coverage: Discard spans for BCBs that are guaranteed to panic/diverge

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -56,6 +56,18 @@ impl CoverageGraph {
 
         this.dominators = Some(dominators::dominators(&this));
 
+        // Initialize the `must_diverge` flag for each BCB. Post-order traversal
+        // ensures that each node's successors have been processed first.
+        for bcb in graph::iterate::post_order_from(&this, this.start_node()) {
+            let terminator = mir_body[this[bcb].last_bb()].terminator();
+
+            // A BCB is assumed to always diverge if its terminator is not a
+            // return, and all of its successors always diverge.
+            let must_diverge = !bcb_filtered_successors(terminator).has_return_arc()
+                && this.successors[bcb].iter().all(|&s| this[s].must_diverge);
+            this[bcb].must_diverge = must_diverge;
+        }
+
         // The coverage graph's entry-point node (bcb0) always starts with bb0,
         // which never has predecessors. Any other blocks merged into bcb0 can't
         // have multiple (coverage-relevant) predecessors, so bcb0 always has
@@ -273,13 +285,15 @@ rustc_index::newtype_index! {
 /// significance.
 #[derive(Debug, Clone)]
 pub(super) struct BasicCoverageBlockData {
+    pub(super) must_diverge: bool,
     pub basic_blocks: Vec<BasicBlock>,
 }
 
 impl BasicCoverageBlockData {
     pub fn from(basic_blocks: Vec<BasicBlock>) -> Self {
         assert!(basic_blocks.len() > 0);
-        Self { basic_blocks }
+        // `must_diverge` is set by a separate postprocessing step.
+        Self { must_diverge: false, basic_blocks }
     }
 
     #[inline(always)]
@@ -302,14 +316,23 @@ enum CoverageSuccessors<'a> {
     /// potentially be combined into the same BCB as that successor.
     Chainable(BasicBlock),
     /// The block cannot be combined into the same BCB as its successor(s).
-    NotChainable(&'a [BasicBlock]),
+    NotChainable { has_return_arc: bool, successors: &'a [BasicBlock] },
 }
 
 impl CoverageSuccessors<'_> {
     fn is_chainable(&self) -> bool {
         match self {
             Self::Chainable(_) => true,
-            Self::NotChainable(_) => false,
+            Self::NotChainable { .. } => false,
+        }
+    }
+
+    /// If true, the block's terminator can return, so it should not be
+    /// assumed to diverge even if all of its successors diverge.
+    fn has_return_arc(&self) -> bool {
+        match *self {
+            Self::Chainable(_) => false,
+            Self::NotChainable { has_return_arc, .. } => has_return_arc,
         }
     }
 }
@@ -321,7 +344,9 @@ impl IntoIterator for CoverageSuccessors<'_> {
     fn into_iter(self) -> Self::IntoIter {
         match self {
             Self::Chainable(bb) => Some(bb).into_iter().chain((&[]).iter().copied()),
-            Self::NotChainable(bbs) => None.into_iter().chain(bbs.iter().copied()),
+            Self::NotChainable { successors, .. } => {
+                None.into_iter().chain(successors.iter().copied())
+            }
         }
     }
 }
@@ -335,11 +360,17 @@ fn bcb_filtered_successors<'a, 'tcx>(terminator: &'a Terminator<'tcx>) -> Covera
     match terminator.kind {
         // A switch terminator can have many coverage-relevant successors.
         // (If there is exactly one successor, we still treat it as not chainable.)
-        SwitchInt { ref targets, .. } => CoverageSuccessors::NotChainable(targets.all_targets()),
+        SwitchInt { ref targets, .. } => CoverageSuccessors::NotChainable {
+            has_return_arc: false,
+            successors: targets.all_targets(),
+        },
 
         // A yield terminator has exactly 1 successor, but should not be chained,
         // because its resume edge has a different execution count.
-        Yield { ref resume, .. } => CoverageSuccessors::NotChainable(std::slice::from_ref(resume)),
+        Yield { ref resume, .. } => CoverageSuccessors::NotChainable {
+            has_return_arc: true,
+            successors: std::slice::from_ref(resume),
+        },
 
         // These terminators have exactly one coverage-relevant successor,
         // and can be chained into it.
@@ -354,13 +385,19 @@ fn bcb_filtered_successors<'a, 'tcx>(terminator: &'a Terminator<'tcx>) -> Covera
         Call { target: maybe_target, .. } | InlineAsm { destination: maybe_target, .. } => {
             match maybe_target {
                 Some(target) => CoverageSuccessors::Chainable(target),
-                None => CoverageSuccessors::NotChainable(&[]),
+                None => CoverageSuccessors::NotChainable { has_return_arc: false, successors: &[] },
             }
         }
 
