Attempt to fix #7954

 Changes to be committed:
	modified:   cranelift/codegen/src/alias_analysis.rs: Use
        dominator tree
        modified:   cranelift/codegen/src/context.rs: Use dominator tree
	modified:   cranelift/codegen/src/dominator_tree.rs: Fix
        dominates and computes and modify logic of domination

cranelift/codegen/src/alias_analysis.rs

@@ -63,7 +63,7 @@
 
 use crate::{
     cursor::{Cursor, FuncCursor},
-    dominator_tree::DominatorTree,
+    dominator_tree::DominatorTreePreorder,
     inst_predicates::{
         has_memory_fence_semantics, inst_addr_offset_type, inst_store_data, visit_block_succs,
     },
@@ -176,7 +176,7 @@ struct MemoryLoc {
 /// An alias-analysis pass.
 pub struct AliasAnalysis<'a> {
     /// The domtree for the function.
-    domtree: &'a DominatorTree,
+    domtree: &'a DominatorTreePreorder,
 
     /// Input state to a basic block.
     block_input: FxHashMap<Block, LastStores>,
@@ -191,7 +191,7 @@ pub struct AliasAnalysis<'a> {
 
 impl<'a> AliasAnalysis<'a> {
     /// Perform an alias analysis pass.
-    pub fn new(func: &Function, domtree: &'a DominatorTree) -> AliasAnalysis<'a> {
+    pub fn new(func: &Function, domtree: &'a DominatorTreePreorder) -> AliasAnalysis<'a> {
         trace!("alias analysis: input is:\n{:?}", func);
         let mut analysis = AliasAnalysis {
             domtree,
@@ -333,7 +333,7 @@ impl<'a> AliasAnalysis<'a> {
                             value.index(),
                             def_inst.index()
                         );
-                        if self.domtree.dominates(def_inst, inst, &func.layout) {
+                        if self.domtree.dominates_inst(def_inst, inst, &func.layout) {
                             trace!(
                                 " -> dominates; value equiv from v{} to v{} inserted",
                                 load_result.index(),

cranelift/codegen/src/context.rs

@@ -10,7 +10,7 @@
 //! single ISA instance.
 
 use crate::alias_analysis::AliasAnalysis;
-use crate::dominator_tree::DominatorTree;
+use crate::dominator_tree::{DominatorTree, DominatorTreePreorder};
 use crate::egraph::EgraphPass;
 use crate::flowgraph::ControlFlowGraph;
 use crate::ir::Function;
@@ -46,6 +46,9 @@ pub struct Context {
     /// Dominator tree for `func`.
     pub domtree: DominatorTree,
 
+    /// Domniator tree preorder, be on look out to merge with domtree, although unlikely
+    pub domtree_preorder: DominatorTreePreorder,
+
     /// Loop analysis of `func`.
     pub loop_analysis: LoopAnalysis,
 
@@ -74,6 +77,7 @@ impl Context {
             func,
             cfg: ControlFlowGraph::new(),
             domtree: DominatorTree::new(),
+            domtree_preorder: DominatorTreePreorder::new(),
             loop_analysis: LoopAnalysis::new(),
             compiled_code: None,
             want_disasm: false,
@@ -300,7 +304,9 @@ impl Context {
 
     /// Compute dominator tree.
     pub fn compute_domtree(&mut self) {
-        self.domtree.compute(&self.func, &self.cfg)
+        self.domtree.compute(&self.func, &self.cfg);
+        self.domtree_preorder
+            .compute(&self.domtree, &self.func.layout);
     }
 
     /// Compute the loop analysis.
@@ -330,7 +336,7 @@ impl Context {
     /// by a store instruction to the same instruction (so-called
     /// "store-to-load forwarding").
     pub fn replace_redundant_loads(&mut self) -> CodegenResult<()> {
-        let mut analysis = AliasAnalysis::new(&self.func, &self.domtree);
+        let mut analysis = AliasAnalysis::new(&self.func, &self.domtree_preorder);
         analysis.compute_and_update_aliases(&mut self.func);
         Ok(())
     }
@@ -362,7 +368,8 @@ impl Context {
         );
         let fisa = fisa.into();
         self.compute_loop_analysis();
-        let mut alias_analysis = AliasAnalysis::new(&self.func, &self.domtree);
+        let mut alias_analysis = AliasAnalysis::new(&self.func, &self.domtree_preorder);
+
         let mut pass = EgraphPass::new(
             &mut self.func,
             &self.domtree,

cranelift/codegen/src/dominator_tree.rs

@@ -482,7 +482,42 @@ impl DominatorTreePreorder {
     pub fn dominates(&self, a: Block, b: Block) -> bool {
         let na = &self.nodes[a];
         let nb = &self.nodes[b];
-        na.pre_number <= nb.pre_number && na.pre_max >= nb.pre_max
+        // debug_assert!(na.pre_number != 0);
+        // debug_assert!(nb.pre_number != 0);
+        if a == b {
+            // check first if they're the same block
+            // if they are then they dominate themselves
+            true
+        } else if nb.pre_number == 0 {
+            // if the supposedly dominated is not reachable (pre_number = 0), is it
+            // really being dominated though ?
+            false
+        } else {
+            na.pre_number <= nb.pre_number && na.pre_max >= nb.pre_max
+        }
+    }
+
+    /// Checks if one instruction dominates another.
+    ///
+    /// From @jameysharp:
+    /// Instruction a dominates instruction b if either of these conditions are true:
+    ///    a and b are in the same block, and a is not after b.
+    ///    Or the block containing a dominates the block containing b.
+    ///
+    ///    "I would prefer to have dominates_inst next to the dominates method that it uses.
+    ///     Then someone looking at one of them can easily see the other one too."
+    ///
+    pub fn dominates_inst(&self, a: Inst, b: Inst, layout: &Layout) -> bool {
+        match (layout.inst_block(a), layout.inst_block(b)) {
+            (Some(block_a), Some(block_b)) => {
+                if block_a == block_b {
+                    layout.pp_cmp(a, b) != Ordering::Greater
+                } else {
+                    self.dominates(block_a, block_b)
+                }
+            }
+            _ => false,
+        }
     }
 
     /// Compare two blocks according to the dominator pre-order.