Change wording of checks and documentation.

Author: iwanders

src/lib.rs

@@ -1120,9 +1120,6 @@ pub enum AllocationKind {
 ///
 /// (This trait's design is inspired by, and derives heavily from, the
 /// trait of the same name in regalloc.rs.)
-///
-/// The ids used in [`Block`] and [`VReg`] must start their numbering
-/// at zero and be sequential.
 pub trait Function {
     // -------------
     // CFG traversal
@@ -1131,7 +1128,13 @@ pub trait Function {
     /// How many instructions are there?
     fn num_insts(&self) -> usize;
 
-    /// How many blocks are there?
+    /// Number of entries to allocate [`Block`] storage for.
+    ///
+    /// The block ids are used as indices into a container, this
+    /// container is allocated with this number of elements. This means
+    /// that the value returned here must be the highest block index in
+    /// the function plus one. The block indices themselves need not be
+    /// consecutive.
     fn num_blocks(&self) -> usize;
 
     /// Get the index of the entry block.
@@ -1201,7 +1204,13 @@ pub trait Function {
     /// value(s).
     fn inst_clobbers(&self, insn: Inst) -> PRegSet;
 
-    /// Get the number of `VReg` in use in this function.
+    /// Number of entries to allocate [`VReg`] storage for.
+    ///
+    /// The VReg ids are used as indices into a container, this
+    /// container is allocated with this number of elements. This
+    /// means that the value returned here must be the highest block
+    /// index in the function plus one. The vreg indices themselves
+    /// need not be consecutive.
     fn num_vregs(&self) -> usize;
 
     /// Get the VRegs for which we should generate value-location

src/postorder.rs

@@ -38,11 +38,12 @@ pub fn calculate<'a, SuccFn: Fn(Block) -> &'a [Block]>(
     while let Some(ref mut state) = stack.last_mut() {
         // Perform one action: push to new succ, skip an already-visited succ, or pop.
         if let Some(&succ) = state.succs.next() {
-            let succ_visit = visited
-                .get_mut(succ.index())
-                .ok_or(RegAllocError::BB(succ))?;
-            if !*succ_visit {
-                *succ_visit = true;
+            if succ.index() >= visited.len() {
+                trace!("Block index {:?} exceeds f.num_blocks().", succ);
+                return Err(RegAllocError::BB(succ));
+            }
+            if !visited[succ.index()] {
+                visited[succ.index()] = true;
                 stack.push(State {
                     block: succ,
                     succs: succ_blocks(succ).iter(),

src/ssa.rs

@@ -17,7 +17,7 @@ pub fn validate_ssa<F: Function>(f: &F, cfginfo: &CFGInfo) -> Result<(), RegAllo
         let block = Block::new(block);
         let mut def = |vreg: VReg, inst| {
             if vreg.vreg() >= defined_in.len() {
-                trace!("VRegs not numbered consecutively {:?}", vreg);
+                trace!("VReg index {:?} exceeds exceeds f.num_vregs().", vreg);
                 return Err(RegAllocError::SSA(vreg, inst));
             }
             if defined_in[vreg.vreg()].is_valid() {