Re: GNAT can't vectorize Real_Matrix multiplication from Ada.Numerics.Real_Arrays. What a surprise!

[Bojan Bozovic <bozovic.bojan@gmail.com>]

On Saturday, February 17, 2018 at 1:55:49 PM UTC+1, Bojan Bozovic wrote:
> -- code
> 
> with Ada.Calendar;
> with Ada.Text_IO;
> with Ada.Numerics.Real_Arrays;
> use Ada.Calendar;
> use Ada.Text_IO;
> use Ada.Numerics.Real_Arrays;
> 
> procedure Matrix_Mul is
> 
>    package F_IO is new Ada.Text_IO.Fixed_IO (Day_Duration);
>    use F_IO;
> 
>    Start_Time, End_Time : Time;
> 
>    procedure Put (X : Real_Matrix) is
>    begin
>       for I in X'Range (1) loop
>          for J in X'Range (2) loop
>             Put (Float'Image (Float (X (I, J))));
>          end loop;
>          New_Line;
>       end loop;
>    end Put;
> 
>    Matrix_A, Matrix_B, Result : Real_Matrix (1 .. 4, 1 .. 4);
>    Elapsed_Time               : Duration;
>    Sum                        : Float;
> begin
>    Matrix_A :=
>      ((1.0, 1.0, 1.0, 1.0),
>       (2.0, 2.0, 2.0, 2.0),
>       (3.0, 3.0, 3.0, 3.0),
>       (4.0, 4.0, 4.0, 4.0));
>    Matrix_B :=
>      ((16.0, 15.0, 14.0, 13.0),
>       (12.0, 11.0, 10.0, 9.0),
>       (8.0, 7.0, 6.0, 5.0),
>       (4.0, 3.0, 2.0, 1.0));
> 
>    Start_Time := Clock;
>    for Iteration in 1 .. 10_000_000 loop
>       Result := Matrix_A * Matrix_B;
>    end loop;
>    End_Time     := Clock;
>    Elapsed_Time := End_Time - Start_Time;
>    Put (Result);
>    New_Line;
>    Put ("Elapsed Time is ");
>    Put (Elapsed_Time);
>    New_Line;
>    Start_Time := Clock;
>    for Iteration in 1 .. 10_000_000 loop
>       for I in Matrix_A'Range (1) loop
>          for J in Matrix_A'Range (2) loop
>             Sum := 0.0;
>             for K in Matrix_A'Range (2) loop
>                Sum := Sum + Matrix_A (I, K) * Matrix_B (K, J);
>             end loop;
>             Result (I, J) := Sum;
>          end loop;
>       end loop;
>    end loop;
>    End_Time     := Clock;
>    Elapsed_Time := End_Time - Start_Time;
>    Put (Result);
>    New_Line;
>    Put ("Elapsed time is ");
>    Put (Elapsed_Time);
>    New_Line;
> end Matrix_Mul;
> -- end code
> 
> Results: FSF GNAT 7.2.0 x64
> C:\Users\Bojan\Documents>gnatmake -O3 -fopt-info -march=skylake matrix_mul.adb -largs -s
> gcc -c -O3 -fopt-info -march=skylake matrix_mul.adb
> matrix_mul.adb:56:41: note: loop turned into non-loop; it never loops.
> matrix_mul.adb:56:41: note: loop with 4 iterations completely unrolled
> matrix_mul.adb:54:38: note: loop turned into non-loop; it never loops.
> matrix_mul.adb:54:38: note: loop with 4 iterations completely unrolled
> matrix_mul.adb:53:35: note: loop turned into non-loop; it never loops.
> matrix_mul.adb:53:35: note: loop with 4 iterations completely unrolled
> matrix_mul.adb:40:15: note: basic block vectorized
> matrix_mul.adb:42:26: note: basic block vectorized
> matrix_mul.adb:18:31: note: basic block vectorized
> matrix_mul.adb:49:9: note: basic block vectorized
