coverage: Simplify counter expressions using simple algebra

Some of these cases currently don't occur in practice, but are included for
completeness, and to avoid having to add them later as branch coverage and
MC/DC coverage start building more complex expressions.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -115,6 +115,43 @@ impl CoverageCounters {
         expressions: &mut IndexVec<ExpressionId, BcbExpression>,
         new_expr: BcbExpression,
     ) -> BcbCounter {
+        // Simplify expressions using basic algebra.
+        //
+        // Some of these cases might not actually occur in practice, depending
+        // on the details of how the instrumentor builds expressions.
+        let BcbExpression { lhs, op, rhs } = new_expr;
+
+        if let BcbCounter::Expression { id } = lhs {
+            let lhs_expr = &expressions[id];
+
+            // Simplify `(a - b) + b` to `a`.
+            if lhs_expr.op == Op::Subtract && op == Op::Add && lhs_expr.rhs == rhs {
+                return lhs_expr.lhs;
+            }
+            // Simplify `(a + b) - b` to `a`.
+            if lhs_expr.op == Op::Add && op == Op::Subtract && lhs_expr.rhs == rhs {
+                return lhs_expr.lhs;
+            }
+            // Simplify `(a + b) - a` to `b`.
+            if lhs_expr.op == Op::Add && op == Op::Subtract && lhs_expr.lhs == rhs {
+                return lhs_expr.rhs;
+            }
+        }
+
+        if let BcbCounter::Expression { id } = rhs {
+            let rhs_expr = &expressions[id];
+
+            // Simplify `a + (b - a)` to `b`.
+            if op == Op::Add && rhs_expr.op == Op::Subtract && lhs == rhs_expr.rhs {
+                return rhs_expr.lhs;
+            }
+            // Simplify `a - (a - b)` to `b`.
+            if op == Op::Subtract && rhs_expr.op == Op::Subtract && lhs == rhs_expr.lhs {
+                return rhs_expr.rhs;
+            }
+        }
+
+        // Simplification failed, so actually create the new expression.
         let id = expressions.push(new_expr);
         BcbCounter::Expression { id }
     }

