Rollup merge of #125108 - Zalathar:info-bitmap-bytes, r=nnethercote

coverage: `CoverageIdsInfo::mcdc_bitmap_bytes` is never needed

This code for recalculating `mcdc_bitmap_bytes` in a query doesn't provide any benefit, because its result won't have changed from the value in `FunctionCoverageInfo` that was computed during the MIR instrumentation pass.

Extracted from #124571, to avoid having this held up by unrelated issues with condition count checks.

`@rustbot` label +A-code-coverage

Author: matthiaskrgr

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -207,13 +207,8 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
                 let cond_bitmap = coverage_context
                                     .try_get_mcdc_condition_bitmap(&instance, decision_depth)
                                     .expect("mcdc cond bitmap should have been allocated for merging into the global bitmap");
-                let bitmap_bytes = bx.tcx().coverage_ids_info(instance.def).mcdc_bitmap_bytes;
+                let bitmap_bytes = function_coverage_info.mcdc_bitmap_bytes;
                 assert!(bitmap_idx < bitmap_bytes, "bitmap index of the decision out of range");
-                assert!(
-                    bitmap_bytes <= function_coverage_info.mcdc_bitmap_bytes,
-                    "bitmap length disagreement: query says {bitmap_bytes} but function info only has {}",
-                    function_coverage_info.mcdc_bitmap_bytes
-                );
 
                 let fn_name = bx.get_pgo_func_name_var(instance);
                 let hash = bx.const_u64(function_coverage_info.function_source_hash);

compiler/rustc_middle/src/mir/coverage.rs

@@ -129,17 +129,11 @@ pub enum CoverageKind {
     /// Marks the point in MIR control flow represented by a evaluated condition.
     ///
     /// This is eventually lowered to `llvm.instrprof.mcdc.condbitmap.update` in LLVM IR.
-    ///
-    /// If this statement does not survive MIR optimizations, the condition would never be
-    /// taken as evaluated.
     CondBitmapUpdate { id: ConditionId, value: bool, decision_depth: u16 },
 
     /// Marks the point in MIR control flow represented by a evaluated decision.
     ///
     /// This is eventually lowered to `llvm.instrprof.mcdc.tvbitmap.update` in LLVM IR.
-    ///
-    /// If this statement does not survive MIR optimizations, the decision would never be
-    /// taken as evaluated.
     TestVectorBitmapUpdate { bitmap_idx: u32, decision_depth: u16 },
 }
 

compiler/rustc_middle/src/mir/query.rs

@@ -362,8 +362,4 @@ pub struct CoverageIdsInfo {
     /// InstrumentCoverage MIR pass, if the highest-numbered counter increments
     /// were removed by MIR optimizations.
     pub max_counter_id: mir::coverage::CounterId,
-
-    /// Coverage codegen for mcdc needs to know the size of the global bitmap so that it can
-    /// set the `bytemap-bytes` argument of the `llvm.instrprof.mcdc.tvbitmap.update` intrinsic.
-    pub mcdc_bitmap_bytes: u32,
 }

compiler/rustc_mir_transform/src/coverage/query.rs

@@ -61,17 +61,7 @@ fn coverage_ids_info<'tcx>(
         .max()
         .unwrap_or(CounterId::ZERO);
 
-    let mcdc_bitmap_bytes = mir_body
-        .coverage_branch_info
-        .as_deref()
-        .map(|info| {
-            info.mcdc_decision_spans
-                .iter()
-                .fold(0, |acc, decision| acc + (1_u32 << decision.conditions_num).div_ceil(8))
-        })
-        .unwrap_or_default();
-
-    CoverageIdsInfo { max_counter_id, mcdc_bitmap_bytes }
+    CoverageIdsInfo { max_counter_id }
 }
 
 fn all_coverage_in_mir_body<'a, 'tcx>(