selftests/rseq: Implement rseq_unqual_scalar_typeof

Allow defining variables and perform cast with a typeof which removes
the volatile and const qualifiers.

This prevents declaring a stack variable with a volatile qualifier
within a macro, which would generate sub-optimal assembler.

This is imported from the "librseq" project.

Signed-off-by: Mathieu Desnoyers <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/rseq/compiler.h

···
 #define RSEQ_COMBINE_TOKENS(_tokena, _tokenb)	\
 	RSEQ__COMBINE_TOKENS(_tokena, _tokenb)
 
+#ifdef __cplusplus
+#define rseq_unqual_scalar_typeof(x)					\
+	std::remove_cv<std::remove_reference<decltype(x)>::type>::type
+#else
+#define rseq_scalar_type_to_expr(type)					\
+	unsigned type: (unsigned type)0,				\
+	signed type: (signed type)0
+
+/*
+ * Use C11 _Generic to express unqualified type from expression. This removes
+ * volatile qualifier from expression type.
+ */
+#define rseq_unqual_scalar_typeof(x)					\
+	__typeof__(							\
+		_Generic((x),						\
+			char: (char)0,					\
+			rseq_scalar_type_to_expr(char),			\
+			rseq_scalar_type_to_expr(short),		\
+			rseq_scalar_type_to_expr(int),			\
+			rseq_scalar_type_to_expr(long),			\
+			rseq_scalar_type_to_expr(long long),		\
+			default: (x)					\
+		)							\
+	)
+#endif
+
 #endif  /* RSEQ_COMPILER_H_ */