mirror of
https://gitlab.freedesktop.org/mesa/mesa.git
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compiler/rust: Fix the DFS loop detection algorithm
The previous algorithm just looked at the dominator's loop header.
However, if you have multiple consecutive loops like:
function_impl {
loop {
// Stuff
}
loop {
// Other stuff
}
}
then it will look like the second loop is contained in the first loop
because the first loop's header dominates the second loop. This isn't
actually what we want. Instead, we want a node N to be considered part
of a loop with header H if H dominates N and H is reachable from N.
Fixes: 741f7067f1 ("nak: Add loop detection to the CFG")
Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36524>
(cherry picked from commit a1d5e8bfdb)
This commit is contained in:
Faith Ekstrand
Eric Engestrom
parent
953a791b53
commit
d583833971
2 changed files with 350 additions and 22 deletions
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@ -74,7 +74,7 @@
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"description": "compiler/rust: Fix the DFS loop detection algorithm",
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"nominated": true,
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"nomination_type": 2,
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"resolution": 0,
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"resolution": 1,
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"main_sha": null,
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"because_sha": "741f7067f1fb5953e84bac0fcb5c6c7df588df48",
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"notes": null
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@ -181,46 +181,100 @@ fn loop_detect_dfs<N>(
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id: usize,
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pre: &mut BitSet,
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post: &mut BitSet,
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loops: &mut BitSet,
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back_edges: &mut Vec<(usize, usize)>,
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) {
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if pre.get(id) {
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if !post.get(id) {
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loops.insert(id);
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}
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return;
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}
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pre.insert(id);
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for s in nodes[id].succ.iter() {
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loop_detect_dfs(nodes, *s, pre, post, loops);
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for &s in nodes[id].succ.iter() {
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if pre.get(s) && !post.get(s) {
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back_edges.push((id, s));
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}
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loop_detect_dfs(nodes, s, pre, post, back_edges);
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}
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post.insert(id);
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}
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/// Computes the set of nodes that reach the given node without going through
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/// stop
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fn reaches_dfs<N>(
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nodes: &Vec<CFGNode<N>>,
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id: usize,
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stop: usize,
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reaches: &mut BitSet,
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) {
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if id == stop || reaches.get(id) {
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return;
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}
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reaches.insert(id);
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// Since we're trying to find the set of things that reach the start node,
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// not the set of things reachable from the start node, walk predecessors.
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for &s in nodes[id].pred.iter() {
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reaches_dfs(nodes, s, stop, reaches);
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}
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}
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fn detect_loops<N>(nodes: &mut Vec<CFGNode<N>>) -> bool {
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let mut dfs_pre = BitSet::new();
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let mut dfs_post = BitSet::new();
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let mut loops = BitSet::new();
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loop_detect_dfs(nodes, 0, &mut dfs_pre, &mut dfs_post, &mut loops);
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let mut back_edges = Vec::new();
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loop_detect_dfs(nodes, 0, &mut dfs_pre, &mut dfs_post, &mut back_edges);
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if back_edges.is_empty() {
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return false;
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}
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// Construct a map from nodes N to loop headers H where there is some
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// back-edge (C, H) such that C is reachable from N without going through H.
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// By running the DFS backwards, we can do this in O(B * E) time where B is
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// the number of back-edges and E is the total number of edges.
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let mut loops = BitSet::new();
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let mut node_loops: Vec<BitSet> = Default::default();
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node_loops.resize_with(nodes.len(), Default::default);
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for (c, h) in back_edges {
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// Stash the loop headers while we're here
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loops.insert(h);
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// re-use dfs_pre for our reaches set
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dfs_pre.clear();
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let reaches = &mut dfs_pre;
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reaches_dfs(nodes, c, h, reaches);
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for n in reaches.iter() {
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node_loops[n].insert(h);
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}
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}
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for i in 0..nodes.len() {
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debug_assert!(nodes[i].lph == usize::MAX);
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let mut has_loop = false;
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nodes[0].lph = usize::MAX;
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for i in 1..nodes.len() {
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if loops.get(i) {
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// This is a loop header
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nodes[i].lph = i;
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has_loop = true;
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} else {
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// Otherwise, we have the same loop header as our dominator
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let dom = nodes[i].dom;
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let dom_lph = nodes[dom].lph;
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nodes[i].lph = dom_lph;
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continue;
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}
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let mut n = i;
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while n != 0 {
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let dom = nodes[n].dom;
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debug_assert!(dom < n);
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if node_loops[i].get(dom) {
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nodes[i].lph = dom;
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break;
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};
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n = dom;
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}
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}
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has_loop
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true
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}
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pub struct CFG<N> {
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@ -306,10 +360,27 @@ impl<N> CFG<N> {
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self.has_loop
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}
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/// Returns true if the given node is a loop header.
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///
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/// A node H is a loop header if there is a back-edge terminating at H.
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pub fn is_loop_header(&self, idx: usize) -> bool {
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self.nodes[idx].lph == idx
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}
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/// Returns the index of the loop header of the innermost loop containing
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/// this node, if any. If this node is not contained in any loops, `None`
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/// is returned.
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///
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/// A node N is considered to be contained to be contained in the loop with
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/// header H if both of the following are true:
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///
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/// 1. H dominates N
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///
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/// 2. There is a back-edge (C, H) in the CFG such that C is reachable
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/// from N without going through H.
