[compiler-rt] [compiler-rt] Reinstate removal of CRT choice flags from CMAKE_*_FLAGS* (PR #67935)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo created https://github.com/llvm/llvm-project/pull/67935

This reverts one part of commit 9f4dfcb795bb0ecf9944553f49371164801cd83f, with a modified comment added about the code.

Ideally, this would only be reinstated temporarily - but given the situation in vcpkg, it looks likely that they would keep passing the duplicate options for quite some time. The conflicting CRT choice usually are benign but only would cause warnings about one option overriding the other, if passing e.g. "/MDd /MT".

However when vcpkg currently sets these options in CMAKE_*_FLAGS_DEBUG, it passes the redundant option /D_DEBUG; thus the compiler finally ends up with e.g. "/D_DEBUG /MDd /MT", which has the effect of defining _DEBUG while using a release mode CRT, which allegedly breaks the build.

There's a PR up for removing this redundant /D_DEBUG option in vcpkg in https://github.com/microsoft/vcpkg/pull/34123. With that in place, this change wouldn't be strictly needed.

>From 9bee19add6575ec9afeed22bffdd372f9eed844a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin at martin.st>
Date: Mon, 2 Oct 2023 00:33:44 +0300
Subject: [PATCH] [compiler-rt] Reinstate removal of CRT choice flags from
 CMAKE_*_FLAGS*

This reverts one part of commit 9f4dfcb795bb0ecf9944553f49371164801cd83f,
with a modified comment added about the code.

Ideally, this would only be reinstated temporarily - but given the
situation in vcpkg, it looks likely that they would keep passing
the duplicate options for quite some time. The conflicting CRT
choice usually are benign but only would cause warnings about
one option overriding the other, if passing e.g. "/MDd /MT".

However when vcpkg currently sets these options in
CMAKE_*_FLAGS_DEBUG, it passes the redundant option /D_DEBUG;
thus the compiler finally ends up with e.g. "/D_DEBUG /MDd /MT",
which has the effect of defining _DEBUG while using a release
mode CRT, which allegedly breaks the build.

There's a PR up for removing this redundant /D_DEBUG option
in vcpkg in https://github.com/microsoft/vcpkg/pull/34123.
With that in place, this change wouldn't be strictly needed.
---
 compiler-rt/CMakeLists.txt | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/compiler-rt/CMakeLists.txt b/compiler-rt/CMakeLists.txt
index cc3190bd7f761e2..514ac250075a8f2 100644
--- a/compiler-rt/CMakeLists.txt
+++ b/compiler-rt/CMakeLists.txt
@@ -375,6 +375,25 @@ endif()
 if(MSVC)
   # FIXME: In fact, sanitizers should support both /MT and /MD, see PR20214.
   set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded)
+
+  # Remove any /M[DT][d] flags, and strip any definitions of _DEBUG.
+  # Since we're using CMAKE_MSVC_RUNTIME_LIBRARY (CMP0091 set to NEW),
+  # these options shouldn't be included in these flags variables. However,
+  # package managers that don't know which mechanism is used for passing
+  # CRT choice flags might be passing them both ways - which leads to
+  # duplicate CRT choice options. Thus make sure to strip out these flags
+  # from these variables, when we're forcing a CRT choice other than what
+  # the user requested here.
+  foreach(flag_var
+    CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
+    CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO
+    CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
+    CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
+    string(REGEX REPLACE "[/-]M[DT]d" "" ${flag_var} "${${flag_var}}")
+    string(REGEX REPLACE "[/-]MD" "" ${flag_var} "${${flag_var}}")
+    string(REGEX REPLACE "[/-]D_DEBUG" "" ${flag_var} "${${flag_var}}")
+  endforeach()
+
   append_list_if(COMPILER_RT_HAS_Oy_FLAG /Oy- SANITIZER_COMMON_CFLAGS)
   append_list_if(COMPILER_RT_HAS_GS_FLAG /GS- SANITIZER_COMMON_CFLAGS)