+        // These terminators have no actual coverage-relevant successors,
+        // but they do return from their enclosing function.
+        CoroutineDrop | Return => {
+            CoverageSuccessors::NotChainable { has_return_arc: true, successors: &[] }
+        }
+
         // These terminators have no coverage-relevant successors.
-        CoroutineDrop | Return | Unreachable | UnwindResume | UnwindTerminate(_) => {
-            CoverageSuccessors::NotChainable(&[])
+        Unreachable | UnwindResume | UnwindTerminate(_) => {
+            CoverageSuccessors::NotChainable { has_return_arc: false, successors: &[] }
         }
     }
 }

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -217,7 +217,11 @@ impl<'a> CoverageSpansGenerator<'a> {
             refined_spans: Vec::with_capacity(basic_coverage_blocks.num_nodes() * 2),
         };
 
-        coverage_spans.to_refined_spans()
+        let mut refined_spans = coverage_spans.to_refined_spans();
+
+        // After refinement, discard spans associated with BCBs that must panic/diverge.
+        refined_spans.retain(|covspan| !basic_coverage_blocks[covspan.bcb].must_diverge);
+        refined_spans
     }
 
     /// Iterate through the sorted `CoverageSpan`s, and return the refined list of merged and

tests/coverage/abort.cov-map

@@ -44,14 +44,13 @@ Number of file 0 mappings: 13
 - Code(Counter(3)) at (prev + 2, 5) to (start + 1, 2)
 
 Function name: abort::might_abort
-Raw bytes (21): 0x[01, 01, 01, 01, 05, 03, 01, 04, 01, 01, 14, 05, 02, 09, 01, 24, 02, 02, 0c, 03, 02]
+Raw bytes (16): 0x[01, 01, 01, 01, 05, 02, 01, 04, 01, 01, 14, 02, 04, 0c, 03, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-Number of file 0 mappings: 3
+Number of file 0 mappings: 2
 - Code(Counter(0)) at (prev + 4, 1) to (start + 1, 20)
-- Code(Counter(1)) at (prev + 2, 9) to (start + 1, 36)
-- Code(Expression(0, Sub)) at (prev + 2, 12) to (start + 3, 2)
+- Code(Expression(0, Sub)) at (prev + 4, 12) to (start + 3, 2)
     = (c0 - c1)
 

tests/coverage/abort.coverage

@@ -3,8 +3,8 @@
    LL|       |
    LL|     12|extern "C" fn might_abort(should_abort: bool) {
    LL|     12|    if should_abort {
-   LL|      0|        println!("aborting...");
-   LL|      0|        panic!("panics and aborts");
+   LL|       |        println!("aborting...");
+   LL|       |        panic!("panics and aborts");
    LL|     12|    } else {
    LL|     12|        println!("Don't Panic");
    LL|     12|    }

tests/coverage/assert.cov-map

@@ -27,14 +27,11 @@ Number of file 0 mappings: 9
 - Code(Counter(4)) at (prev + 2, 5) to (start + 1, 2)
 
 Function name: assert::might_fail_assert
-Raw bytes (21): 0x[01, 01, 01, 01, 05, 03, 01, 04, 01, 02, 0f, 02, 02, 25, 00, 3d, 05, 01, 01, 00, 02]
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 04, 01, 02, 0f, 05, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 1
-- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-Number of file 0 mappings: 3
+Number of expressions: 0
+Number of file 0 mappings: 2
 - Code(Counter(0)) at (prev + 4, 1) to (start + 2, 15)
-- Code(Expression(0, Sub)) at (prev + 2, 37) to (start + 0, 61)
-    = (c0 - c1)
-- Code(Counter(1)) at (prev + 1, 1) to (start + 0, 2)
+- Code(Counter(1)) at (prev + 3, 1) to (start + 0, 2)
 

tests/coverage/assert.coverage

@@ -4,7 +4,6 @@
    LL|      4|fn might_fail_assert(one_plus_one: u32) {
    LL|      4|    println!("does 1 + 1 = {}?", one_plus_one);
    LL|      4|    assert_eq!(1 + 1, one_plus_one, "the argument was wrong");
-                                                  ^1
    LL|      3|}
    LL|       |
    LL|      1|fn main() -> Result<(), u8> {

