[clang] [llvm] Enable ClangIR Build By Default (PR #221291)

[Erich Keane via cfe-commits]

https://github.com/erichkeane updated https://github.com/llvm/llvm-project/pull/221291

>From e3116fa653a9dac54d2a58932130f2ec2d175c0c Mon Sep 17 00:00:00 2001
From: erichkeane <ekeane at nvidia.com>
Date: Fri, 4 Sep 2026 09:23:26 -0700
Subject: [PATCH 1/2] Enable ClangIR Build By Default-

This is subject to an RFC:

https://discourse.llvm.org/t/rfc-enable-clangir-build-by-default/91730

This patch enables building ClangIR (and thus MLIR) by default in clang
builds.  The project has hit a maturity level where we believe this is
a good time to do it.  See the RFC for details.
---
 clang/CMakeLists.txt       |  6 ++----
 clang/docs/ReleaseNotes.md |  5 +++++
 llvm/CMakeLists.txt        | 11 +++++++++++
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
index fc71cf45dbb34c..3416e67eb79910 100644
--- a/clang/CMakeLists.txt
+++ b/clang/CMakeLists.txt
@@ -190,15 +190,13 @@ if(CLANG_ENABLE_LIBXML2)
   endif()
 endif()
 
+cmake_dependent_option(CLANG_ENABLE_CIR "Build ClangIR support into the compiler" ON "NOT CLANG_BUILT_STANDALONE" OFF)
+
 if(CLANG_ENABLE_CIR)
   if (CLANG_BUILT_STANDALONE)
     message(FATAL_ERROR
       "ClangIR is not yet supported in the standalone build.")
   endif()
-  if (NOT "${LLVM_ENABLE_PROJECTS}" MATCHES "MLIR|mlir")
-    message(FATAL_ERROR
-      "Cannot build ClangIR without MLIR in LLVM_ENABLE_PROJECTS")
-  endif()
 endif()
 
 include(CheckIncludeFile)
diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index ca5c63fbaa17d7..ce458ce2e0ecc7 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -837,6 +837,11 @@ The `alpha.cplusplus.UseAfterLifetimeEnd` checker was renamed to `alpha.core.Use
 
 #### Improvements
 
+### Clang IR Improvements
+
+- Clang IR has now been enabled in builds by default, and should work with most workloads. However this is still an experimental flag and a work-in-progress.
+
+
 ## Additional Information
 
 A wide variety of additional information is available on the [Clang web
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 44299d51d784a1..7f0e8fc7a9915e 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -201,6 +201,17 @@ if ("flang" IN_LIST LLVM_ENABLE_PROJECTS)
   endif ()
 endif()
 
+# Clang defaults to CLANG_ENABLE_CIR now, so we need to auto-add MLIR unless it
+# was explicitly disabled.
+if ("clang" IN_LIST LLVM_ENABLE_PROJECTS)
+  if (NOT DEFINED CLANG_ENABLE_CIR OR CLANG_ENABLE_CIR)
+    if (NOT "mlir" IN_LIST LLVM_ENABLE_PROJECTS)
+      message(STATUS "Enabling MLIR as a dependency of ClangIR")
+      list(APPEND LLVM_ENABLE_PROJECTS "mlir")
+    endif()
+  endif()
+endif()
+
 if ("lldb" IN_LIST LLVM_ENABLE_PROJECTS)
   if (NOT "clang" IN_LIST LLVM_ENABLE_PROJECTS)
     message(STATUS "Enabling clang as a dependency of lldb")

>From 33e5ee57b3112cd62890b36419c22156ec338247 Mon Sep 17 00:00:00 2001
From: erichkeane <ekeane at nvidia.com>
Date: Mon, 14 Sep 2026 10:21:37 -0700
Subject: [PATCH 2/2] Add to release-notes-template, plus clarify release notes

---
 clang/docs/ReleaseNotes.md          | 2 +-
 clang/docs/ReleaseNotesTemplate.txt | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index cbe99b9a22b603..21d3be7313dcde 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -908,7 +908,7 @@ The `alpha.cplusplus.UseAfterLifetimeEnd` checker was renamed to `alpha.core.Use
 
 ### Clang IR Improvements
 
-- Clang IR has now been enabled in builds by default, and should work with most workloads. However this is still an experimental flag and a work-in-progress.
+- Clang builds now build Clang IR support by default. Clang IR is expected to work with most Linux/X86-64 workloads. Normal builds are unchanged, however those who wish to test out the experimental `-fclangir` flag can now do so with a Clang build.
 
 
 ## Additional Information
diff --git a/clang/docs/ReleaseNotesTemplate.txt b/clang/docs/ReleaseNotesTemplate.txt
index df98f9adc03387..b7920ab029d0ea 100644
--- a/clang/docs/ReleaseNotesTemplate.txt
+++ b/clang/docs/ReleaseNotesTemplate.txt
@@ -193,6 +193,8 @@ latest release, please see the [Clang Web Site](https://clang.llvm.org) or the
 
 #### Improvements
 
+### Clang IR Improvements
+
 ## Additional Information
 
 A wide variety of additional information is available on the [Clang web