[compiler-rt] [compiler-rt] Don't link builtins against the CRT on Windows (PR #70675)

[David Truby via llvm-commits]

https://github.com/DavidTruby created https://github.com/llvm/llvm-project/pull/70675

compiler-rt/builtins doesn't depend on anything from the CRT but
currently links against it and embeds a `/defaultlib:msvcrt` in the
`.lib` file, forcing anyone linking against it to also link against that
specific CRT. This isn't necessary as the end user can just choose which
CRT they want to use independently.


>From 97ec9805366e47301750ca3c0ed56bf2ea3e50b3 Mon Sep 17 00:00:00 2001
From: David Truby <david at truby.dev>
Date: Mon, 30 Oct 2023 15:49:56 +0000
Subject: [PATCH] [compiler-rt] Don't link builtins against the CRT on Windows

compiler-rt/builtins doesn't depend on anything from the CRT but
currently links against it and embeds a `/defaultlib:msvcrt` in the
`.lib` file, forcing anyone linking against it to also link against that
specific CRT. This isn't necessary as the end user can just choose which
CRT they want to use independently.
---
 compiler-rt/cmake/builtin-config-ix.cmake | 1 +
 compiler-rt/lib/builtins/CMakeLists.txt   | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/compiler-rt/cmake/builtin-config-ix.cmake b/compiler-rt/cmake/builtin-config-ix.cmake
index e91e3923a756c53..9de05bb298efa13 100644
--- a/compiler-rt/cmake/builtin-config-ix.cmake
+++ b/compiler-rt/cmake/builtin-config-ix.cmake
@@ -18,6 +18,7 @@ builtin_check_c_compiler_flag(-fno-profile-generate COMPILER_RT_HAS_FNO_PROFILE_
 builtin_check_c_compiler_flag(-fno-profile-instr-generate COMPILER_RT_HAS_FNO_PROFILE_INSTR_GENERATE_FLAG)
 builtin_check_c_compiler_flag(-fno-profile-instr-use COMPILER_RT_HAS_FNO_PROFILE_INSTR_USE_FLAG)
 builtin_check_c_compiler_flag(-Wno-pedantic         COMPILER_RT_HAS_WNO_PEDANTIC)
+builtin_check_c_compiler_flag(/Zl COMPILER_RT_HAS_ZL_FLAG)
 
 builtin_check_c_compiler_source(COMPILER_RT_HAS_ATOMIC_KEYWORD
 "
diff --git a/compiler-rt/lib/builtins/CMakeLists.txt b/compiler-rt/lib/builtins/CMakeLists.txt
index a705486e2d3c842..fd36f298fb8f7e2 100644
--- a/compiler-rt/lib/builtins/CMakeLists.txt
+++ b/compiler-rt/lib/builtins/CMakeLists.txt
@@ -761,6 +761,12 @@ else ()
 
   append_list_if(COMPILER_RT_HAS_STD_C11_FLAG -std=c11 BUILTIN_CFLAGS)
 
+  # Don't link the the CRT on Windows
+  if (MSVC)
+    set(CMAKE_MSVC_RUNTIME_LIBRARY "")
+    append_list_if(COMPILER_RT_HAS_ZL_FLAG /Zl BUILTIN_CFLAGS)
+  endif()
+
   # These flags would normally be added to CMAKE_C_FLAGS by the llvm
   # cmake step. Add them manually if this is a standalone build.
   if(COMPILER_RT_STANDALONE_BUILD)