tests/coverage/bad_counter_ids.cov-map

@@ -24,16 +24,13 @@ Number of file 0 mappings: 2
 - Code(Zero) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: bad_counter_ids::eq_bad_message
-Raw bytes (21): 0x[01, 01, 01, 01, 00, 03, 01, 28, 01, 02, 0f, 02, 02, 20, 00, 2b, 00, 01, 01, 00, 02]
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 28, 01, 02, 0f, 00, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 1
-- expression 0 operands: lhs = Counter(0), rhs = Zero
-Number of file 0 mappings: 3
+Number of expressions: 0
+Number of file 0 mappings: 2
 - Code(Counter(0)) at (prev + 40, 1) to (start + 2, 15)
-- Code(Expression(0, Sub)) at (prev + 2, 32) to (start + 0, 43)
-    = (c0 - Zero)
-- Code(Zero) at (prev + 1, 1) to (start + 0, 2)
+- Code(Zero) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: bad_counter_ids::eq_good
 Raw bytes (14): 0x[01, 01, 00, 02, 01, 0f, 01, 02, 1f, 05, 03, 01, 00, 02]
@@ -45,14 +42,13 @@ Number of file 0 mappings: 2
 - Code(Counter(1)) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: bad_counter_ids::eq_good_message
-Raw bytes (19): 0x[01, 01, 00, 03, 01, 14, 01, 02, 0f, 00, 02, 20, 00, 2b, 05, 01, 01, 00, 02]
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 14, 01, 02, 0f, 05, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
-Number of file 0 mappings: 3
+Number of file 0 mappings: 2
 - Code(Counter(0)) at (prev + 20, 1) to (start + 2, 15)
-- Code(Zero) at (prev + 2, 32) to (start + 0, 43)
-- Code(Counter(1)) at (prev + 1, 1) to (start + 0, 2)
+- Code(Counter(1)) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: bad_counter_ids::ne_bad
 Raw bytes (14): 0x[01, 01, 00, 02, 01, 2d, 01, 02, 1f, 00, 03, 01, 00, 02]
@@ -64,14 +60,13 @@ Number of file 0 mappings: 2
 - Code(Zero) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: bad_counter_ids::ne_bad_message
-Raw bytes (19): 0x[01, 01, 00, 03, 01, 32, 01, 02, 0f, 05, 02, 20, 00, 2b, 00, 01, 01, 00, 02]
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 32, 01, 02, 0f, 00, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
-Number of file 0 mappings: 3
+Number of file 0 mappings: 2
 - Code(Counter(0)) at (prev + 50, 1) to (start + 2, 15)
-- Code(Counter(1)) at (prev + 2, 32) to (start + 0, 43)
-- Code(Zero) at (prev + 1, 1) to (start + 0, 2)
+- Code(Zero) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: bad_counter_ids::ne_good
 Raw bytes (16): 0x[01, 01, 01, 01, 00, 02, 01, 19, 01, 02, 1f, 02, 03, 01, 00, 02]
@@ -85,14 +80,13 @@ Number of file 0 mappings: 2
     = (c0 - Zero)
 
 Function name: bad_counter_ids::ne_good_message
-Raw bytes (21): 0x[01, 01, 01, 01, 00, 03, 01, 1e, 01, 02, 0f, 00, 02, 20, 00, 2b, 02, 01, 01, 00, 02]
+Raw bytes (16): 0x[01, 01, 01, 01, 00, 02, 01, 1e, 01, 02, 0f, 02, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Zero
-Number of file 0 mappings: 3
+Number of file 0 mappings: 2
 - Code(Counter(0)) at (prev + 30, 1) to (start + 2, 15)
-- Code(Zero) at (prev + 2, 32) to (start + 0, 43)
-- Code(Expression(0, Sub)) at (prev + 1, 1) to (start + 0, 2)
+- Code(Expression(0, Sub)) at (prev + 3, 1) to (start + 0, 2)
     = (c0 - Zero)
 

tests/coverage/bad_counter_ids.coverage

@@ -20,7 +20,6 @@
    LL|      1|fn eq_good_message() {
    LL|      1|    println!("b");
    LL|      1|    assert_eq!(Foo(1), Foo(1), "message b");
-                                             ^0
    LL|      1|}
    LL|       |
    LL|      1|fn ne_good() {
@@ -31,7 +30,6 @@
    LL|      1|fn ne_good_message() {
    LL|      1|    println!("d");
    LL|      1|    assert_ne!(Foo(1), Foo(3), "message d");
-                                             ^0
    LL|      1|}
    LL|       |
    LL|      1|fn eq_bad() {

tests/coverage/coroutine.cov-map

@@ -14,33 +14,20 @@ Number of file 0 mappings: 4
     = (c1 + (c0 - c1))
 
