Reland - Report coverage 0 of dead blocks

Fixes: #84018

With `-Z instrument-coverage`, coverage reporting of dead blocks
(for example, blocks dropped because a conditional branch is dropped,
based on const evaluation) is now supported.

Note, this PR relands an earlier, reverted PR that failed when compiling
generators. Generators clone blocks, so CFG simplification is used to
remove the original `BasicBlocks`; and since they were cloned, the
original blocks should not be saved. (Saving them resulted in duplicate
coverage counters, and `llvm-cov` failed with a "Malformed coverage
data" error.

If `instrument-coverage` is enabled,
`simplify::remove_dead_blocks_with_coverage()`,
with `DroppedCoverage::SaveUnreachable`, finds all dropped coverage
`Statement`s and adds their `code_region`s as `Unreachable` coverage
`Statement`s to the `START_BLOCK`, so they are still included in the
coverage map.

Check out the resulting changes in the test coverage reports in this PR.

Author: richkadel

compiler/rustc_codegen_ssa/src/coverageinfo/map.rs

@@ -28,6 +28,7 @@ pub struct Expression {
 /// only whitespace or comments). According to LLVM Code Coverage Mapping documentation, "A count
 /// for a gap area is only used as the line execution count if there are no other regions on a
 /// line."
+#[derive(Debug)]
 pub struct FunctionCoverage<'tcx> {
     instance: Instance<'tcx>,
     source_hash: u64,
@@ -113,6 +114,14 @@ impl<'tcx> FunctionCoverage<'tcx> {
             expression_id, lhs, op, rhs, region
         );
         let expression_index = self.expression_index(u32::from(expression_id));
+        debug_assert!(
+            expression_index.as_usize() < self.expressions.len(),
+            "expression_index {} is out of range for expressions.len() = {}
+            for {:?}",
+            expression_index.as_usize(),
+            self.expressions.len(),
+            self,
+        );
         if let Some(previous_expression) = self.expressions[expression_index].replace(Expression {
             lhs,
             op,
@@ -150,6 +159,13 @@ impl<'tcx> FunctionCoverage<'tcx> {
             self.instance
         );
 
+        if !self.counters.iter().any(|region| region.is_some()) {
+            // FIXME(richkadel): How is this possible and why is it OK? I verified that the first
+            // test program that triggers this still works, and `llvm-cov` does not fail, so I need
+            // to allow it.
+            debug!("no counters for {:?}", self);
+        }
+
         let counter_regions = self.counter_regions();
         let (counter_expressions, expression_regions) = self.expressions_with_regions();
         let unreachable_regions = self.unreachable_regions();

compiler/rustc_mir/src/transform/const_goto.rs

@@ -47,7 +47,7 @@ impl<'tcx> MirPass<'tcx> for ConstGoto {
         // if we applied optimizations, we potentially have some cfg to cleanup to
         // make it easier for further passes
         if should_simplify {
-            simplify_cfg(body);
+            simplify_cfg(tcx, body);
             simplify_locals(body, tcx);
         }
     }

compiler/rustc_mir/src/transform/deduplicate_blocks.rs

@@ -26,7 +26,7 @@ impl<'tcx> MirPass<'tcx> for DeduplicateBlocks {
         if has_opts_to_apply {
             let mut opt_applier = OptApplier { tcx, duplicates };
             opt_applier.visit_body(body);
-            simplify_cfg(body);
+            simplify_cfg(tcx, body);
         }
     }
 }

compiler/rustc_mir/src/transform/early_otherwise_branch.rs

@@ -164,7 +164,7 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
         // Since this optimization adds new basic blocks and invalidates others,
         // clean up the cfg to make it nicer for other passes
         if should_cleanup {
-            simplify_cfg(body);
+            simplify_cfg(tcx, body);
         }
     }
 }

compiler/rustc_mir/src/transform/generator.rs

@@ -964,7 +964,7 @@ fn create_generator_drop_shim<'tcx>(
 
     // Make sure we remove dead blocks to remove
     // unrelated code from the resume part of the function
-    simplify::remove_dead_blocks(&mut body);
+    simplify::remove_dead_blocks_with_coverage(tcx, &mut body, simplify::DroppedCoverage::Allow);
 
     dump_mir(tcx, None, "generator_drop", &0, &body, |_, _| Ok(()));
 
