Implement common subexpression elimination

Signed-off-by: Lucas Steuernagel <[email protected]>

docs/optimizer.rst

@@ -124,3 +124,59 @@ a allocation. For example:
 This optimization pass can be disabled by running `solang --no-vector-to-slice`. You can see the difference between
 having this optimization pass on by comparing the output of `solang --no-vector-to-slice --emit cfg foo.sol` with
 `solang --emit cfg foo.sol`.
+
+Unused Variable Elimination
+___________________________
+
+During the semantic analysis, Solang detects unused variables and raises warnings for them.
+During codegen, we remove all assignments that have been made to this unused variable. There is an example below:
+
+.. code-block:: javascript
+
+    contract test {
+
+        function test1(int a) public pure returns (int) {
+            int x = 5;
+            x++;
+            if (a > 0) {
+                x = 5;
+            }
+
+            a = (x=3) + a*4;
+
+            return a;
+        }
+    }
+
+The variable 'x' will be removed from the function, as it has never been used. The removal won't affect any
+expressions inside the function.
+
+
+Common Subexpression Elimination
+________________________________
+Solang performs common subexpression elimination by doing two passes over the CFG (Control
+Flow Graph). During the first one, it builds a graph to track existing expressions and detect repeated ones.
+During the second pass, it replaces the repeated expressions by a temporary variable, which assumes the value
+of the expression. To disable this feature, use `solang --no-cse`.
+
+Check out the example below. It contains multiple common subexpressions:
+
+.. code-block:: javascript
+
+    contract {
+
+        function csePass(int a, int b) {
+            int x = a*b-5;
+            if (x > 0) {
+                x = a*b-19;
+            } else {
+                x = a*b*a;
+            }
+
+            return x+a*b;
+        }
+    }
+
+The expression `a*b` is repeated throughout the function and will be saved to a temporary variable.
+This temporary will be placed wherever there is an expression `a*b`. You can see the pass in action when you compile
+this contract and check the CFG, using `solang --emit cfg`.

src/bin/solang.rs

@@ -150,11 +150,17 @@ fn main() {
                 .long("no-vector-to-slice")
                 .display_order(4),
         )
+        .arg(
+            Arg::with_name("COMMONSUBEXPRESSIONELIMINATION")
+                .help("Disable common subexpression elimination")
+                .long("no-cse")
+                .display_order(5),
+        )
         .arg(
             Arg::with_name("MATHOVERFLOW")
                 .help("Enable math overflow checking")
                 .long("math-overflow")
-                .display_order(5),
+                .display_order(6),
         )
         .arg(
             Arg::with_name("LANGUAGESERVER")
@@ -307,6 +313,7 @@ fn main() {
             strength_reduce: !matches.is_present("STRENGTHREDUCE"),
             constant_folding: !matches.is_present("CONSTANTFOLDING"),
             vector_to_slice: !matches.is_present("VECTORTOSLICE"),
+            common_subexpression_elimination: !matches.is_present("COMMONEXPRESSIONELIMINATION"),
             math_overflow_check,
             opt_level,
         };