tests/coverage/abort.cov-map

@@ -1,45 +1,37 @@
 Function name: abort::main
-Raw bytes (105): 0x[01, 01, 12, 01, 47, 05, 09, 03, 0d, 42, 11, 03, 0d, 11, 3e, 42, 11, 03, 0d, 3b, 15, 11, 3e, 42, 11, 03, 0d, 15, 36, 3b, 15, 11, 3e, 42, 11, 03, 0d, 05, 09, 0d, 01, 0e, 01, 01, 1b, 03, 02, 0b, 00, 18, 42, 01, 0c, 00, 19, 11, 00, 1a, 02, 0a, 3e, 02, 0a, 00, 0b, 3b, 02, 0c, 00, 19, 15, 00, 1a, 00, 31, 36, 00, 31, 00, 32, 33, 04, 0c, 00, 19, 05, 00, 1a, 00, 31, 09, 00, 31, 00, 32, 47, 01, 09, 00, 17, 0d, 02, 05, 01, 02]
+Raw bytes (89): 0x[01, 01, 0a, 01, 27, 05, 09, 03, 0d, 22, 11, 03, 0d, 03, 0d, 22, 15, 03, 0d, 03, 0d, 05, 09, 0d, 01, 0e, 01, 01, 1b, 03, 02, 0b, 00, 18, 22, 01, 0c, 00, 19, 11, 00, 1a, 02, 0a, 0e, 02, 0a, 00, 0b, 22, 02, 0c, 00, 19, 15, 00, 1a, 00, 31, 1a, 00, 31, 00, 32, 22, 04, 0c, 00, 19, 05, 00, 1a, 00, 31, 09, 00, 31, 00, 32, 27, 01, 09, 00, 17, 0d, 02, 05, 01, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 18
-- expression 0 operands: lhs = Counter(0), rhs = Expression(17, Add)
+Number of expressions: 10
+- expression 0 operands: lhs = Counter(0), rhs = Expression(9, Add)
 - expression 1 operands: lhs = Counter(1), rhs = Counter(2)
 - expression 2 operands: lhs = Expression(0, Add), rhs = Counter(3)
-- expression 3 operands: lhs = Expression(16, Sub), rhs = Counter(4)
+- expression 3 operands: lhs = Expression(8, Sub), rhs = Counter(4)
 - expression 4 operands: lhs = Expression(0, Add), rhs = Counter(3)
-- expression 5 operands: lhs = Counter(4), rhs = Expression(15, Sub)
-- expression 6 operands: lhs = Expression(16, Sub), rhs = Counter(4)
+- expression 5 operands: lhs = Expression(0, Add), rhs = Counter(3)
+- expression 6 operands: lhs = Expression(8, Sub), rhs = Counter(5)
 - expression 7 operands: lhs = Expression(0, Add), rhs = Counter(3)
-- expression 8 operands: lhs = Expression(14, Add), rhs = Counter(5)
-- expression 9 operands: lhs = Counter(4), rhs = Expression(15, Sub)
-- expression 10 operands: lhs = Expression(16, Sub), rhs = Counter(4)
-- expression 11 operands: lhs = Expression(0, Add), rhs = Counter(3)
-- expression 12 operands: lhs = Counter(5), rhs = Expression(13, Sub)
-- expression 13 operands: lhs = Expression(14, Add), rhs = Counter(5)
-- expression 14 operands: lhs = Counter(4), rhs = Expression(15, Sub)
-- expression 15 operands: lhs = Expression(16, Sub), rhs = Counter(4)
-- expression 16 operands: lhs = Expression(0, Add), rhs = Counter(3)
-- expression 17 operands: lhs = Counter(1), rhs = Counter(2)
+- expression 8 operands: lhs = Expression(0, Add), rhs = Counter(3)
+- expression 9 operands: lhs = Counter(1), rhs = Counter(2)
 Number of file 0 mappings: 13
 - Code(Counter(0)) at (prev + 14, 1) to (start + 1, 27)
 - Code(Expression(0, Add)) at (prev + 2, 11) to (start + 0, 24)
     = (c0 + (c1 + c2))
-- Code(Expression(16, Sub)) at (prev + 1, 12) to (start + 0, 25)
+- Code(Expression(8, Sub)) at (prev + 1, 12) to (start + 0, 25)
     = ((c0 + (c1 + c2)) - c3)
 - Code(Counter(4)) at (prev + 0, 26) to (start + 2, 10)
-- Code(Expression(15, Sub)) at (prev + 2, 10) to (start + 0, 11)
+- Code(Expression(3, Sub)) at (prev + 2, 10) to (start + 0, 11)
     = (((c0 + (c1 + c2)) - c3) - c4)
-- Code(Expression(14, Add)) at (prev + 2, 12) to (start + 0, 25)
-    = (c4 + (((c0 + (c1 + c2)) - c3) - c4))
+- Code(Expression(8, Sub)) at (prev + 2, 12) to (start + 0, 25)
+    = ((c0 + (c1 + c2)) - c3)
 - Code(Counter(5)) at (prev + 0, 26) to (start + 0, 49)
-- Code(Expression(13, Sub)) at (prev + 0, 49) to (start + 0, 50)
-    = ((c4 + (((c0 + (c1 + c2)) - c3) - c4)) - c5)
-- Code(Expression(12, Add)) at (prev + 4, 12) to (start + 0, 25)
-    = (c5 + ((c4 + (((c0 + (c1 + c2)) - c3) - c4)) - c5))
+- Code(Expression(6, Sub)) at (prev + 0, 49) to (start + 0, 50)
+    = (((c0 + (c1 + c2)) - c3) - c5)
+- Code(Expression(8, Sub)) at (prev + 4, 12) to (start + 0, 25)
+    = ((c0 + (c1 + c2)) - c3)
 - Code(Counter(1)) at (prev + 0, 26) to (start + 0, 49)
 - Code(Counter(2)) at (prev + 0, 49) to (start + 0, 50)
-- Code(Expression(17, Add)) at (prev + 1, 9) to (start + 0, 23)
+- Code(Expression(9, Add)) at (prev + 1, 9) to (start + 0, 23)
     = (c1 + c2)
 - Code(Counter(3)) at (prev + 2, 5) to (start + 1, 2)
 

tests/coverage/async.cov-map

@@ -7,19 +7,17 @@ Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 9, 1) to (start + 0, 25)
 