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///
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/// This matches the definitions given in
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/// https://www.cs.cornell.edu/courses/cs4120/2023sp/notes.html?id=cflow
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pub fn loop_header_index(&self, idx: usize) -> Option<usize> {
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let lph = self.nodes[idx].lph;
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if lph == usize::MAX {
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@ -407,3 +478,260 @@ impl<K, N> Default for CFGBuilder<K, N> {
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CFGBuilder::new()
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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fn test_loop_nesting(edges: &[(usize, usize)], expected: &[Option<usize>]) {
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let mut builder = CFGBuilder::new();
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for i in 0..expected.len() {
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builder.add_node(i, i);
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}
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for &(a, b) in edges {
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builder.add_edge(a, b);
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}
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let cfg = builder.as_cfg();
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let mut lphs = Vec::new();
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lphs.resize(expected.len(), None);
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for (i, idx) in cfg.iter().enumerate() {
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let lph = cfg.loop_header_index(i);
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lphs[*idx.deref()] = lph.map(|h| cfg[h]);
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}
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assert_eq!(&lphs, expected);
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}
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#[test]
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fn test_loop_simple() {
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// block 0
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// loop {
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// block 1
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// if ... {
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// block 2
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// break;
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// }
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// block 3
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// }
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// block 4
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test_loop_nesting(
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&[(0, 1), (1, 2), (1, 3), (2, 4), (3, 1)],
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&[None, Some(1), None, Some(1), None, None],
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);
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}
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#[test]
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fn test_loop_simple_nested() {
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// loop {
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// block 0
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// loop {
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// block 1
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// if ... {
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// block 2
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// break;
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// }
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// block 3
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// }
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// block 4
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// if ... {
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// block 5
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// break;
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// }
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// block 6
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// }
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// block 7
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test_loop_nesting(
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&[
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(0, 1),
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(1, 2),
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(1, 3),
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(2, 4),
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(3, 1),
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(4, 5),
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(4, 6),
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(5, 7),
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(6, 0),
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],
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&[
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Some(0),
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Some(1),
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Some(0),
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Some(1),
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Some(0),
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None,
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Some(0),
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None,
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],
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);
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}
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#[test]
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fn test_loop_two_continues() {
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// loop {
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// block 0
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// if ... {
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// block 1
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// continue;
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// }
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// block 2
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// if ... {
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// block 3
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// break;
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// }
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// block 4
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// }
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// block 5
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test_loop_nesting(
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&[(0, 1), (0, 2), (1, 0), (2, 3), (2, 4), (3, 5), (4, 0)],
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&[Some(0), Some(0), Some(0), None, Some(0), None],
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);
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}
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#[test]
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fn test_loop_two_breaks() {
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// loop {
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// block 0
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// if ... {
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// block 1
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// break;
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// }
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// block 2
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// if ... {
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// block 3
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// break;
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// }
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// block 4
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// }
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// block 5
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test_loop_nesting(
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&[(0, 1), (0, 2), (1, 5), (2, 3), (2, 4), (3, 5), (4, 0)],
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&[Some(0), None, Some(0), None, Some(0), None],
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);
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}
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#[test]
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fn test_loop_predicated_continue() {
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// loop {
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// block 0
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// continue_if(...);
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// block 1
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// if ... {
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// block 2
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// break;
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// }
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// block 3
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// }
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// block 4
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test_loop_nesting(
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&[(0, 0), (0, 1), (1, 2), (1, 3), (2, 4), (3, 0)],
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&[Some(0), Some(0), None, Some(0), None],
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);
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}
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#[test]
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fn test_loop_predicated_break() {
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// block 0
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// loop {
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// block 1
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// break_if(...);
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// block 2
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// if ... {
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// block 3
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// break;
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// }
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// block 4
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// }
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// block 5
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test_loop_nesting(
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&[(0, 1), (1, 2), (1, 5), (2, 3), (2, 4), (3, 5), (4, 1)],
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&[None, Some(1), Some(1), None, Some(1), None],
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);
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}
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#[test]
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fn test_loop_complex() {
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// loop {
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// block 0
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// loop {
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// block 1
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// if ... {
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// block 2
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// break;
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// }
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// block 3
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// }
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// loop {
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// block 4
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// break_if(..);
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// }
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// block 5
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// if ... {
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// block 6
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// break;
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// }
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// block 7
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// }
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// block 8
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test_loop_nesting(
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&[
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(0, 1),
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(1, 2),
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(1, 3),
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(2, 4),
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(3, 1),
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(4, 5),
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(4, 4),
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(5, 6),
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(5, 7),
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(6, 8),
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(7, 0),
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],
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&[
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Some(0),
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Some(1),
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Some(0),
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Some(1),
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Some(4),
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Some(0),
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None,
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Some(0),
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None,
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],
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);
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}
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#[test]
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fn test_simple_irreducible() {
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// block 0
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// if ... {
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// block 1
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// }
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// block 2
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// if ... {
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// block 3
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// }
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// block 4
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test_loop_nesting(
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&[(0, 1), (0, 2), (1, 2), (2, 3), (2, 4), (3, 4)],
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&[None, None, None, None, None, None],
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);
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}
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#[test]
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fn test_loop_irreducible() {
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// block 0
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// goto_if(...) label;
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// loop {
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// block 1
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// break_if(...);
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// label:
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// block 2
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// }
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// block 3
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test_loop_nesting(
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&[(0, 1), (0, 2), (1, 3), (1, 2), (2, 1)],
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&[None, Some(1), None, None, None, None],
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);
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}
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}
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