 Function name: coroutine::main
-Raw bytes (65): 0x[01, 01, 08, 07, 0d, 05, 09, 11, 15, 1e, 19, 11, 15, 15, 19, 1e, 19, 11, 15, 09, 01, 0f, 01, 02, 16, 01, 07, 0b, 00, 2e, 11, 01, 2b, 00, 2d, 03, 01, 0e, 00, 35, 11, 02, 0b, 00, 2e, 1e, 01, 22, 00, 27, 1a, 00, 2c, 00, 2e, 17, 01, 0e, 00, 35, 1a, 02, 01, 00, 02]
+Raw bytes (41): 0x[01, 01, 01, 09, 0d, 07, 01, 0f, 01, 02, 16, 01, 07, 0b, 00, 2e, 09, 01, 2b, 00, 2d, 09, 03, 0b, 00, 2e, 02, 01, 22, 00, 27, 11, 00, 2c, 00, 2e, 11, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 8
-- expression 0 operands: lhs = Expression(1, Add), rhs = Counter(3)
-- expression 1 operands: lhs = Counter(1), rhs = Counter(2)
-- expression 2 operands: lhs = Counter(4), rhs = Counter(5)
-- expression 3 operands: lhs = Expression(7, Sub), rhs = Counter(6)
-- expression 4 operands: lhs = Counter(4), rhs = Counter(5)
-- expression 5 operands: lhs = Counter(5), rhs = Counter(6)
-- expression 6 operands: lhs = Expression(7, Sub), rhs = Counter(6)
-- expression 7 operands: lhs = Counter(4), rhs = Counter(5)
-Number of file 0 mappings: 9
+Number of expressions: 1
+- expression 0 operands: lhs = Counter(2), rhs = Counter(3)
+Number of file 0 mappings: 7
 - Code(Counter(0)) at (prev + 15, 1) to (start + 2, 22)
 - Code(Counter(0)) at (prev + 7, 11) to (start + 0, 46)
-- Code(Counter(4)) at (prev + 1, 43) to (start + 0, 45)
-- Code(Expression(0, Add)) at (prev + 1, 14) to (start + 0, 53)
-    = ((c1 + c2) + c3)
-- Code(Counter(4)) at (prev + 2, 11) to (start + 0, 46)
-- Code(Expression(7, Sub)) at (prev + 1, 34) to (start + 0, 39)
-    = (c4 - c5)
-- Code(Expression(6, Sub)) at (prev + 0, 44) to (start + 0, 46)
-    = ((c4 - c5) - c6)
-- Code(Expression(5, Add)) at (prev + 1, 14) to (start + 0, 53)
-    = (c5 + c6)
-- Code(Expression(6, Sub)) at (prev + 2, 1) to (start + 0, 2)
-    = ((c4 - c5) - c6)
+- Code(Counter(2)) at (prev + 1, 43) to (start + 0, 45)
+- Code(Counter(2)) at (prev + 3, 11) to (start + 0, 46)
+- Code(Expression(0, Sub)) at (prev + 1, 34) to (start + 0, 39)
+    = (c2 - c3)
+- Code(Counter(4)) at (prev + 0, 44) to (start + 0, 46)
+- Code(Counter(4)) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: coroutine::main::{closure#0}
 Raw bytes (14): 0x[01, 01, 00, 02, 01, 11, 1c, 01, 1f, 05, 02, 10, 01, 06]

tests/coverage/coroutine.coverage

@@ -22,11 +22,11 @@
    LL|       |
    LL|      1|    match Pin::new(&mut coroutine).resume(()) {
    LL|      1|        CoroutineState::Yielded(Ok(1)) => {}
-   LL|      0|        _ => panic!("unexpected return from resume"),
+   LL|       |        _ => panic!("unexpected return from resume"),
    LL|       |    }
    LL|      1|    match Pin::new(&mut coroutine).resume(()) {
    LL|      1|        CoroutineState::Complete("foo") => {}
-   LL|      0|        _ => panic!("unexpected return from resume"),
+   LL|       |        _ => panic!("unexpected return from resume"),
    LL|       |    }
    LL|      1|}