 Function name: async::c::{closure#0}
-Raw bytes (28): 0x[01, 01, 02, 01, 05, 05, 02, 04, 01, 09, 19, 01, 0e, 05, 02, 09, 00, 0a, 02, 02, 09, 00, 0a, 07, 02, 01, 00, 02]
+Raw bytes (26): 0x[01, 01, 01, 01, 05, 04, 01, 09, 19, 01, 0e, 05, 02, 09, 00, 0a, 02, 02, 09, 00, 0a, 01, 02, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 2
+Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
 Number of file 0 mappings: 4
 - Code(Counter(0)) at (prev + 9, 25) to (start + 1, 14)
 - Code(Counter(1)) at (prev + 2, 9) to (start + 0, 10)
 - Code(Expression(0, Sub)) at (prev + 2, 9) to (start + 0, 10)
     = (c0 - c1)
-- Code(Expression(1, Add)) at (prev + 2, 1) to (start + 0, 2)
-    = (c1 + (c0 - c1))
+- Code(Counter(0)) at (prev + 2, 1) to (start + 0, 2)
 
 Function name: async::d
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 11, 01, 00, 14]
@@ -188,19 +186,17 @@ Number of file 0 mappings: 9
     = ((c1 + c2) + c3)
 
 Function name: async::j::c
-Raw bytes (28): 0x[01, 01, 02, 01, 05, 05, 02, 04, 01, 37, 05, 01, 12, 05, 02, 0d, 00, 0e, 02, 0a, 0d, 00, 0e, 07, 02, 05, 00, 06]
+Raw bytes (26): 0x[01, 01, 01, 01, 05, 04, 01, 37, 05, 01, 12, 05, 02, 0d, 00, 0e, 02, 0a, 0d, 00, 0e, 01, 02, 05, 00, 06]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 2
+Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
 Number of file 0 mappings: 4
 - Code(Counter(0)) at (prev + 55, 5) to (start + 1, 18)
 - Code(Counter(1)) at (prev + 2, 13) to (start + 0, 14)
 - Code(Expression(0, Sub)) at (prev + 10, 13) to (start + 0, 14)
     = (c0 - c1)
-- Code(Expression(1, Add)) at (prev + 2, 5) to (start + 0, 6)
-    = (c1 + (c0 - c1))
+- Code(Counter(0)) at (prev + 2, 5) to (start + 0, 6)
 
 Function name: async::j::d
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 46, 05, 00, 17]

tests/coverage/async2.cov-map

@@ -7,17 +7,15 @@ Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 13, 1) to (start + 0, 23)
 
 Function name: async2::async_func::{closure#0}
-Raw bytes (26): 0x[01, 01, 01, 05, 00, 04, 01, 0d, 17, 03, 09, 05, 03, 0a, 02, 06, 00, 02, 06, 00, 07, 03, 01, 01, 00, 02]
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 0d, 17, 03, 09, 05, 03, 0a, 02, 06, 00, 02, 06, 00, 07, 01, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 1
-- expression 0 operands: lhs = Counter(1), rhs = Zero
+Number of expressions: 0
 Number of file 0 mappings: 4
 - Code(Counter(0)) at (prev + 13, 23) to (start + 3, 9)
 - Code(Counter(1)) at (prev + 3, 10) to (start + 2, 6)
 - Code(Zero) at (prev + 2, 6) to (start + 0, 7)
-- Code(Expression(0, Add)) at (prev + 1, 1) to (start + 0, 2)
-    = (c1 + Zero)
+- Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
 
 Function name: async2::async_func_just_println
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 15, 01, 00, 24]
@@ -44,15 +42,13 @@ Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 25, 1) to (start + 7, 2)
 
 Function name: async2::non_async_func
-Raw bytes (26): 0x[01, 01, 01, 05, 00, 04, 01, 05, 01, 03, 09, 05, 03, 0a, 02, 06, 00, 02, 06, 00, 07, 03, 01, 01, 00, 02]
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 05, 01, 03, 09, 05, 03, 0a, 02, 06, 00, 02, 06, 00, 07, 01, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 1
-- expression 0 operands: lhs = Counter(1), rhs = Zero
+Number of expressions: 0
 Number of file 0 mappings: 4
 - Code(Counter(0)) at (prev + 5, 1) to (start + 3, 9)
 - Code(Counter(1)) at (prev + 3, 10) to (start + 2, 6)
 - Code(Zero) at (prev + 2, 6) to (start + 0, 7)
-- Code(Expression(0, Add)) at (prev + 1, 1) to (start + 0, 2)
-    = (c1 + Zero)
+- Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
 