@@ -1137,7 +1137,7 @@ fn create_generator_resume_function<'tcx>(
 
     // Make sure we remove dead blocks to remove
     // unrelated code from the drop part of the function
-    simplify::remove_dead_blocks(body);
+    simplify::remove_dead_blocks_with_coverage(tcx, body, simplify::DroppedCoverage::Allow);
 
     dump_mir(tcx, None, "generator_resume", &0, body, |_, _| Ok(()));
 }

compiler/rustc_mir/src/transform/inline.rs

@@ -57,7 +57,7 @@ impl<'tcx> MirPass<'tcx> for Inline {
         if inline(tcx, body) {
             debug!("running simplify cfg on {:?}", body.source);
             CfgSimplifier::new(body).simplify();
-            remove_dead_blocks(body);
+            remove_dead_blocks(tcx, body);
         }
     }
 }

compiler/rustc_mir/src/transform/match_branches.rs

@@ -167,7 +167,7 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
         }
 
         if should_cleanup {
-            simplify_cfg(body);
+            simplify_cfg(tcx, body);
         }
     }
 }

compiler/rustc_mir/src/transform/multiple_return_terminators.rs

@@ -38,6 +38,6 @@ impl<'tcx> MirPass<'tcx> for MultipleReturnTerminators {
             }
         }
 
-        simplify::remove_dead_blocks(body)
+        simplify::remove_dead_blocks(tcx, body)
     }
 }

compiler/rustc_mir/src/transform/remove_unneeded_drops.rs

@@ -36,7 +36,7 @@ impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops {
         // if we applied optimizations, we potentially have some cfg to cleanup to
         // make it easier for further passes
         if should_simplify {
-            simplify_cfg(body);
+            simplify_cfg(tcx, body);
         }
     }
 }

compiler/rustc_mir/src/transform/simplify.rs

@@ -29,6 +29,7 @@
 
 use crate::transform::MirPass;
 use rustc_index::vec::{Idx, IndexVec};
+use rustc_middle::mir::coverage::*;
 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
@@ -46,9 +47,17 @@ impl SimplifyCfg {
     }
 }
 
-pub fn simplify_cfg(body: &mut Body<'_>) {
+pub fn simplify_cfg(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) {
+    simplify_cfg_with_coverage(tcx, body, DroppedCoverage::AssertNone);
+}
+
+pub fn simplify_cfg_with_coverage(
+    tcx: TyCtxt<'tcx>,
+    body: &mut Body<'_>,
+    dropped_coverage: DroppedCoverage,
+) {
     CfgSimplifier::new(body).simplify();
-    remove_dead_blocks(body);
+    remove_dead_blocks_with_coverage(tcx, body, dropped_coverage);
 
     // FIXME: Should probably be moved into some kind of pass manager
     body.basic_blocks_mut().raw.shrink_to_fit();
@@ -59,12 +68,31 @@ impl<'tcx> MirPass<'tcx> for SimplifyCfg {
         Cow::Borrowed(&self.label)
     }
 
-    fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         debug!("SimplifyCfg({:?}) - simplifying {:?}", self.label, body.source);
-        simplify_cfg(body);
+        simplify_cfg_with_coverage(tcx, body, DroppedCoverage::SaveUnreachable);
     }
 }
 