> C:/MSYS64/MINGW64/lib/gcc/x86_64-w64-mingw32/7.2.0/adainclude/a-tifiio.adb:706:10: note: basic block vectorized
> matrix_mul.adb:64:17: note: basic block vectorized
> matrix_mul.adb:18:31: note: basic block vectorized
> matrix_mul.adb:68:9: note: basic block vectorized
> C:/MSYS64/MINGW64/lib/gcc/x86_64-w64-mingw32/7.2.0/adainclude/a-tifiio.adb:706:10: note: basic block vectorized
> gnatbind -x matrix_mul.ali
> gnatlink matrix_mul.ali -O3 -fopt-info -march=skylake -s
> 
> C:\Users\Bojan\Documents>matrix_mul
>  4.00000E+01 3.60000E+01 3.20000E+01 2.80000E+01
>  8.00000E+01 7.20000E+01 6.40000E+01 5.60000E+01
>  1.20000E+02 1.08000E+02 9.60000E+01 8.40000E+01
>  1.60000E+02 1.44000E+02 1.28000E+02 1.12000E+02
> 
> Elapsed Time is      1.338206667
>  4.00000E+01 3.60000E+01 3.20000E+01 2.80000E+01
>  8.00000E+01 7.20000E+01 6.40000E+01 5.60000E+01
>  1.20000E+02 1.08000E+02 9.60000E+01 8.40000E+01
>  1.60000E+02 1.44000E+02 1.28000E+02 1.12000E+02
> 
> Elapsed time is      0.000000445
> 
> C:\Users\Bojan\Documents>set path=C:\GNAT\2017\BIN;%path%
> 
> Results GPL GNAT/2017 from AdaCore.
> 
> C:\Users\Bojan\Documents>gnatmake -O3 -fopt-info -mavx2 matrix_mul.adb -largs -s
> gcc -c -O3 -fopt-info -mavx2 matrix_mul.adb
> matrix_mul.adb:56:41: note: loop turned into non-loop; it never loops.
> matrix_mul.adb:56:41: note: loop with 4 iterations completely unrolled
> matrix_mul.adb:54:38: note: loop turned into non-loop; it never loops.
> matrix_mul.adb:54:38: note: loop with 4 iterations completely unrolled
> matrix_mul.adb:53:35: note: loop turned into non-loop; it never loops.
> matrix_mul.adb:53:35: note: loop with 4 iterations completely unrolled
> matrix_mul.adb:40:15: note: basic block vectorized
> matrix_mul.adb:68:9: note: basic block vectorized
> gnatbind -x matrix_mul.ali
> gnatlink matrix_mul.ali -O3 -fopt-info -mavx2 -s
> 
> C:\Users\Bojan\Documents>matrix_mul
>  4.00000E+01 3.60000E+01 3.20000E+01 2.80000E+01
>  8.00000E+01 7.20000E+01 6.40000E+01 5.60000E+01
>  1.20000E+02 1.08000E+02 9.60000E+01 8.40000E+01
>  1.60000E+02 1.44000E+02 1.28000E+02 1.12000E+02
> 
> Elapsed Time is      2.145337334
>  4.00000E+01 3.60000E+01 3.20000E+01 2.80000E+01
>  8.00000E+01 7.20000E+01 6.40000E+01 5.60000E+01
>  1.20000E+02 1.08000E+02 9.60000E+01 8.40000E+01
>  1.60000E+02 1.44000E+02 1.28000E+02 1.12000E+02
> 
> Elapsed time is      0.000000444
> 
> C:\Users\Bojan\Documents>gcc --version
> gcc (GCC) 6.3.1 20170510 (for GNAT GPL 2017 20170515)
> Copyright (C) 2016 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.
> See your AdaCore support agreement for details of warranty and support.
> If you do not have a current support agreement, then there is absolutely
> no warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
> PURPOSE.
> 
> Should I submit this as bug to AdaCore? My computer is Intel Core i3-6100U (Skylake AVX2). Please try to reproduce.

Compiler is smart enough to not iterate 10 million times over constant values, but there is still optimization problem.