k_current_get(): make it a "const" function

This function always returns the same value for a given thread.
Add the const attribute to it so the compiler won't call it over and
over needlessly each time _current is referenced, making for far more
efficient code.

The __attribute_const__ symbol is used to mimic the Linux equivalent.
We want to make it clear that this is distinct from the const keyword.

Fix the test_x86_cpu_scrubs_regs where the compiler wasn't told that a
bunch of registers are being clobbered as highlighted by this change.

Signed-off-by: Nicolas Pitre <[email protected]>

Author: Nicolas Pitre

doc/conf.py

@@ -180,6 +180,7 @@
     "__used",
     "__unused",
     "__weak",
+    "__attribute_const__",
     "__DEPRECATED_MACRO",
     "FUNC_NORETURN",
     "__subsystem",

include/kernel.h

@@ -528,7 +528,7 @@ __syscall void k_wakeup(k_tid_t thread);
  * @return ID of current thread.
  *
  */
-__syscall k_tid_t k_current_get(void);
+__syscall k_tid_t k_current_get(void) __attribute_const__;
 
 /**
  * @brief Abort a thread.

include/toolchain/gcc.h

@@ -190,6 +190,9 @@ do {                                                                    \
 #ifndef __deprecated
 #define __deprecated	__attribute__((deprecated))
 #endif
+#ifndef __attribute_const__
+#define __attribute_const__ __attribute__((__const__))
+#endif
 #define ARG_UNUSED(x) (void)(x)
 
 #define likely(x)   __builtin_expect((bool)!!(x), true)

tests/arch/x86/cpu_scrubs_regs/src/main.c

@@ -27,23 +27,26 @@ void z_impl_test_cpu_write_reg(void)
 #if CONFIG_X86
 #ifndef CONFIG_X86_64
 	__asm__ volatile (
-		"movl $0xDEADBEEF, %eax;\n\t"
-		"movl $0xDEADBEEF, %ebx;\n\t"
-		"movl $0xDEADBEEF, %ecx;\n\t"
-		"movl $0xDEADBEEF, %edx;\n\t"
-		"movl $0xDEADBEEF, %edi;\n\t"
+		"movl $0xDEADBEEF, %%eax;\n\t"
+		"movl $0xDEADBEEF, %%ebx;\n\t"
+		"movl $0xDEADBEEF, %%ecx;\n\t"
+		"movl $0xDEADBEEF, %%edx;\n\t"
+		"movl $0xDEADBEEF, %%edi;\n\t"
+		: : : "eax", "ebx", "ecx", "edx", "edi"
 		);
 #else
 	__asm__ volatile (
-		"movq $0xDEADBEEF, %rax;\n\t"
-		"movq $0xDEADBEEF, %rcx;\n\t"
-		"movq $0xDEADBEEF, %rdx;\n\t"
-		"movq $0xDEADBEEF, %rsi;\n\t"
-		"movq $0xDEADBEEF, %rdi;\n\t"
-		"movq $0xDEADBEEF, %r8;\n\t"
-		"movq $0xDEADBEEF, %r9;\n\t"
-		"movq $0xDEADBEEF, %r10;\n\t"
-		"movq $0xDEADBEEF, %r11;\n\t"
+		"movq $0xDEADBEEF, %%rax;\n\t"
+		"movq $0xDEADBEEF, %%rcx;\n\t"
+		"movq $0xDEADBEEF, %%rdx;\n\t"
+		"movq $0xDEADBEEF, %%rsi;\n\t"
+		"movq $0xDEADBEEF, %%rdi;\n\t"
+		"movq $0xDEADBEEF, %%r8;\n\t"
+		"movq $0xDEADBEEF, %%r9;\n\t"
+		"movq $0xDEADBEEF, %%r10;\n\t"
+		"movq $0xDEADBEEF, %%r11;\n\t"
+		: : : "rax", "rcx", "rdx", "rsi", "rdi",
+		      "r8",  "r9",  "r10", "r11"
 		);
 #endif
 #endif