[clang] [CMake][Release] Stop linking against stage1 runtimes (PR #164017)

[Tom Stellard via cfe-commits]

https://github.com/tstellar created https://github.com/llvm/llvm-project/pull/164017

This was causing a build failure on Darwin and a test failure on Linux.  This config is not widely used or well tested, so I don't think the potential and likely small performance gains and the portability from this are worth the maintenance costs.

>From e0d52ed006f7bc15ffb6d68ab4cb4bbc7fcd499b Mon Sep 17 00:00:00 2001
From: Tom Stellard <tstellar at redhat.com>
Date: Wed, 15 Oct 2025 11:04:53 -0700
Subject: [PATCH] [CMake][Release] Stop linking against stage1 runtimes

This was causing a build failure on Darwin and a test failure
on Linux.  This config is not widely used or well tested, so
I don't think the potential and likely small performance gains
and the portability from this are worth the maintenance costs.
---
 clang/cmake/caches/Release.cmake | 39 ++++++++++++++++++++++++--------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/clang/cmake/caches/Release.cmake b/clang/cmake/caches/Release.cmake
index a523cc561b3f9..9aa7178537397 100644
--- a/clang/cmake/caches/Release.cmake
+++ b/clang/cmake/caches/Release.cmake
@@ -44,6 +44,19 @@ set(LLVM_RELEASE_ENABLE_LTO THIN CACHE STRING "")
 set(LLVM_RELEASE_ENABLE_PGO ON CACHE BOOL "")
 set(LLVM_RELEASE_ENABLE_RUNTIMES ${DEFAULT_RUNTIMES} CACHE STRING "")
 set(LLVM_RELEASE_ENABLE_PROJECTS ${DEFAULT_PROJECTS} CACHE STRING "")
+
+# This option enables linking stage2 clang statically with the runtimes
+# (libc++ and compiler-rt) from stage1.  In theory this will give the
+# binaries better performance and make them more portable.  However,
+# this configuration is not well tested and causes build failures with
+# the flang-rt tests cases, since the -stclib=libc++ flag does not
+# get propagated to the runtimes build.  There is also a separate
+# issue on Darwin where clang will use the local libc++ headers, but
+# link with system libc++ which can cause some incompatibilities.
+# See https://github.com/llvm/llvm-project/issues/77653
+# Because of these problems, this option will default to OFF.
+set(LLVM_RELEASE_LINK_LOCAL_RUNTIMES OFF CACHE BOOL "")
+
 # Note we don't need to add install here, since it is one of the pre-defined
 # steps.
 set(LLVM_RELEASE_FINAL_STAGE_TARGETS "clang;package;check-all;check-llvm;check-clang" CACHE STRING "")
@@ -55,8 +68,12 @@ set(CLANG_ENABLE_BOOTSTRAP ON CACHE BOOL "")
 
 set(STAGE1_PROJECTS "clang")
 
+# Need to build compiler-rt in order to use PGO for later stages.
+set(STAGE1_RUNTIMES "compiler-rt")
 # Build all runtimes so we can statically link them into the stage2 compiler.
-set(STAGE1_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind")
+if(LLVM_RELEASE_LINK_LOCAL_RUNTIMES)
+  list(APPEND STAGE1_RUNTIMES "libcxx;libcxxabi;libunwind")
+endif()
 
 if (LLVM_RELEASE_ENABLE_PGO)
   list(APPEND STAGE1_PROJECTS "lld")
@@ -118,11 +135,13 @@ set_instrument_and_final_stage_var(LLVM_ENABLE_LTO "${LLVM_RELEASE_ENABLE_LTO}"
 if (LLVM_RELEASE_ENABLE_LTO)
   set_instrument_and_final_stage_var(LLVM_ENABLE_LLD "ON" BOOL)
 endif()
-set_instrument_and_final_stage_var(LLVM_ENABLE_LIBCXX "ON" BOOL)
-set_instrument_and_final_stage_var(LLVM_STATIC_LINK_CXX_STDLIB "ON" BOOL)
-set(RELEASE_LINKER_FLAGS "-rtlib=compiler-rt --unwindlib=libunwind")
-if(NOT ${CMAKE_HOST_SYSTEM_NAME} MATCHES "Darwin")
-  set(RELEASE_LINKER_FLAGS "${RELEASE_LINKER_FLAGS} -static-libgcc")
+if(LLVM_RELEASE_LINK_LOCAL_RUNTIMES)
+  set_instrument_and_final_stage_var(LLVM_ENABLE_LIBCXX "ON" BOOL)
+  set_instrument_and_final_stage_var(LLVM_STATIC_LINK_CXX_STDLIB "ON" BOOL)
+  set(RELEASE_LINKER_FLAGS "-rtlib=compiler-rt --unwindlib=libunwind")
+  if(NOT ${CMAKE_HOST_SYSTEM_NAME} MATCHES "Darwin")
+    set(RELEASE_LINKER_FLAGS "${RELEASE_LINKER_FLAGS} -static-libgcc")
+  endif()
 endif()
 
 # Set flags for bolt
@@ -130,9 +149,11 @@ if (${CMAKE_HOST_SYSTEM_NAME} MATCHES "Linux")
   set(RELEASE_LINKER_FLAGS "${RELEASE_LINKER_FLAGS} -Wl,--emit-relocs,-znow")
 endif()
 
-set_instrument_and_final_stage_var(CMAKE_EXE_LINKER_FLAGS ${RELEASE_LINKER_FLAGS} STRING)
-set_instrument_and_final_stage_var(CMAKE_SHARED_LINKER_FLAGS ${RELEASE_LINKER_FLAGS} STRING)
-set_instrument_and_final_stage_var(CMAKE_MODULE_LINKER_FLAGS ${RELEASE_LINKER_FLAGS} STRING)
+if (RELEASE_LINKER_FLAGS)
+  set_instrument_and_final_stage_var(CMAKE_EXE_LINKER_FLAGS ${RELEASE_LINKER_FLAGS} STRING)
+  set_instrument_and_final_stage_var(CMAKE_SHARED_LINKER_FLAGS ${RELEASE_LINKER_FLAGS} STRING)
+  set_instrument_and_final_stage_var(CMAKE_MODULE_LINKER_FLAGS ${RELEASE_LINKER_FLAGS} STRING)
+endif()
 
 # Final Stage Config (stage2)
 set_final_stage_var(LLVM_ENABLE_RUNTIMES "${LLVM_RELEASE_ENABLE_RUNTIMES}" STRING)