How to construct isl sets.

README.md

@@ -1 +1,253 @@
-# How to use isl via the C++ / Python interface
+Warning: This is a proposal, which does not document the current isl 
+
+# How to use isl via the C++ and Python interface
+
+## Constructing an integer set or map (isl::set / isl::map)
+
+### Explicit Interface (today)
+
+We first describe how the current C++ interface should be used to construct
+isl sets and maps, just proposing a small number of extensions beyond what exists
+today. The resulting code is still somehow verbose, but very explicit.
+
+Example:
+
+*{ [N, M] -> { [i,j] : 2 * M + 3 * N <= 2 * i + j + 10 }*
+
+We create an integer set as follows. We first create a set of identifiers for
+each of the needed dimensions (`isl::id Id_N(ctx, "N"`). We then introduce
+initial expressions for all identifiers and required constants (`isl::pw_aff
+N(Id_N), Ten(ctx, 10)`). From these initial expressions the actual affine
+expressions are constructed (`isl::pw_aff LHS = Two.mul(M)`). Pairs of
+expressions are combined with the operators lt_set (<), le_set (<=), ge_set
+(>=), gt_set (>), eq_set (=), ne_set (!=) into a parameteric set of constraints
+(`isl::set PSet = LHS.le_set(RHS)`). Finally, a non-parameteric set is
+constructed from 1) a parameteric set specifying its constraints and 2) a list
+of identifiers that specify the parameter dimensions that should be promoted to
+set dimensions (`isl::set Set({Id_i, Id_j}, PSet)`).  Similary, a map can be
+constructed by providing two lists of identifiers defining the input and output
+dimensions (`isl::map Map({Id_i}, {Id_j}, PSet)`)
+
+
+
+
+```
+// Identifiers
+isl::id Id_N(ctx, "N"), Id_M(ctx, "M"), Id_i(ctx, "i"), Id_j(ctx, "j");
+
+// One (piece-wise) affine expression per identifier
+// [N] -> { [(N)]}, [N] -> { [(M)]}, [i] -> { [(i)]}, [j] -> { [(j)]}
+isl::pw_aff N(Id_N), M(Id_M), i(Id_i), j(Id_j);
+
+// One (piece-wise) affine expression per constant
+// {[(10)]}, {[(2)]}, {[(3)]}
+isl::pw_aff Ten(ctx, 10), Two(ctx, 2), Three(ctx, 3);
+
+// Build the left and right hand side of the expression
+// [M, N] -> { [(2 * M + 3 * M)] }
+isl::pw_aff LHS = Two.mul(M).add(Three.mul(N));
+
+// [M, N] -> { [(2 * i + j + 10)] }
+isl::pw_aff RHS = Two.mul(i).add(j).add(Ten);
+
+// [N, M, i, j] -> { : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::set PSet = LHS.le_set(RHS);
+
+// [N, M] -> { [i, j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::set Set({Id_i, Id_j}, PSet);
+
+// [N, M] -> { [i] -> [j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::map Map({Id_i}, {Id_j}, PSet);
+```
+
+#### New functions
+
+```
+__isl_constructor
+isl_pw_aff *isl_pw_aff_param_from_id(isl_id *identifier);
+__isl_constructor
+isl_pw_aff *isl_pw_aff_val_from_val(isl_val *value);
+__isl_constructor
+isl_pw_aff *isl_pw_aff_val_from_si(int value);
+```
+See: https://github.com/PollyLabs/isl/pull/25
+
+```
+__isl_constructor
+isl_set *isl_set_from_id_list_params(isl_id_list *dims, isl_set *pset);
+__isl_constructor
+isl_map *isl_map_from_id_list_params(isl_id_list *input_dims, isl_id_list *output_dims, isl_set *pset);
+```
+See: https://github.com/PollyLabs/isl/pull/26
+
+#### Notes
+
+##### Why not use isl_aff
+
+Currently we always need to use isl_pw_aff, as isl_aff does not allow for
+parameter auto-alignment. This should be changed, but will require more work.
+We should likely write the documentation in terms of isl_pw_aff for now.
+
+##### How to introduce parameters
+
+We can either use a constructor (in the proposal):
+
+```
+isl::set PSet("[N, M, i, j] -> { : 2 * M + 3 * N <= 2 * i + j + 10 }")
+
+// { [N, M] -> { [i,j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::set Set({i,j}, PSet);
+// { [N, M] -> { [i]->[j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::map Map({i}, {j}, PSet);
+```
+
+or a set of member functions.
