coverage: Replace max_decision_depth with num_condition_bitmaps

This clearly distinguishes individual decision-depth indices from the total
number of condition bitmaps to allocate.

Author: Zalathar

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -110,7 +110,7 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
 
         // Create locals for condition bitmaps named `mcdc.addr.{i}`.
         let mut cond_bitmaps = vec![];
-        for i in 0..=function_coverage_info.mcdc_max_decision_depth {
+        for i in 0..function_coverage_info.mcdc_num_condition_bitmaps {
             let align = Align::FOUR;
             let cond_bitmap = self.alloca(Size::from_bytes(4), align);
             llvm::set_value_name(cond_bitmap, format!("mcdc.addr.{i}").as_bytes());

compiler/rustc_middle/src/mir/coverage.rs

@@ -277,7 +277,7 @@ pub struct FunctionCoverageInfo {
     pub mappings: Vec<Mapping>,
     /// The depth of the deepest decision is used to know how many
     /// temp condbitmaps should be allocated for the function.
-    pub mcdc_max_decision_depth: u16,
+    pub mcdc_num_condition_bitmaps: usize,
 }
 
 /// Branch information recorded during THIR-to-MIR lowering, and stored in MIR.

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -102,23 +102,23 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
 
     inject_mcdc_statements(mir_body, &basic_coverage_blocks, &coverage_spans);
 
-    let mcdc_max_decision_depth = coverage_spans
+    let mcdc_num_condition_bitmaps = coverage_spans
         .mappings
         .iter()
         .filter_map(|bcb_mapping| match bcb_mapping.kind {
             BcbMappingKind::MCDCDecision { decision_depth, .. } => Some(decision_depth),
             _ => None,
         })
         .max()
-        .unwrap_or(0);
+        .map_or(0, |max| usize::from(max) + 1);
 
     mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
         function_source_hash: hir_info.function_source_hash,
         num_counters: coverage_counters.num_counters(),
         mcdc_bitmap_bytes: coverage_spans.test_vector_bitmap_bytes(),
         expressions: coverage_counters.into_expressions(),
         mappings,
-        mcdc_max_decision_depth,
+        mcdc_num_condition_bitmaps,
     }));
 }