linux-kernel - [RFC PATCH 1/3] selftests/rseq: Implement rseq_unqual_scalar

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Date:   Wed,  3 May 2023 16:13:22 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [RFC PATCH 1/3] 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 <mathieu.desnoyers@...icios.com>
---
 tools/testing/selftests/rseq/compiler.h | 27 +++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/rseq/compiler.h b/tools/testing/selftests/rseq/compiler.h
index f47092bddeba..8dc7f881e253 100644
--- a/tools/testing/selftests/rseq/compiler.h
+++ b/tools/testing/selftests/rseq/compiler.h
@@ -33,4 +33,31 @@
 #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
+/*
+ * 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,					\
+			unsigned char: (unsigned char)0,		\
+			signed char: (signed char)0,			\
+			unsigned short: (unsigned short)0,		\
+			signed short: (signed short)0,			\
+			unsigned int: (unsigned int)0,			\
+			signed int: (signed int)0,			\
+			unsigned long: (unsigned long)0,		\
+			signed long: (signed long)0,			\
+			unsigned long long: (unsigned long long)0,	\
+			signed long long: (signed long long)0,		\
+			default: (x)					\
+		)							\
+	)
+#endif
+
 #endif  /* RSEQ_COMPILER_H_ */
-- 
2.25.1