+/// How to handle `Coverage` statements in dropped `BasicBlocks`.
+pub enum DroppedCoverage {
+    /// Allow the `Coverage` statement(s) in a dropped `BasicBlock` to be
+    /// dropped as well. (This may be because the `Coverage` statement was
+    /// already cloned.)
+    Allow,
+
+    /// Assert that there are none. The current or most recent MIR pass(es)
+    /// should not have done anything that would have resulted in an
+    /// unreachable block with a `Coverage` statement.
+    AssertNone,
+
+    /// Assume the `Coverage` statement(s) in a dropped `BacicBlock` are part
+    /// of a dead code region. Save them in the `START_BLOCK` but mark `Counter`
+    /// coverage as `is_dead` so they are not incremented. (Expressions from
+    /// dead blocks should already add up to zero.)
+    SaveUnreachable,
+}
+
 pub struct CfgSimplifier<'a, 'tcx> {
     basic_blocks: &'a mut IndexVec<BasicBlock, BasicBlockData<'tcx>>,
     pred_count: IndexVec<BasicBlock, u32>,
@@ -286,7 +314,15 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
     }
 }
 
-pub fn remove_dead_blocks(body: &mut Body<'_>) {
+pub fn remove_dead_blocks(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) {
+    remove_dead_blocks_with_coverage(tcx, body, DroppedCoverage::AssertNone);
+}
+
+pub fn remove_dead_blocks_with_coverage(
+    tcx: TyCtxt<'tcx>,
+    body: &mut Body<'_>,
+    dropped_coverage: DroppedCoverage,
+) {
     let reachable = traversal::reachable_as_bitset(body);
     let num_blocks = body.basic_blocks().len();
     if num_blocks == reachable.count() {
@@ -306,6 +342,16 @@ pub fn remove_dead_blocks(body: &mut Body<'_>) {
         }
         used_blocks += 1;
     }
+
+    if tcx.sess.instrument_coverage() {
+        use DroppedCoverage::*;
+        match dropped_coverage {
+            Allow => {}
+            AssertNone => assert_no_unreachable_coverage(basic_blocks, used_blocks),
+            SaveUnreachable => save_unreachable_coverage(basic_blocks, used_blocks),
+        }
+    }
+
     basic_blocks.raw.truncate(used_blocks);
 
     for block in basic_blocks {
@@ -315,6 +361,47 @@ pub fn remove_dead_blocks(body: &mut Body<'_>) {
     }
 }
 
+fn assert_no_unreachable_coverage(
+    basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>,
+    first_dead_block: usize,
+) {
+    for dead_block in first_dead_block..basic_blocks.len() {
+        for statement in &basic_blocks[BasicBlock::new(dead_block)].statements {
+            assert!(
+                !matches!(statement.kind, StatementKind::Coverage(..)),
+                "Dead blocks should not have `Coverage` statements at this stage."
+            );
+        }
+    }
+}
+
+fn save_unreachable_coverage(
+    basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>,
+    first_dead_block: usize,
+) {
+    // retain coverage info for dead blocks, so coverage reports will still
+    // report `0` executions for the uncovered code regions.
+    let mut dropped_coverage = Vec::new();
+    for dead_block in first_dead_block..basic_blocks.len() {
+        for statement in basic_blocks[BasicBlock::new(dead_block)].statements.iter() {
+            if let StatementKind::Coverage(coverage) = &statement.kind {
+                if let Some(code_region) = &coverage.code_region {
+                    dropped_coverage.push((statement.source_info, code_region.clone()));
+                }
+            }
+        }
+    }
+    for (source_info, code_region) in dropped_coverage {
+        basic_blocks[START_BLOCK].statements.push(Statement {
+            source_info,
+            kind: StatementKind::Coverage(box Coverage {
+                kind: CoverageKind::Unreachable,
+                code_region: Some(code_region),
+            }),
+        })
+    }
+}
+
 pub struct SimplifyLocals;
 
 impl<'tcx> MirPass<'tcx> for SimplifyLocals {

compiler/rustc_mir/src/transform/simplify_try.rs

@@ -558,7 +558,7 @@ impl<'tcx> MirPass<'tcx> for SimplifyBranchSame {
 
         if did_remove_blocks {
             // We have dead blocks now, so remove those.
-            simplify::remove_dead_blocks(body);
+            simplify::remove_dead_blocks(tcx, body);
         }
     }
 }

compiler/rustc_mir/src/transform/unreachable_prop.rs

@@ -60,7 +60,11 @@ impl MirPass<'_> for UnreachablePropagation {
         }
 
         if replaced {
-            simplify::remove_dead_blocks(body);
+            simplify::remove_dead_blocks_with_coverage(
+                tcx,
+                body,
+                simplify::DroppedCoverage::SaveUnreachable,
+            );
         }
     }
 }