tests/coverage/async_block.cov-map

@@ -1,32 +1,28 @@
 Function name: async_block::main
-Raw bytes (38): 0x[01, 01, 02, 01, 05, 03, 05, 06, 01, 05, 01, 00, 0b, 05, 01, 09, 00, 0a, 03, 00, 0e, 00, 13, 05, 00, 14, 01, 16, 05, 07, 0a, 02, 06, 06, 03, 01, 00, 02]
+Raw bytes (36): 0x[01, 01, 01, 01, 05, 06, 01, 05, 01, 00, 0b, 05, 01, 09, 00, 0a, 03, 00, 0e, 00, 13, 05, 00, 14, 01, 16, 05, 07, 0a, 02, 06, 01, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 2
+Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 1 operands: lhs = Expression(0, Add), rhs = Counter(1)
 Number of file 0 mappings: 6
 - Code(Counter(0)) at (prev + 5, 1) to (start + 0, 11)
 - Code(Counter(1)) at (prev + 1, 9) to (start + 0, 10)
 - Code(Expression(0, Add)) at (prev + 0, 14) to (start + 0, 19)
     = (c0 + c1)
 - Code(Counter(1)) at (prev + 0, 20) to (start + 1, 22)
 - Code(Counter(1)) at (prev + 7, 10) to (start + 2, 6)
-- Code(Expression(1, Sub)) at (prev + 3, 1) to (start + 0, 2)
-    = ((c0 + c1) - c1)
+- Code(Counter(0)) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: async_block::main::{closure#0}
-Raw bytes (28): 0x[01, 01, 02, 01, 05, 05, 02, 04, 01, 07, 1c, 01, 17, 05, 01, 18, 02, 0e, 02, 02, 14, 02, 0e, 07, 03, 09, 00, 0a]
+Raw bytes (26): 0x[01, 01, 01, 01, 05, 04, 01, 07, 1c, 01, 17, 05, 01, 18, 02, 0e, 02, 02, 14, 02, 0e, 01, 03, 09, 00, 0a]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 2
+Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
 Number of file 0 mappings: 4
 - Code(Counter(0)) at (prev + 7, 28) to (start + 1, 23)
 - Code(Counter(1)) at (prev + 1, 24) to (start + 2, 14)
 - Code(Expression(0, Sub)) at (prev + 2, 20) to (start + 2, 14)
     = (c0 - c1)
-- Code(Expression(1, Add)) at (prev + 3, 9) to (start + 0, 10)
-    = (c1 + (c0 - c1))
+- Code(Counter(0)) at (prev + 3, 9) to (start + 0, 10)
 

tests/coverage/branch/generics.cov-map

@@ -1,10 +1,9 @@
 Function name: generics::print_size::<()>
-Raw bytes (35): 0x[01, 01, 02, 01, 05, 05, 02, 05, 01, 06, 01, 01, 24, 20, 05, 02, 01, 08, 00, 24, 05, 00, 25, 02, 06, 02, 02, 0c, 02, 06, 07, 03, 01, 00, 02]
+Raw bytes (33): 0x[01, 01, 01, 01, 05, 05, 01, 06, 01, 01, 24, 20, 05, 02, 01, 08, 00, 24, 05, 00, 25, 02, 06, 02, 02, 0c, 02, 06, 01, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 2
+Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
 Number of file 0 mappings: 5
 - Code(Counter(0)) at (prev + 6, 1) to (start + 1, 36)
 - Branch { true: Counter(1), false: Expression(0, Sub) } at (prev + 1, 8) to (start + 0, 36)
@@ -13,16 +12,14 @@ Number of file 0 mappings: 5
 - Code(Counter(1)) at (prev + 0, 37) to (start + 2, 6)
 - Code(Expression(0, Sub)) at (prev + 2, 12) to (start + 2, 6)
     = (c0 - c1)
-- Code(Expression(1, Add)) at (prev + 3, 1) to (start + 0, 2)
-    = (c1 + (c0 - c1))
+- Code(Counter(0)) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: generics::print_size::<u32>
-Raw bytes (35): 0x[01, 01, 02, 01, 05, 05, 02, 05, 01, 06, 01, 01, 24, 20, 05, 02, 01, 08, 00, 24, 05, 00, 25, 02, 06, 02, 02, 0c, 02, 06, 07, 03, 01, 00, 02]
+Raw bytes (33): 0x[01, 01, 01, 01, 05, 05, 01, 06, 01, 01, 24, 20, 05, 02, 01, 08, 00, 24, 05, 00, 25, 02, 06, 02, 02, 0c, 02, 06, 01, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 2
+Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
 Number of file 0 mappings: 5
 - Code(Counter(0)) at (prev + 6, 1) to (start + 1, 36)
 - Branch { true: Counter(1), false: Expression(0, Sub) } at (prev + 1, 8) to (start + 0, 36)
@@ -31,16 +28,14 @@ Number of file 0 mappings: 5
 - Code(Counter(1)) at (prev + 0, 37) to (start + 2, 6)
 - Code(Expression(0, Sub)) at (prev + 2, 12) to (start + 2, 6)
     = (c0 - c1)
-- Code(Expression(1, Add)) at (prev + 3, 1) to (start + 0, 2)
-    = (c1 + (c0 - c1))
+- Code(Counter(0)) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: generics::print_size::<u64>
-Raw bytes (35): 0x[01, 01, 02, 01, 05, 05, 02, 05, 01, 06, 01, 01, 24, 20, 05, 02, 01, 08, 00, 24, 05, 00, 25, 02, 06, 02, 02, 0c, 02, 06, 07, 03, 01, 00, 02]
+Raw bytes (33): 0x[01, 01, 01, 01, 05, 05, 01, 06, 01, 01, 24, 20, 05, 02, 01, 08, 00, 24, 05, 00, 25, 02, 06, 02, 02, 0c, 02, 06, 01, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 2
+Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
 Number of file 0 mappings: 5
 - Code(Counter(0)) at (prev + 6, 1) to (start + 1, 36)
 - Branch { true: Counter(1), false: Expression(0, Sub) } at (prev + 1, 8) to (start + 0, 36)
@@ -49,6 +44,5 @@ Number of file 0 mappings: 5
 - Code(Counter(1)) at (prev + 0, 37) to (start + 2, 6)
 - Code(Expression(0, Sub)) at (prev + 2, 12) to (start + 2, 6)
     = (c0 - c1)
-- Code(Expression(1, Add)) at (prev + 3, 1) to (start + 0, 2)
-    = (c1 + (c0 - c1))
+- Code(Counter(0)) at (prev + 3, 1) to (start + 0, 2)
 

