[llvm-branch-commits] [mlir] [mlir][Interfaces][NFC] Document that `RegionBranchTerminatorOpInterface` is mandatory (PR #174978)

[Matthias Springer via llvm-branch-commits]

https://github.com/matthias-springer updated https://github.com/llvm/llvm-project/pull/174978

>From 94c1209c3bc0e05516edfb3eb2284d36b75f85d7 Mon Sep 17 00:00:00 2001
From: Matthias Springer <me at m-sp.org>
Date: Thu, 8 Jan 2026 12:58:02 +0000
Subject: [PATCH] [mlir][Interfaces] Document that
 `RegionBranchTerminatorOpInterface` is mandatory

---
 .../mlir/Interfaces/ControlFlowInterfaces.td  | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/mlir/include/mlir/Interfaces/ControlFlowInterfaces.td b/mlir/include/mlir/Interfaces/ControlFlowInterfaces.td
index ff99e220c179f..8ee62216b105d 100644
--- a/mlir/include/mlir/Interfaces/ControlFlowInterfaces.td
+++ b/mlir/include/mlir/Interfaces/ControlFlowInterfaces.td
@@ -127,7 +127,8 @@ def RegionBranchOpInterface : OpInterface<"RegionBranchOpInterface"> {
 
     A "region branch point" indicates a point from which a branch originates. It
     can indicate:
-    1. A terminator in any of the immediately nested region of this op.
+    1. A `RegionBranchTerminatorOpInterface` terminator in any of the
+       immediately nested region of this op.
     2. `RegionBranchPoint::parent()`: the branch originates from outside of the
        op, i.e., when first executing this op.
 
@@ -147,6 +148,15 @@ def RegionBranchOpInterface : OpInterface<"RegionBranchOpInterface"> {
     results must have the same type. `areTypesCompatible` can be implemented to
     allow non-equal types.
 
+    Note: This interface works in conjunction with
+    `RegionBranchTerminatorOpInterface`. All immediately nested block
+    terminators that model branching between regions must implement the
+    `RegionBranchTerminatorOpInterface`. Otherwise, analyses/transformations
+    may miss control flow edges and produce incorrect results. Not every block
+    terminator is necessarily a region branch terminator: e.g., in the presence
+    of unstructured control flow, a block terminator could indicate a branch to
+    a different block within the same region.
+
     Example:
 
     ```
@@ -379,7 +389,14 @@ def RegionBranchTerminatorOpInterface :
   let description = [{
     This interface provides information for branching terminator operations
     in the presence of a parent `RegionBranchOpInterface` implementation. It
-    specifies which operands are passed to which region successor.
+    acts as a marker for valid region branch points and specifies which
+    operands are passed to which region successor.
+
+    Note: If an operation does not implement the
+    `RegionBranchTerminatorOpInterface`, then that op has no region successors.
+    (However, there may be other block terminators in the same region that
+    implement the `RegionBranchTerminatorOpInterface`, so the enclosing region
+    may have region successors.)
   }];
   let cppNamespace = "::mlir";