mcdc-coverage: Generate mcdc.parameters intrinsic call at the beginning of a function

Author: RenjiSann

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -106,9 +106,18 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
                 "marker statement {kind:?} should have been removed by CleanupPostBorrowck"
             ),
             CoverageKind::MCDCBitmapRequire { needed_bytes } => {
-                // FIXME(dprn): TODO
-                let _ = needed_bytes;
-                warn!("call to correct intrinsic");
+                // We need to explicitly drop the `RefMut` before calling into `instrprof_mcdc_parameters`,
+                // as that needs an exclusive borrow.
+                drop(coverage_map);
+
+                let fn_name = bx.get_pgo_func_name_var(instance);
+                let hash = bx.const_u64(function_coverage_info.function_source_hash);
+                let num_bitmap_bytes = bx.const_u32(needed_bytes);
+                debug!(
+                    "codegen intrinsic instrprof.mcdc.parameters(fn_name={:?}, hash={:?}, bitmap_bytes={:?})",
+                    fn_name, hash, num_bitmap_bytes,
+                );
+                bx.instrprof_mcdc_parameters(fn_name, hash, num_bitmap_bytes);
             }
             CoverageKind::CounterIncrement { id } => {
                 func_coverage.mark_counter_id_seen(id);

compiler/rustc_mir_transform/messages.ftl

@@ -34,6 +34,8 @@ mir_transform_mutation_layout_constrained_borrow_label = borrow of layout constr
 mir_transform_mutation_layout_constrained_borrow_note = references to fields of layout constrained fields lose the constraints. Coupled with interior mutability, the field can be changed to invalid values
 mir_transform_mutation_layout_constrained_label = mutation of layout constrained field
 mir_transform_mutation_layout_constrained_note = mutating layout constrained fields cannot statically be checked for valid values
+mir_transform_mcdc_too_many_conditions = Unsupported MCDC Decision: Number of conditions ({$num_conditions}) exceeds max ({$max_conditions}). Expression will be skipped.
+
 mir_transform_operation_will_panic = this operation will panic at runtime
 
 mir_transform_requires_unsafe = {$details} is unsafe and requires unsafe {$op_in_unsafe_fn_allowed ->

compiler/rustc_mir_transform/src/coverage/mcdc.rs

@@ -0,0 +1,60 @@
+use rustc_macros::Diagnostic;
+use rustc_middle::{
+    mir::{
+        self,
+        coverage::{CoverageKind, DecisionSpan},
+        Statement,
+    },
+    ty::TyCtxt,
+};
+use rustc_span::Span;
+
+#[derive(Diagnostic)]
+#[diag(mir_transform_mcdc_too_many_conditions)]
+pub(crate) struct MCDCTooManyConditions {
+    #[primary_span]
+    pub span: Span,
+    pub num_conditions: u32,
+    pub max_conditions: u32,
+}
+
+const MAX_COND_DECISION: u32 = 6;
+
+/// If MCDC coverage is enabled, add MCDC instrumentation to the function.
+/// Will do the following things :
+/// 1. Add an instruction in the first basic block for the codegen to call
+///     the `instrprof.mcdc.parameters` instrinsic.
+pub fn instrument_function_mcdc<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir::Body<'tcx>) {
+    if !tcx.sess.instrument_coverage_mcdc() {
+        return;
+    }
+
+    let Some(branch_info) = mir_body.coverage_branch_info.as_deref() else {
+        return;
+    };
+
+    // Compute the total sum of needed bytes for the current body.
+    let needed_bytes: u32 = branch_info
+        .decision_spans
+        .iter()
+        .filter_map(|DecisionSpan { span, num_conditions }| {
+            if *num_conditions > MAX_COND_DECISION {
+                tcx.dcx().emit_warn(MCDCTooManyConditions {
+                    span: *span,
+                    num_conditions: *num_conditions,
+                    max_conditions: MAX_COND_DECISION,
+                });
+                None
+            } else {
+                Some(1 << num_conditions)
+            }
+        })
+        .sum();
+
+    let entry_bb = &mut mir_body.basic_blocks_mut()[mir::START_BLOCK];
+    let mcdc_parameters_statement = Statement {
+        source_info: entry_bb.terminator().source_info,
+        kind: mir::StatementKind::Coverage(CoverageKind::MCDCBitmapRequire { needed_bytes }),
+    };
+    entry_bb.statements.insert(0, mcdc_parameters_statement);
+}

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -3,6 +3,7 @@ pub mod query;
 mod counters;
 mod graph;
 mod spans;
+mod mcdc;
 
 #[cfg(test)]
 mod tests;
@@ -100,6 +101,10 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
         &coverage_counters,
     );
 
+    ////////////////////////////////////////////////////
+    // Add the MCDC instrumentation. This is a no-op if MCDC coverage is disabled.
+    mcdc::instrument_function_mcdc(tcx, mir_body);
+
     mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
         function_source_hash: hir_info.function_source_hash,
         num_counters: coverage_counters.num_counters(),