
.   [7, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9, "STEP",
"FORWARD", {"i": "0", "second":
"1", "total": "0", "first":
"0", "next":
"1"}]
.   [8, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29, "STEP",
"BACKWARD", {}]
.   [9, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "1",
"second": "1", "total": "0",
"first":
"1"}]
.   [10, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9, "STEP",
"FORWARD", {"i": "1", "second":
"1", "total": "0", "first":
"1", "next":
"2"}]
.   [11, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [12, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "2",
"second": "2", "total": "0",
"first":
"1"}]
.   [13, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9, "STEP",
"FORWARD", {"i": "2", "second":
"2", "total": "0", "first":
"1", "next":
"3"}]
.   [14, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [15, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "3",
"second": "3", "total": "0",
"first":
"2"}]
.   [16, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9, "STEP",
"FORWARD", {"i": "3", "second":
"3", "total": "0", "first":
"2", "next":
"5"}]
.   [17, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [18, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 15, 20,
"BREAKPOINT", "FORWARD", {"i": "4",
"second": "5", "total": "0",
"first":
"3"}]
.   [19, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9, "STEP",
"FORWARD", {"i": "4", "second":
"5", "total": "0", "first":
"3", "next":
"8"}]
.   [20, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 13, 29,
"STEP", "BACKWARD", {}]
.   [21, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 16, 9, "STEP",
"FORWARD", {"i": "5", "second":
"8", "total": "0", "first":
"5", "next":
"8"}]
.   [22, "Fibonacci(int, int&)",
"tests/fibonacci/test.cpp", 20, 1,
"BREAKPOINT", "FORWARD", {}]
[23, "main", "tests/fibonacci/test.cpp", 26, 9,
"BREAKPOINT", "FUNC", {}]
## END (23 steps) ##

The difference here that really sticks out as a problem to me is reported
by the heuristic like this:

  test.cpp:15-18 [total] [21/21]
    expected encountered values:
      0

    missing values:
      1 [+6]
      2 [+6]
      4 [+6]
      7 [+3]

So, "total" evaluates to 0 for the duration of Fibonacci(int,
int&) which
definitely smells wrong to me.  Prior to that it was evaluating to 0, 1, 2
and 4 in order (we stopped seeing 7 after SROA).

-Greg
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> That is good to hear / expected, since we went though great lengths to
ensure that SROA preserves variable debug info.
> 
> Have you looked at the location-less phi instructions?
> Does empty mean line 0 or do you mean they have no debug location
whatsoever?
Most of them have line 0, only 12 are locationless.
> Are they empty for good reasons or does it look more like an oversight in
the implementation?
How can I approach this? My guess is that all these phi's are with line 0
because they are made
of stores and allocas that aren't actual code, and are instead generated by
clang. Like here
https://reviews.llvm.org/D47097 at test1