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

>
> 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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Thanks,
These are very helpful.

As I understand it, SROA and LICM render some variables
"useless" by optimizing the code to not use them. Hence we can't
debug them.

FWIW, I've raised the LICM issue here:
https://bugs.llvm.org/show_bug.cgi?id=37682

On 31 May 2018 at 13:28, Anast Gramm <anastasis.gramm2 at gmail.com>
wrote:
> Thanks,
> These are very helpful.
>
> As I understand it, SROA and LICM render some variables
> "useless" by optimizing the code to not use them. Hence we
can't debug
> them.
>
>
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