[CMake] Support setting default fused FP contract via CMake

[phosek]

Summary

D74436 changed the default fused FP contract which broke several
projects and it may take a while to address those issues. This
change allows setting the default fused FP contract via CMake, so
vendors can change their defaults rather than having to include the
-ffp-contract=off flag in every project to revert to the previous
behavior while the issues that were uncovered by D74436 are being
addressed.

[hans]

I just got back from vacation, so am probably missing some of the discussion elsewhere, but I'm a bit skeptical of this patch. It seems it would be confusing for users if clang generates code with different floating-point behaviour depending on how it was built.

[rjmccall]

I agree. This isn't the right approach. The patch in question is currently reverted; if you'd like to argue to not restore it, that conversation should be part of that review.

clang/CMakeLists.txt

 set(CLANG_LINK_CLANG_DYLIB ${LLVM_LINK_LLVM_DYLIB} CACHE BOOL
     "Link tools against libclang-cpp.so")
 
 if (NOT LLVM_LINK_LLVM_DYLIB AND CLANG_LINK_CLANG_DYLIB)
   message(FATAL_ERROR "Cannot set CLANG_LINK_CLANG_DYLIB=ON when "
                       "LLVM_LINK_LLVM_DYLIB=OFF")
 endif()
 
+set(CLANG_DEFAULT_FFP_CONTRACT "on" CACHE STRING
+  "Default fused FP contract (\"fast\", \"on\", \"off\", \"fast-honor-pragmas\")")
+if (NOT(CLANG_DEFAULT_FFP_CONTRACT STREQUAL "fast" OR
+        CLANG_DEFAULT_FFP_CONTRACT STREQUAL "on" OR
+        CLANG_DEFAULT_FFP_CONTRACT STREQUAL "off" OR
+        CLANG_DEFAULT_FFP_CONTRACT STREQUAL "fast-honor-pragmas"))
+  message(WARNING "Resetting default fused FP contract")
+  set(CLANG_DEFAULT_RTLIB "on" CACHE STRING
+    "Default fused FP contract (\"fast\", \"on\", \"off\", \"fast-honor-pragmas\", empty for platform default)" FORCE)
+endif()
+
 # The libdir suffix must exactly match whatever LLVM's configuration used.
 set(CLANG_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}")
 
 set(CLANG_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(CLANG_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
 if( CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR AND NOT MSVC_IDE )
   message(FATAL_ERROR "In-source builds are not allowed. "

clang/cmake/caches/Fuchsia-stage2.cmake

 
 set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
 if(NOT APPLE)
   # TODO: Remove this once we switch to ld64.lld.
   set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
   set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
 endif()
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
+set(CLANG_DEFAULT_FFP_CONTRACT off CACHE STRING "")
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
 set(CLANG_ENABLE_STATIC_ANALYZER ON CACHE BOOL "")
 set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")
 
 set(ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER ON CACHE BOOL "")
 set(ENABLE_LINKER_BUILD_ID ON CACHE BOOL "")
 set(ENABLE_X86_RELAX_RELOCATIONS ON CACHE BOOL "")
 

clang/cmake/caches/Fuchsia.cmake

 
 set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
 if(NOT APPLE)
   # TODO: Remove this once we switch to ld64.lld.
   set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
   set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
 endif()
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
+set(CLANG_DEFAULT_FFP_CONTRACT off CACHE STRING "")
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
 set(CLANG_ENABLE_STATIC_ANALYZER ON CACHE BOOL "")
 set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")
 
 set(ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER ON CACHE BOOL "")
 set(ENABLE_LINKER_BUILD_ID ON CACHE BOOL "")
 set(ENABLE_X86_RELAX_RELOCATIONS ON CACHE BOOL "")
 

clang/include/clang/Config/config.h.cmake

 #define CLANG_DEFAULT_UNWINDLIB "${CLANG_DEFAULT_UNWINDLIB}"
 
 /* Default objcopy to use */
 #define CLANG_DEFAULT_OBJCOPY "${CLANG_DEFAULT_OBJCOPY}"
 
 /* Default OpenMP runtime used by -fopenmp. */
 #define CLANG_DEFAULT_OPENMP_RUNTIME "${CLANG_DEFAULT_OPENMP_RUNTIME}"
 
+/* Default fused FP contract */
+#define CLANG_DEFAULT_FFP_CONTRACT "${CLANG_DEFAULT_FFP_CONTRACT}"
+
 /* Default architecture for OpenMP offloading to Nvidia GPUs. */
 #define CLANG_OPENMP_NVPTX_DEFAULT_ARCH "${CLANG_OPENMP_NVPTX_DEFAULT_ARCH}"
 
 /* Default architecture for SystemZ. */
 #define CLANG_SYSTEMZ_DEFAULT_ARCH "${CLANG_SYSTEMZ_DEFAULT_ARCH}"
 
 /* Multilib suffix for libdir. */
 #define CLANG_LIBDIR_SUFFIX "${CLANG_LIBDIR_SUFFIX}"

clang/lib/Driver/ToolChains/Clang.cpp

 #include "Hexagon.h"
 #include "MSP430.h"
 #include "PS4CPU.h"
 #include "clang/Basic/CharInfo.h"
 #include "clang/Basic/CodeGenOptions.h"
 #include "clang/Basic/LangOptions.h"
 #include "clang/Basic/ObjCRuntime.h"
 #include "clang/Basic/Version.h"
+#include "clang/Config/config.h"
 #include "clang/Driver/Distro.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/InputInfo.h"
 #include "clang/Driver/Options.h"
 #include "clang/Driver/SanitizerArgs.h"
 #include "clang/Driver/XRayArgs.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Config/llvm-config.h"
static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
   StringRef FPEvalMethod = "";
   const llvm::DenormalMode DefaultDenormalFPMath =
       TC.getDefaultDenormalModeForType(Args, JA);
   const llvm::DenormalMode DefaultDenormalFP32Math =
       TC.getDefaultDenormalModeForType(Args, JA, &llvm::APFloat::IEEEsingle());
 
   llvm::DenormalMode DenormalFPMath = DefaultDenormalFPMath;
   llvm::DenormalMode DenormalFP32Math = DefaultDenormalFP32Math;
-  StringRef FPContract = "on";
+  StringRef FPContract = CLANG_DEFAULT_FFP_CONTRACT;
   bool StrictFPModel = false;
 
 
   if (const Arg *A = Args.getLastArg(options::OPT_flimited_precision_EQ)) {
     CmdArgs.push_back("-mlimit-float-precision");
     CmdArgs.push_back(A->getValue());
   }