tests/coverage/branch/if-let.cov-map

@@ -1,10 +1,9 @@
 Function name: if_let::if_let
-Raw bytes (45): 0x[01, 01, 02, 05, 09, 09, 02, 07, 01, 0c, 01, 01, 10, 20, 02, 09, 03, 0c, 00, 13, 02, 00, 11, 00, 12, 05, 00, 16, 00, 1b, 02, 00, 1c, 02, 06, 09, 02, 0c, 02, 06, 07, 03, 05, 01, 02]
+Raw bytes (43): 0x[01, 01, 01, 05, 09, 07, 01, 0c, 01, 01, 10, 20, 02, 09, 03, 0c, 00, 13, 02, 00, 11, 00, 12, 05, 00, 16, 00, 1b, 02, 00, 1c, 02, 06, 09, 02, 0c, 02, 06, 05, 03, 05, 01, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 2
+Number of expressions: 1
 - expression 0 operands: lhs = Counter(1), rhs = Counter(2)
-- expression 1 operands: lhs = Counter(2), rhs = Expression(0, Sub)
 Number of file 0 mappings: 7
 - Code(Counter(0)) at (prev + 12, 1) to (start + 1, 16)
 - Branch { true: Expression(0, Sub), false: Counter(2) } at (prev + 3, 12) to (start + 0, 19)
@@ -16,8 +15,7 @@ Number of file 0 mappings: 7
 - Code(Expression(0, Sub)) at (prev + 0, 28) to (start + 2, 6)
     = (c1 - c2)
 - Code(Counter(2)) at (prev + 2, 12) to (start + 2, 6)
-- Code(Expression(1, Add)) at (prev + 3, 5) to (start + 1, 2)
-    = (c2 + (c1 - c2))
+- Code(Counter(1)) at (prev + 3, 5) to (start + 1, 2)
 
 Function name: if_let::if_let_chain
 Raw bytes (66): 0x[01, 01, 04, 01, 05, 05, 09, 0f, 0d, 05, 09, 0a, 01, 17, 01, 00, 33, 20, 02, 05, 01, 0c, 00, 13, 02, 00, 11, 00, 12, 01, 00, 16, 00, 17, 20, 0d, 09, 01, 10, 00, 17, 0d, 00, 15, 00, 16, 02, 00, 1a, 00, 1b, 0d, 01, 05, 03, 06, 0f, 03, 0c, 02, 06, 0b, 03, 05, 01, 02]