[llvm-branch-commits] [libunwind] PR for llvm/llvm-project#81084 (PR #81085)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/81085

resolves llvm/llvm-project#81084

>From e6c6fe1a84fff2014be2b8f91f8905f109a15f0f Mon Sep 17 00:00:00 2001
From: YunQiang Su <syq at debian.org>
Date: Thu, 8 Feb 2024 09:15:53 +0800
Subject: [PATCH] MIPS/libunwind: Use -mfp64 if compiler is FPXX (#68521)

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.

It's not due to the code bug, but rather the nature of FPXX.
FPXX is an ABI which uses only a common subset of FR=1(FP64) and FR=0
(FP32).
So that FPXX binaries can link with both FP64 and FP32 ones, aka.
    FPXX + FP32 -> FP32
    FPXX + FP64 -> FP64

While for libunwind, we should save/restore all of FPRs. If we use FPXX,
we can only save/restore a common subset of FPRs, instead of superset.

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.

Co-authored-by: YunQiang Su <yunqiang.su at cipunited.com>
(cherry picked from commit 4520b478d2512b0f39764e0464dcb4cb961845b5)
---
 libunwind/CMakeLists.txt | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/libunwind/CMakeLists.txt b/libunwind/CMakeLists.txt
index bb1b052f61d875..806d5a783ec39c 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
@@ -96,6 +97,20 @@ 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
 #===============================================================================
@@ -179,6 +194,10 @@ 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