ssa-range-prop: fix ICE when encountering self-domiating bb

compiler/rustc_data_structures/src/graph/dominators/mod.rs

@@ -386,6 +386,24 @@ impl<Node: Idx> Dominators<Node> {
             }
         }
     }
+
+    /// Returns true if `a` **strictly** dominates `b`
+    ///
+    /// # Panics
+    ///
+    /// Panics if `b` is unreachable
+    #[inline]
+    pub fn strictly_dominates(&self, a: Node, b: Node) -> bool {
+        match &self.kind {
+            Kind::Path => a.index() < b.index(),
+            Kind::General(g) => {
+                let a = g.time[a];
+                let b = g.time[b];
+                assert!(b.start != 0, "node {b:?} is not reachable");
+                a.start < b.start && b.finish < a.finish
+            }
+        }
+    }
 }
 
 /// Describes the number of vertices discovered at the time when processing of a particular vertex

compiler/rustc_middle/src/mir/mod.rs

@@ -1609,6 +1609,11 @@ impl Location {
             dominators.dominates(self.block, other.block)
         }
     }
+
+    #[inline]
+    pub fn strictly_dominates(&self, other: Location, dominators: &Dominators<BasicBlock>) -> bool {
+        self.block != other.block && dominators.strictly_dominates(self.block, other.block)
+    }
 }
 
 /// `DefLocation` represents the location of a definition - either an argument or an assignment

compiler/rustc_mir_transform/src/ssa_range_prop.rs

@@ -163,8 +163,7 @@ impl<'tcx> MutVisitor<'tcx> for RangeSet<'tcx, '_, '_> {
                     && self.is_ssa(place) =>
             {
                 let successor = Location { block: *target, statement_index: 0 };
-                if location.dominates(successor, &self.dominators) {
-                    assert_ne!(location.block, successor.block);
+                if location.strictly_dominates(successor, &self.dominators) {
                     let val = *expected as u128;
                     let range = WrappingRange { start: val, end: val };
                     self.insert_range(place, successor, range);

tests/ui/mir/ssa-range-prop-bb-self-domination.rs

@@ -0,0 +1,22 @@
+/// Regression test for <https://github.com/rust-lang/rust/issues/155836>.
+///
+/// SsaRangeProp pass used to fail the assert when encountering self-dominating block
+/// e.g. small loops like in `a`
+
+//@ compile-flags: -Copt-level=2
+//@ build-pass
+
+use std::hint::black_box;
+fn a(d: u8) {
+    loop {
+        1 % d;
+    }
+}
+pub fn e(d: u8) {
+    if d == 0 || black_box(false) {
+        a(d);
+    }
+}
+fn main() {
+    e(1);
+}