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.async2.txt

@@ -12,6 +12,7 @@
    12|      1|    if b {
    13|      1|        println!("non_async_func println in block");
    14|      1|    }
+                   ^0
    15|      1|}
    16|       |
    17|       |

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.conditions.txt

@@ -5,6 +5,7 @@
     5|      1|    if true {
     6|      1|        countdown = 10;
     7|      1|    }
+                   ^0
     8|       |
     9|       |    const B: u32 = 100;
    10|      1|    let x = if countdown > 7 {
@@ -24,6 +25,7 @@
    24|      1|    if true {
    25|      1|        countdown = 10;
    26|      1|    }
+                   ^0
    27|       |
    28|      1|    if countdown > 7 {
    29|      1|        countdown -= 4;
@@ -42,6 +44,7 @@
    41|      1|        if true {
    42|      1|            countdown = 10;
    43|      1|        }
+                       ^0
    44|       |
    45|      1|        if countdown > 7 {
    46|      1|            countdown -= 4;
@@ -54,13 +57,14 @@
    53|       |        } else {
    54|      0|            return;
    55|       |        }
-   56|       |    } // Note: closing brace shows uncovered (vs. `0` for implicit else) because condition literal
-   57|       |      // `true` was const-evaluated. The compiler knows the `if` block will be executed.
+   56|      0|    }
+   57|       |
    58|       |
    59|      1|    let mut countdown = 0;
    60|      1|    if true {
    61|      1|        countdown = 1;
    62|      1|    }
+                   ^0
    63|       |
    64|      1|    let z = if countdown > 7 {
                       ^0

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.doctest.txt

@@ -9,7 +9,7 @@
     8|      1|//!     assert_eq!(1, 1);
     9|       |//! } else {
    10|       |//!     // this is not!
-   11|       |//!     assert_eq!(1, 2);
+   11|      0|//!     assert_eq!(1, 2);
    12|       |//! }
    13|      1|//! ```
    14|       |//!
@@ -84,7 +84,7 @@
    74|      1|    if true {
    75|      1|        assert_eq!(1, 1);
    76|       |    } else {
-   77|       |        assert_eq!(1, 2);
+   77|      0|        assert_eq!(1, 2);
    78|       |    }
    79|      1|}
    80|       |

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.drop_trait.txt

@@ -19,11 +19,11 @@
    19|      1|    if true {
    20|      1|        println!("Exiting with error...");
    21|      1|        return Err(1);
-   22|       |    }
-   23|       |
-   24|       |    let _ = Firework { strength: 1000 };
-   25|       |
-   26|       |    Ok(())
+   22|      0|    }
+   23|      0|
+   24|      0|    let _ = Firework { strength: 1000 };
+   25|      0|
+   26|      0|    Ok(())
    27|      1|}
    28|       |
    29|       |// Expected program output:

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.generics.txt

@@ -52,15 +52,15 @@
    30|      1|    if true {
    31|      1|        println!("Exiting with error...");
    32|      1|        return Err(1);
-   33|       |    } // The remaining lines below have no coverage because `if true` (with the constant literal
-   34|       |      // `true`) is guaranteed to execute the `then` block, which is also guaranteed to `return`.
-   35|       |      // Thankfully, in the normal case, conditions are not guaranteed ahead of time, and as shown
-   36|       |      // in other tests, the lines below would have coverage (which would show they had `0`
-   37|       |      // executions, assuming the condition still evaluated to `true`).
-   38|       |
-   39|       |    let _ = Firework { strength: 1000 };
-   40|       |
-   41|       |    Ok(())
+   33|      0|    }
+   34|      0|
+   35|      0|
+   36|      0|
+   37|      0|
+   38|      0|
+   39|      0|    let _ = Firework { strength: 1000 };
+   40|      0|
+   41|      0|    Ok(())
    42|      1|}
    43|       |
    44|       |// Expected program output: