x86: enable soft float support for Zephyr SDK

This adds the correct compiler and linker flags to
support software floating point operations. The flags
need to be added to TOOLCHAIN_*_FLAGS for GCC to find
the correct library (when calling GCC with
--print-libgcc-file-name).

Note that software floating point needs to be turned
on for Newlib. This is due to Newlib having floating
point numbers in its various printf() functions which
results in floating point instructions being emitted
from toolchain. These instructions are placed very
early in the functions which results in them being
executed even though the format string contains
no floating point conversions. Without using CONFIG_FPU
to enable hardware floating point support, any calls to
printf() like functions will result in exceptions
complaining FPU is not available. Although forcing
CONFIG_FPU=y with newlib is an option, and because
the OS doesn't know which threads would call these
printf() functions, Zephyr has to assume all threads
are using FPU and thus incurring performance penalty as
every context switching now needs to save FPU registers.
A compromise here is to use soft float instead. Newlib
with soft float enabled does not have floating point
instructions and yet can still support its printf()
like functions.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

arch/x86/core/Kconfig.ia32

@@ -86,7 +86,8 @@ config X86_PAE
 	  more memory for paging structures.
 
 menu "Architecture Floating Point Options"
-depends on CPU_HAS_FPU
+
+if CPU_HAS_FPU
 
 config SSE
 	bool "SSE registers"
@@ -138,6 +139,15 @@ config LAZY_FPU_SHARING
 	  malware to read the contents of all floating point registers, see
 	  CVE-2018-3665.
 
+endif # CPU_HAS_FPU
+
+config X86_FP_USE_SOFT_FLOAT
+	bool
+	prompt "Use Software Floating Point Operations" if !(NEWLIB_LIBC && !FPU)
+	default y if NEWLIB_LIBC && !FPU
+	help
+	  Enable using software floating point operations.
+
 endmenu
 
 config DCACHE_LINE_SIZE

arch/x86/core/ia32.cmake

@@ -33,3 +33,8 @@ zephyr_library_sources_ifdef(
 
 # Last since we declare default exception handlers here
 zephyr_library_sources(ia32/fatal.c)
+
+zephyr_library_sources_ifdef(
+  CONFIG_X86_FP_USE_SOFT_FLOAT
+  ia32/soft_float_stubs.c
+)

arch/x86/core/ia32/soft_float_stubs.c

@@ -0,0 +1,280 @@
+/*
+ * Copyright (c) 2021 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <kernel.h>
+#include <toolchain.h>
+
+/**
+ * @file
+ * @brief Provide soft float function stubs for long double operations.
+ *
+ * GCC soft float does not support long double so these need to be
+ * stubbed out.
+ *
+ * The function names come from the GCC public documentation.
+ */
+
+extern void abort(void);
+
+__weak void __addtf3(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __addxf3(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __subtf3(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __subxf3(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __multf3(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __mulxf3(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __divtf3(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __divxf3(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __negtf2(long double a)
+{
+	k_oops();
+}
+
+__weak void __negxf2(long double a)
+{
+	k_oops();
+}
+
+__weak void __extendsftf2(float a)
+{
+	k_oops();
+}
+
+__weak void __extendsfxf2(float a)
+{
+	k_oops();
+}
+
+__weak void __extenddftf2(double a)
+{
+	k_oops();
+}
+
+__weak void __extenddfxf2(double a)
+{
+	k_oops();
+}
+
+__weak void __truncxfdf2(long double a)
+{
+	k_oops();
+}
+
+__weak void __trunctfdf2(long double a)
+{
+	k_oops();
+}
+
+__weak void __truncxfsf2(long double a)
+{
+	k_oops();
+}
+
+__weak void __trunctfsf2(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixtfsi(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixxfsi(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixtfdi(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixxfdi(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixtfti(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixxfti(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixunstfsi(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixunsxfsi(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixunstfdi(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixunsxfdi(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixunstfti(long double a)
+{
+	k_oops();
+}
+
+__weak void __fixunsxfti(long double a)
+{
+	k_oops();
+}
+
+__weak void __floatsitf(int i)
+{
+	k_oops();
+}
+
+__weak void __floatsixf(int i)
+{
+	k_oops();
+}
+
+__weak void __floatditf(long i)
+{
+	k_oops();
+}
+
+__weak void __floatdixf(long i)
+{
+	k_oops();
+}
+
+__weak void __floattitf(long long i)
+{
+	k_oops();
+}
+
+__weak void __floattixf(long long i)
+{
+	k_oops();
+}
+
+__weak void __floatunsitf(unsigned int i)
+{
+	k_oops();
+}
+
+__weak void __floatunsixf(unsigned int i)
+{
+	k_oops();
+}
+
+__weak void __floatunditf(unsigned long i)
+{
+	k_oops();
+}
+
+__weak void __floatundixf(unsigned long i)
+{
+	k_oops();
+}
+
+__weak void __floatuntitf(unsigned long long i)
+{
+	k_oops();
+}
+
+__weak void __floatuntixf(unsigned long long i)
+{
+	k_oops();
+}
+
+__weak void __cmptf2(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __unordtf2(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __eqtf2(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __netf2(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __getf2(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __lttf2(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __letf2(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __gttf2(long double a, long double b)
+{
+	k_oops();
+}
+
+__weak void __powitf2(long double a, int b)
+{
+	k_oops();
+}
+
+__weak void __powixf2(long double a, int b)
+{
+	k_oops();
+}

cmake/compiler/gcc/target_x86.cmake

@@ -8,6 +8,11 @@ else()
   string(PREPEND CMAKE_ASM_FLAGS "-m32 ")
   string(PREPEND CMAKE_C_FLAGS   "-m32 ")
   string(PREPEND CMAKE_CXX_FLAGS "-m32 ")
+
+  if(CONFIG_X86_FP_USE_SOFT_FLOAT)
+    list(APPEND TOOLCHAIN_C_FLAGS  -msoft-float)
+    list(APPEND TOOLCHAIN_LD_FLAGS -msoft-float)
+  endif()
 endif()
 
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