[libunwind] MIPS/libunwind: Use -mfp64 if compiler is FPXX (PR #68521)

[YunQiang Su via cfe-commits]

https://github.com/wzssyqa updated https://github.com/llvm/llvm-project/pull/68521

>From 6053822322fd1594b46bf77c8b168ad802a7c534 Mon Sep 17 00:00:00 2001
From: YunQiang Su <yunqiang.su at cipunited.com>
Date: Sun, 8 Oct 2023 07:12:45 -0400
Subject: [PATCH] MIPS/libunwind: Use -mfp64 if compiler is FPXX

Libunwind supports FP64 and FP32 modes, but not FPXX.
The reason is that, FP64 and FP32 have different way to save/restore
FPRs. If libunwind is built as FPXX, we have no idea which one should
we use.

If libunwind is built as FP64, it will interoperatable with FPXX/FP64
APPs, and if it is built as FP32, it will interoperatable with
FP32/FPXX. Currently most of O32 APPs are FPXX or FP64, while few are FP32.

So if the compiler is FPXX, which is the default value of most toolchain,
let's switch it to FP64.
---
 libunwind/CMakeLists.txt | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/libunwind/CMakeLists.txt b/libunwind/CMakeLists.txt
index 84f8ce296a7410b..387d4b43b5746a6 100644
--- a/libunwind/CMakeLists.txt
+++ b/libunwind/CMakeLists.txt
@@ -21,6 +21,7 @@ set(LIBUNWIND_LIBCXX_PATH "${CMAKE_CURRENT_LIST_DIR}/../libcxx" CACHE PATH
         "Specify path to libc++ source.")
 
 include(GNUInstallDirs)
+include(CheckSymbolExists)
 
 #===============================================================================
 # Setup CMake Options
@@ -101,6 +102,20 @@ endif()
 option(LIBUNWIND_HIDE_SYMBOLS
   "Do not export any symbols from the static library." ${LIBUNWIND_DEFAULT_HIDE_SYMBOLS})
 
+# If toolchain is FPXX, we switch to FP64 to save the full FPRs. See:
+# https://web.archive.org/web/20180828210612/https://dmz-portal.mips.com/wiki/MIPS_O32_ABI_-_FR0_and_FR1_Interlinking
+check_symbol_exists(__mips_hard_float "" __MIPSHF)
+check_symbol_exists(_ABIO32 "" __MIPS_O32)
+if (__MIPSHF AND __MIPS_O32)
+  file(WRITE ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/mips_is_fpxx.c
+    "#if __mips_fpr != 0\n"
+    "# error\n"
+    "#endif\n")
+  try_compile(MIPS_FPABI_FPXX ${CMAKE_BINARY_DIR}
+    ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/mips_is_fpxx.c
+    CMAKE_FLAGS -DCMAKE_C_LINK_EXECUTABLE='echo')
+endif()
+
 #===============================================================================
 # Configure System
 #===============================================================================
@@ -184,6 +199,10 @@ if (WIN32)
   add_compile_flags_if_supported(-Wno-dll-attribute-on-redeclaration)
 endif()
 
+if (MIPS_FPABI_FPXX)
+  add_compile_flags(-mfp64)
+endif()
+
 # Get feature flags.
 # Exceptions
 # Catches C++ exceptions only and tells the compiler to assume that extern C