[Mesa-dev] [PATCH 2/2] nir: Remove fake edges in the CF handling code

[Jason Ekstrand]

When NIR was first introduced, Connor added this fake-edge hack to work
around issues related to unreachable blocks.  Thanks to GLSL IR's jump
lowering code, the only unreachable code you can have is a block after an
infinite loop.  With SPIR-V, we didn't have the jump lowering code so we
could also end up with the "if (...) { break; } else { continue; }" case
which generates an unreachable block after the if.  Because of this, most
of NIR had to be fixed up for handling unreachable blocks.  The only
remaining case of not handling unreachable blocks was specifically the
block-after-infinite-loop case in dead_cf which was fixed by the previous
commit.  We can now delete the fake edge hack.

Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
---
 src/compiler/nir/nir_control_flow.c | 59 ++-----------------------------------
 1 file changed, 2 insertions(+), 57 deletions(-)

diff --git a/src/compiler/nir/nir_control_flow.c b/src/compiler/nir/nir_control_flow.c
index a485e71..05cc9e9 100644
--- a/src/compiler/nir/nir_control_flow.c
+++ b/src/compiler/nir/nir_control_flow.c
@@ -39,8 +39,6 @@
  * 3. Two basic blocks cannot be directly next to each other.
  * 4. If a basic block has a jump instruction, there must be only one and it
  *    must be at the end of the block.
- * 5. The CFG must always be connected - this means that we must insert a fake
- *    CFG edge for loops with no break statement.
  *
  * The purpose of the second one is so that we have places to insert code during
  * GCM, as well as eliminating the possibility of critical edges.
@@ -135,25 +133,6 @@ link_non_block_to_block(nir_cf_node *node, nir_block *block)
       }
    } else {
       assert(node->type == nir_cf_node_loop);
-
-      /*
-       * We can only get to this codepath if we're inserting a new loop, or
-       * at least a loop with no break statements; we can't insert break
-       * statements into a loop when we haven't inserted it into the CFG
-       * because we wouldn't know which block comes after the loop
-       * and therefore, which block should be the successor of the block with
-       * the break). Therefore, we need to insert a fake edge (see invariant
-       * #5).
-       */
-
-      nir_loop *loop = nir_cf_node_as_loop(node);
-
-      nir_cf_node *last = nir_loop_last_cf_node(loop);
-      assert(last->type == nir_cf_node_block);
-      nir_block *last_block =  nir_cf_node_as_block(last);
-
-      last_block->successors[1] = block;
-      block_add_pred(block, last_block);
    }
 }
 
@@ -520,13 +499,6 @@ nir_handle_add_jump(nir_block *block)
          assert(after->type == nir_cf_node_block);
          nir_block *after_block = nir_cf_node_as_block(after);
          link_blocks(block, after_block, NULL);
-
-         /* If we inserted a fake link, remove it */
-         nir_cf_node *last = nir_loop_last_cf_node(loop);
-         assert(last->type == nir_cf_node_block);
-         nir_block *last_block =  nir_cf_node_as_block(last);
-         if (last_block->successors[1] != NULL)
-            unlink_blocks(last_block, after_block);
       }
    } else {
       assert(jump_instr->type == nir_jump_return);
@@ -551,8 +523,8 @@ remove_phi_src(nir_block *block, nir_block *pred)
    }
 }
 
-/* Removes the successor of a block with a jump, and inserts a fake edge for
- * infinite loops. Note that the jump to be eliminated may be free-floating.
+/* Removes the successor of a block with a jump. Note that the jump to be
+ * eliminated may be free-floating.
  */
 
 static void
@@ -568,33 +540,6 @@ unlink_jump(nir_block *block, nir_jump_type type, bool add_normal_successors)
    unlink_block_successors(block);
    if (add_normal_successors)
       block_add_normal_succs(block);
-
-   /* If we've just removed a break, and the block we were jumping to (after
-    * the loop) now has zero predecessors, we've created a new infinite loop.
-    *
-    * NIR doesn't allow blocks (other than the start block) to have zero
-    * predecessors.  In particular, dominance assumes all blocks are reachable.
-    * So, we insert a "fake link" by making successors[1] point after the loop.
-    *
-    * Note that we have to do this after unlinking/recreating the block's
-    * successors.  If we removed a "break" at the end of the loop, then
-    * block == last_block, so block->successors[0] would already be "next",
-    * and adding a fake link would create two identical successors.  Doing
-    * this afterward works, as we'll have changed block->successors[0] to
-    * be the top of the loop.
-    */
-   if (type == nir_jump_break && next->predecessors->entries == 0) {
-      nir_loop *loop =
-         nir_cf_node_as_loop(nir_cf_node_prev(&next->cf_node));
-
-      /* insert fake link */
-      nir_cf_node *last = nir_loop_last_cf_node(loop);
-      assert(last->type == nir_cf_node_block);
-      nir_block *last_block = nir_cf_node_as_block(last);
-
-      last_block->successors[1] = next;
-      block_add_pred(next, last_block);
-   }
 }
 
 void
-- 
2.5.0.400.gff86faf