ExpressionSimplify can not handle y - x + x.

[carlj@…]

I have recently had issues with ExpressionSimplify not simplifying expressions on the form y - x + x to y.

To be explicit, the expressions were on the form:

  DAE.BINARY(
     DAE.BINARY(y,DAE.SUB(type),x),
     DAE.ADD(type),
     x
  )

I have fixed this by adding the following case to ExpressionSimplify.simplifyBinaryCommutativeWork:

    case(DAE.ADD(ty = tp),DAE.BINARY(e1,DAE.SUB(_),e2),e3)
      equation
        true = Expression.expEqual(e2,e3);
      then e1;

However, since the module is largely undocumented, I leave it to you to decide if this is the proper way to do it.

[Per Östlund]

Could you please give a small model that shows this issue? The compiler will normally simplify such expressions, so it would be good if you can give an example of when it doesn't.

[carlj@…]

You can't recreate the issue? Give ExpressionSimplify an expression on the form above and you will see that it will not be simplified.

[Per Östlund]

Replying to carlj@…:

You can't recreate the issue? Give ExpressionSimplify an expression on the form above and you will see that it will not be simplified.

Running the following model through the compiler:

model M
  Real x, y;
  Real z = y - x + x;
equation
  z = y - x + x;
end M;

gives the flattened model:

class M
  Real x;
  Real y;
  Real z = y;
equation
  z = y;
end M;

If you do not get the same output I would guess that your compiler is broken. This sometimes happens when compiling with RML, when some depencies doesn't get recompiled when they shouldn't. make clean and a rebuild usually solves such issues.

[carlj@…]

This is not about the flattening not working, it is about ExpressionSimplify not doing what it should.

[Adrian Pop]

Sorry. I don't follow. What do you mean ExpressionSimplify doesn't do what it should?
As far as I can see from the model above, it does simplify y - x + x to y.

Can you explain better what the problem is?

[Per Östlund]

Are you by any chance calling ExpressionSimplify.simplify1 instead of ExpressionSimplify.simplify? simplify1 only does basic simplification, simplify2 does more advanced stuff. simplify calls both simplify1 and simplify2.

[Martin Sjölund]

runScript("LoadCompilerSources.mos");getErrorString();

setCommandLineOptions("+d=rml");
x := DAE.CREF(DAE.CREF_IDENT("x",DAE.T_REAL_DEFAULT,{}),DAE.T_REAL_DEFAULT);
y := DAE.CREF(DAE.CREF_IDENT("y",DAE.T_REAL_DEFAULT,{}),DAE.T_REAL_DEFAULT);
sub := DAE.SUB(DAE.T_REAL_DEFAULT);getErrorString();
add := DAE.ADD(DAE.T_REAL_DEFAULT);getErrorString();
exp1 := DAE.BINARY(x,sub,y);getErrorString();
exp := DAE.BINARY(exp1,add,y);getErrorString();
res := ExpressionSimplify.simplify2(exp);getErrorString();
ExpressionDump.printExpStr(exp);getErrorString();
ExpressionDump.printExpStr(res);getErrorString();

Prints:

"x - y + y"
""
"x + 0.0 * y"
""

[Martin Sjölund]

And

(res,changed) := ExpressionSimplify.simplify(exp);getErrorString();
ExpressionDump.printExpStr(exp);getErrorString();
ExpressionDump.printExpStr(res);getErrorString();

Prints:

"x - y + y"
""
"x"
""

[carlj@…]

perost, yes that was certainly the issue. I now see that a simplification of this kind is not considered a simple simplification.

Sjoelund, very convincing.

I was wrong, you were right.

adrpo, the model might convince you, but it does not convince me (since I am not familiar with how the instantiation handles these things).