[gcc r13-2285] Make graphds_scc pass the node order back to callers

[gcc r13-2285] Make graphds_scc pass the node order back to callers

[Richard Sandiford]

https://gcc.gnu.org/g:2bac880a19633b96608548b412739379df1c10dc

commit r13-2285-g2bac880a19633b96608548b412739379df1c10dc
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Tue Aug 30 15:43:46 2022 +0100

    Make graphds_scc pass the node order back to callers
    
    As a side-effect, graphds_scc constructs a vector in which all
    nodes in an SCC are listed consecutively.  This can be useful
    information, so that the patch adds an optional pass-back parameter
    for it.  The interface is similar to the one for graphds_dfs.
    
    gcc/
            * graphds.cc (graphds_scc): Add a pass-back parameter for the
            final node order.
            * graphds.h (graphds_scc): Update prototype accordingly.

Diff:
---
 gcc/graphds.cc | 13 ++++++++++---
 gcc/graphds.h  |  3 ++-
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/gcc/graphds.cc b/gcc/graphds.cc
index 91a2ca5c225..2a108fd475f 100644
--- a/gcc/graphds.cc
+++ b/gcc/graphds.cc
@@ -281,7 +281,14 @@ graphds_dfs (struct graph *g, int *qs, int nq, vec<int> *qt,
    numbers assigned by the previous pass.  If SUBGRAPH is not NULL, it
    specifies the subgraph of G whose strongly connected components we want
    to determine.  If SKIP_EDGE_P is not NULL, it points to a callback function.
-   Edge E will be skipped if callback function returns true.
+   Edge E will be skipped if callback function returns true.  If SCC_GROUPING
+   is not null, the nodes will be added to it in the following order:
+
+   - If SCC A is a direct or indirect predecessor of SCC B in the SCC dag,
+     A's nodes come before B's nodes.
+
+   - All of an SCC's nodes are listed consecutively, although the order
+     of the nodes within an SCC is not really meaningful.
 
    After running this function, v->component is the number of the strongly
    connected component for each vertex of G.  Returns the number of the
@@ -289,7 +296,7 @@ graphds_dfs (struct graph *g, int *qs, int nq, vec<int> *qt,
 
 int
 graphds_scc (struct graph *g, bitmap subgraph,
-	     skip_edge_callback skip_edge_p)
+	     skip_edge_callback skip_edge_p, vec<int> *scc_grouping)
 {
   int *queue = XNEWVEC (int, g->n_vertices);
   vec<int> postorder = vNULL;
@@ -317,7 +324,7 @@ graphds_scc (struct graph *g, bitmap subgraph,
 
   for (i = 0; i < nq; i++)
     queue[i] = postorder[nq - i - 1];
-  comp = graphds_dfs (g, queue, nq, NULL, true, subgraph, skip_edge_p);
+  comp = graphds_dfs (g, queue, nq, scc_grouping, true, subgraph, skip_edge_p);
 
   free (queue);
   postorder.release ();
diff --git a/gcc/graphds.h b/gcc/graphds.h
index c54d8767fa7..e0e4d802cbb 100644
--- a/gcc/graphds.h
+++ b/gcc/graphds.h
@@ -58,7 +58,8 @@ void identify_vertices (struct graph *, int, int);
 typedef bool (*skip_edge_callback) (struct graph_edge *);
 int graphds_dfs (struct graph *, int *, int,
 		 vec<int> *, bool, bitmap, skip_edge_callback = NULL);
-int graphds_scc (struct graph *, bitmap, skip_edge_callback = NULL);
+int graphds_scc (struct graph *, bitmap, skip_edge_callback = NULL,
+		 vec<int> * = NULL);
 void graphds_domtree (struct graph *, int, int *, int *, int *);
 typedef void (*graphds_edge_callback) (struct graph *,
 				       struct graph_edge *, void *);