+
+```
+isl::set PSet("[N, M, i, j] -> { : 2 * M + 3 * N <= 2 * i + j + 10 }")
+
+// { [N, M] -> { [i,j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::set PSet.inputs(i,j);
+// { [N, M] -> { [i]->[j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::map PSet.inputs(i).outputs(j);
+```
+
+### Explicit Interface for isl::aff (future)
+
+After isl_aff supports parameter auto-alignment it will be possible to write the
+above example (and other examples which do not require isl_pw_affs) as follows:
+
+```
+// Identifiers
+isl::id Id_N(ctx, "N"), Id_M(ctx, "M"), Id_i(ctx, "i"), Id_j(ctx, "j");
+
+// One (piece-wise) affine expression per identifier
+// [N] -> { [(N)]}, [N] -> { [(M)]}, [i] -> { [(i)]}, [j] -> { [(j)]}
+isl::aff N(Id_N), M(Id_M), i(Id_i), j(Id_j);
+
+// One (piece-wise) affine expression per constant
+// {[(10)]}, {[(2)]}, {[(3)]}
+isl::aff Ten(ctx, 10), Two(ctx, 2), Three(ctx, 3);
+
+// Build the left and right hand side of the expression
+// [M, N] -> { [(2 * M + 3 * M)] }
+isl::aff LHS = Two.mul(M).add(Three.mul(N));
+
+// [M, N] -> { [(2 * i + j + 10)] }
+isl::aff RHS = Two.mul(i).add(j).add(Ten);
+
+// [N, M, i, j] -> { : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::constraint C = LHS.le_constraint(RHS);
+
+// [N, M] -> { [i, j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::set Set({Id_i, Id_j}, C);
+
+// [N, M] -> { [i] -> [j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::map Map({Id_i}, {Id_j}, C);
+```
+
+#### TODOs:
+
+- Introduce parameter auto-alignment to isl_aff
+- Introduce isl_aff_[op]_constraint methods
+- Enable conversion from a constraint to an isl_set
+
+### Streamlined interface (future)
+
+In certain use cases a more streamlined interface might be useful. Here is
+an example which includes:
+
+  - operator overloading
+  - automatic conversion from isl::id to isl::aff
+  - automatic conversion from int to isl::aff
+  - a default context
+
+```
+isl::id N("N"), M("M"), i("i"), j("j');
+
+isl::aff LHS = 2 * M + 3 * N;  // natural precedence works
+isl::aff RHS = 2 * i + j + 10;
+
+// { [N, M] -> { [i,j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::set Set({i,j}, LHS <= RHS);
+
+// { [N, M] -> { [i]->[j] : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::map Map({i}, {j}, LHS <= RHS);
+```
+
+#### Extensions
+
+##### Use of a thread-local context
+
+Instead of always providing a ctx object, the bindings could provide a thread
+local ctx.
+
+
+Explicit context:
+```
+isl::id N(ctx, "N");
+```
+
+Implicit context:
+```
+isl::id N("N");
+```
+
+##### Overloading of operators
+
+Instead of calling the explicit interface, operator overloading can be used.
+
+Without overloading:
+```
+isl::pw_aff = A.add(B).add(Three.mul(C));
+```
+
+With overloading
+```
+isl::pw_aff = A + B + 3 * C;
+```
+
+*Warning*: Overloading of the comparision operators may cause confusion as the
+           result is not a boolean expression.
+
+A solution might be to have these operators in a separate sub-namespace to
+avoid surprising behavior of operator overloads. Another one might be to use
+'markers'.
+
+```
+
+// [N, M, i, j] -> { : 2 * M + 3 * N <= 2 * i + j + 10 }
+isl::set PSet = isl::set_maker(LHS) <= RHS;
+isl::set Set({i}, PSet);
+```
+
+##### More efficient construction of parameter isl::aff's
+
+When constructing an affine expression for a parameter, the explicit interface
+requires two steps. First the construction of an isl::id and then its conversion
+to a isl::aff. It would be nice if just one step would be needed. There
+are two options:
+
+1) Construction of isl::aff's from strings.
+
+```
+isl::aff A = ...
+
+// [N] -> { [(N)] }
+isl::aff N(ctx, "N");
+
+isl::aff X = A.add(N);
+```
+
+2) Automatic conversion from isl::id to aff
+
+```
+isl::aff A = ...
+
+// [N] -> { [(N)] }
+isl::id N(ctx, "N");
+
+isl::aff X = A.add(N);
+```
+
+