arch: arm: aarch32: Allow selecting compiler instruction set

This commit introduces the `COMPILER_ISA_THUMB2` symbol to allow
choosing either the ARM or Thumb instruction set for C code
compilation.

In addition, this commit introduces the `ASSEMBLER_ISA_THUMB2` helper
symbol to specify the default target instruction set for the assembler.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

arch/arm/core/aarch32/Kconfig

@@ -76,6 +76,38 @@ config ISA_ARM
 	  processor start-up. Much of its functionality was subsumed into T32 with
 	  the introduction of Thumb-2 technology.
 
+config ASSEMBLER_ISA_THUMB2
+	bool
+	default y if ISA_THUMB2 && !ISA_ARM
+	depends on !ISA_ARM
+	help
+	  This helper symbol specifies the default target instruction set for
+	  the assembler.
+
+	  When only the Thumb-2 ISA is supported (i.e. on Cortex-M cores), the
+	  assembler must use the Thumb-2 instruction set.
+
+	  When both the Thumb-2 and ARM ISAs are supported (i.e. on Cortex-A
+	  and Cortex-R cores), the assembler must use the ARM instruction set
+	  because the architecture assembly code makes use of the ARM
+	  instructions.
+
+config COMPILER_ISA_THUMB2
+	bool "Compile C/C++ functions using Thumb-2 instruction set"
+	depends on ISA_THUMB2
+	default y
+	help
+	  This option configures the compiler to compile all C/C++ functions
+	  using the Thumb-2 instruction set.
+
+	  N.B. The scope of this symbol is not necessarily limited to the C and
+	       C++ languages; in fact, this symbol refers to all forms of
+	       "compiled" code.
+
+	       When an additional natively-compiled language support is added
+	       in the future, this symbol shall also specify the Thumb-2
+	       instruction set for that language.
+
 config NUM_IRQS
 	int
 

arch/arm/core/aarch32/thread.c

@@ -135,9 +135,13 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
 	thread->callee_saved.psp = (u32_t)pInitCtx;
 #if defined(CONFIG_CPU_CORTEX_R)
 	pInitCtx->basic.lr = (u32_t)pInitCtx->basic.pc;
-	thread->callee_saved.spsr = A_BIT | T_BIT | MODE_SYS;
-	thread->callee_saved.lr = (u32_t)pInitCtx->basic.pc;
+	thread->callee_saved.spsr = A_BIT | MODE_SYS;
+#if defined(CONFIG_COMPILER_ISA_THUMB2)
+	thread->callee_saved.spsr |= T_BIT;
 #endif
+
+	thread->callee_saved.lr = (u32_t)pInitCtx->basic.pc;
+#endif /* CONFIG_CPU_CORTEX_R */
 	thread->arch.basepri = 0;
 
 #if defined(CONFIG_USERSPACE) || defined(CONFIG_FP_SHARING)

cmake/compiler/gcc/target_arm.cmake

@@ -7,14 +7,13 @@ if(CONFIG_ARM64)
     -mcpu=${GCC_M_CPU}
     )
 else()
-  list(APPEND TOOLCHAIN_C_FLAGS
-    -mthumb
-    -mcpu=${GCC_M_CPU}
-    )
-  list(APPEND TOOLCHAIN_LD_FLAGS
-    -mthumb
-    -mcpu=${GCC_M_CPU}
-    )
+  list(APPEND TOOLCHAIN_C_FLAGS   -mcpu=${GCC_M_CPU})
+  list(APPEND TOOLCHAIN_LD_FLAGS  -mcpu=${GCC_M_CPU})
+
+  if(CONFIG_COMPILER_ISA_THUMB2)
+    list(APPEND TOOLCHAIN_C_FLAGS   -mthumb)
+    list(APPEND TOOLCHAIN_LD_FLAGS  -mthumb)
+  endif()
 
   # Defines a mapping from GCC_M_CPU to FPU
 

include/toolchain/gcc.h

@@ -232,28 +232,24 @@ do {                                                                    \
 
 #if defined(CONFIG_ARM) && !defined(CONFIG_ARM64)
 
-#if defined(CONFIG_ISA_THUMB2)
+#if defined(CONFIG_ASSEMBLER_ISA_THUMB2)
 
 #define FUNC_CODE() .thumb;
 #define FUNC_INSTR(a)
 
-#elif defined(CONFIG_ISA_ARM)
+#else
 
 #define FUNC_CODE() .code 32
 #define FUNC_INSTR(a)
 
-#else
-
-#error unknown instruction set
-
-#endif /* ISA */
+#endif /* CONFIG_ASSEMBLER_ISA_THUMB2 */
 
 #else
 
 #define FUNC_CODE()
 #define FUNC_INSTR(a)
 
-#endif /* !CONFIG_ARM */
+#endif /* CONFIG_ARM && !CONFIG_ARM64 */
 
 #endif /* _ASMLANGUAGE */
 
@@ -359,14 +355,16 @@ do {                                                                    \
 
 #endif /* _ASMLANGUAGE */
 
-#if defined(CONFIG_ARM) && defined(_ASMLANGUAGE)
-#if defined(CONFIG_ISA_THUMB2)
+#if defined(_ASMLANGUAGE)
+#if defined(CONFIG_ARM) && !defined(CONFIG_ARM64)
+#if defined(CONFIG_ASSEMBLER_ISA_THUMB2)
 /* '.syntax unified' is a gcc-ism used in thumb-2 asm files */
 #define _ASM_FILE_PROLOGUE .text; .syntax unified; .thumb
 #else
 #define _ASM_FILE_PROLOGUE .text; .code 32
-#endif
-#endif
+#endif /* CONFIG_ASSEMBLER_ISA_THUMB2 */
+#endif /* CONFIG_ARM && !CONFIG_ARM64 */
+#endif /* _ASMLANGUAGE */
 
 /*
  * These macros generate absolute symbols for GCC