Implement algebraic simplifications.

Author: cjgillot

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -309,7 +309,10 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
     ) -> (FlatSet<ScalarInt>, FlatSet<bool>) {
         let left = self.eval_operand(left, state);
         let right = self.eval_operand(right, state);
+
         match (left, right) {
+            (FlatSet::Bottom, _) | (_, FlatSet::Bottom) => (FlatSet::Bottom, FlatSet::Bottom),
+            // Both sides are known, do the actual computation.
             (FlatSet::Elem(left), FlatSet::Elem(right)) => {
                 match self.ecx.overflowing_binary_op(op, &left, &right) {
                     Ok((Scalar::Int(val), overflow, _)) => {
@@ -318,11 +321,36 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
                     _ => (FlatSet::Top, FlatSet::Top),
                 }
             }
-            (FlatSet::Bottom, _) | (_, FlatSet::Bottom) => (FlatSet::Bottom, FlatSet::Bottom),
-            (_, _) => {
-                // Could attempt some algebraic simplifications here.
-                (FlatSet::Top, FlatSet::Top)
+            // Exactly one side is known, attempt some algebraic simplifications.
+            (FlatSet::Elem(const_arg), _) | (_, FlatSet::Elem(const_arg)) => {
+                let layout = const_arg.layout;
+                if !matches!(layout.abi, rustc_target::abi::Abi::Scalar(..)) {
+                    return (FlatSet::Top, FlatSet::Top);
+                }
+
+                let arg_scalar = const_arg.to_scalar();
+                let Ok(arg_scalar) = arg_scalar.try_to_int() else {
+                    return (FlatSet::Top, FlatSet::Top);
+                };
+                let Ok(arg_value) = arg_scalar.to_bits(layout.size) else {
+                    return (FlatSet::Top, FlatSet::Top);
+                };
+
+                match op {
+                    BinOp::BitAnd if arg_value == 0 => (FlatSet::Elem(arg_scalar), FlatSet::Bottom),
+                    BinOp::BitOr
+                        if arg_value == layout.size.truncate(u128::MAX)
+                            || (layout.ty.is_bool() && arg_value == 1) =>
+                    {
+                        (FlatSet::Elem(arg_scalar), FlatSet::Bottom)
+                    }
+                    BinOp::Mul if layout.ty.is_integral() && arg_value == 0 => {
+                        (FlatSet::Elem(arg_scalar), FlatSet::Elem(false))
+                    }
+                    _ => (FlatSet::Top, FlatSet::Top),
+                }
             }
+            (FlatSet::Top, FlatSet::Top) => (FlatSet::Top, FlatSet::Top),
         }
     }
 

tests/mir-opt/dataflow-const-prop/boolean_identities.rs

@@ -0,0 +1,10 @@
+// unit-test: DataflowConstProp
+
+// EMIT_MIR boolean_identities.test.DataflowConstProp.diff
+pub fn test(x: bool, y: bool) -> bool {
+    (y | true) & (x & false)
+}
+
+fn main() {
+    test(true, false);
+}

tests/mir-opt/dataflow-const-prop/boolean_identities.test.DataflowConstProp.diff

@@ -0,0 +1,33 @@
+- // MIR for `test` before DataflowConstProp
++ // MIR for `test` after DataflowConstProp
+  
+  fn test(_1: bool, _2: bool) -> bool {
+      debug x => _1;
+      debug y => _2;
+      let mut _0: bool;
+      let mut _3: bool;
+      let mut _4: bool;
+      let mut _5: bool;
+      let mut _6: bool;
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _2;
+-         _3 = BitOr(move _4, const true);
++         _3 = const true;
+          StorageDead(_4);
+          StorageLive(_5);
+          StorageLive(_6);
+          _6 = _1;
+-         _5 = BitAnd(move _6, const false);
++         _5 = const false;
+          StorageDead(_6);
+-         _0 = BitAnd(move _3, move _5);
++         _0 = const false;
+          StorageDead(_5);
+          StorageDead(_3);
+          return;
+      }
+  }
+  

tests/mir-opt/dataflow-const-prop/mult_by_zero.rs

@@ -0,0 +1,10 @@
+// unit-test: DataflowConstProp
+
+// EMIT_MIR mult_by_zero.test.DataflowConstProp.diff
+fn test(x : i32) -> i32 {
+  x * 0
+}
+
+fn main() {
+    test(10);
+}

tests/mir-opt/dataflow-const-prop/mult_by_zero.test.DataflowConstProp.diff

@@ -0,0 +1,18 @@
+- // MIR for `test` before DataflowConstProp
++ // MIR for `test` after DataflowConstProp
+  
+  fn test(_1: i32) -> i32 {
+      debug x => _1;
+      let mut _0: i32;
+      let mut _2: i32;
+  
+      bb0: {
+          StorageLive(_2);
+          _2 = _1;
+-         _0 = Mul(move _2, const 0_i32);
++         _0 = const 0_i32;
+          StorageDead(_2);
+          return;
+      }
+  }
+