Make coverage counter IDs count up from 0, not 1

Operand types are now tracked explicitly, so there is no need to reserve ID 0
for the special always-zero counter.

As part of the renumbering, this change fixes an off-by-one error in the way
counters were counted by the `coverageinfo` query. As a result, functions
should now have exactly the number of counters they actually need, instead of
always having an extra counter that is never used.

compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs

@@ -1,4 +1,4 @@
-use rustc_middle::mir::coverage::{CounterValueReference, MappedExpressionIndex};
+use rustc_middle::mir::coverage::{CounterId, MappedExpressionIndex};
 
 /// Must match the layout of `LLVMRustCounterKind`.
 #[derive(Copy, Clone, Debug)]
@@ -36,11 +36,9 @@ impl Counter {
         Self { kind: CounterKind::Zero, id: 0 }
     }
 
-    /// Constructs a new `Counter` of kind `CounterValueReference`, and converts
-    /// the given 1-based counter_id to the required 0-based equivalent for
-    /// the `Counter` encoding.
-    pub fn counter_value_reference(counter_id: CounterValueReference) -> Self {
-        Self { kind: CounterKind::CounterValueReference, id: counter_id.zero_based_index() }
+    /// Constructs a new `Counter` of kind `CounterValueReference`.
+    pub fn counter_value_reference(counter_id: CounterId) -> Self {
+        Self { kind: CounterKind::CounterValueReference, id: counter_id.as_u32() }
     }
 
     /// Constructs a new `Counter` of kind `Expression`.

compiler/rustc_codegen_llvm/src/coverageinfo/map_data.rs

@@ -3,7 +3,7 @@ pub use super::ffi::*;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::bug;
 use rustc_middle::mir::coverage::{
-    CodeRegion, CounterValueReference, ExpressionId, MappedExpressionIndex, Op, Operand,
+    CodeRegion, CounterId, ExpressionId, MappedExpressionIndex, Op, Operand,
 };
 use rustc_middle::ty::Instance;
 use rustc_middle::ty::TyCtxt;
@@ -32,7 +32,7 @@ pub struct FunctionCoverage<'tcx> {
     instance: Instance<'tcx>,
     source_hash: u64,
     is_used: bool,
-    counters: IndexVec<CounterValueReference, Option<CodeRegion>>,
+    counters: IndexVec<CounterId, Option<CodeRegion>>,
     expressions: IndexVec<ExpressionId, Option<Expression>>,
     unreachable_regions: Vec<CodeRegion>,
 }
@@ -80,7 +80,7 @@ impl<'tcx> FunctionCoverage<'tcx> {
     }
 
     /// Adds a code region to be counted by an injected counter intrinsic.
-    pub fn add_counter(&mut self, id: CounterValueReference, region: CodeRegion) {
+    pub fn add_counter(&mut self, id: CounterId, region: CodeRegion) {
         if let Some(previous_region) = self.counters[id].replace(region.clone()) {
             assert_eq!(previous_region, region, "add_counter: code region for id changed");
         }
@@ -192,18 +192,7 @@ impl<'tcx> FunctionCoverage<'tcx> {
         type NewIndexes = IndexSlice<ExpressionId, Option<MappedExpressionIndex>>;
         let id_to_counter = |new_indexes: &NewIndexes, operand: Operand| match operand {
             Operand::Zero => Some(Counter::zero()),
-            Operand::Counter(id) => {
-                debug_assert!(
-                    id.index() > 0,
-                    "Operand::Counter indexes for counters are 1-based, but this id={}",
-                    id.index()
-                );
-                // Note: Some codegen-injected Counters may be only referenced by `Expression`s,
-                // and may not have their own `CodeRegion`s,
-                let index = CounterValueReference::from(id.index());
-                // Note, the conversion to LLVM `Counter` adjusts the index to be zero-based.
-                Some(Counter::counter_value_reference(index))
-            }
+            Operand::Counter(id) => Some(Counter::counter_value_reference(id)),
             Operand::Expression(id) => {
                 self.expressions
                     .get(id)

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -16,9 +16,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_llvm::RustString;
 use rustc_middle::bug;
-use rustc_middle::mir::coverage::{
-    CodeRegion, CounterValueReference, CoverageKind, ExpressionId, Op, Operand,
-};
+use rustc_middle::mir::coverage::{CodeRegion, CounterId, CoverageKind, ExpressionId, Op, Operand};
 use rustc_middle::mir::Coverage;
 use rustc_middle::ty;
 use rustc_middle::ty::layout::{FnAbiOf, HasTyCtxt};
@@ -33,7 +31,7 @@ mod ffi;
 pub(crate) mod map_data;
 pub mod mapgen;
 
-const UNUSED_FUNCTION_COUNTER_ID: CounterValueReference = CounterValueReference::START;
+const UNUSED_FUNCTION_COUNTER_ID: CounterId = CounterId::START;
 
 const VAR_ALIGN_BYTES: usize = 8;
 
@@ -125,7 +123,7 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
                     let fn_name = bx.get_pgo_func_name_var(instance);
                     let hash = bx.const_u64(function_source_hash);
                     let num_counters = bx.const_u32(coverageinfo.num_counters);
-                    let index = bx.const_u32(id.zero_based_index());
+                    let index = bx.const_u32(id.as_u32());
                     debug!(
                         "codegen intrinsic instrprof.increment(fn_name={:?}, hash={:?}, num_counters={:?}, index={:?})",
                         fn_name, hash, num_counters, index,
@@ -178,7 +176,7 @@ impl<'tcx> Builder<'_, '_, 'tcx> {
     fn add_coverage_counter(
         &mut self,
         instance: Instance<'tcx>,
-        id: CounterValueReference,
+        id: CounterId,
         region: CodeRegion,
     ) -> bool {
         if let Some(coverage_context) = self.coverage_context() {