linux-kernel - [PATCH 2/4] selftests/rseq: Implement rseq_unqual_scalar

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Date:   Tue, 27 Jun 2023 11:29:21 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Shuah Khan <shuah@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [PATCH 2/4] 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 | 26 +++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tools/testing/selftests/rseq/compiler.h b/tools/testing/selftests/rseq/compiler.h
index f47092bddeba..49d62fbd6dda 100644
--- a/tools/testing/selftests/rseq/compiler.h
+++ b/tools/testing/selftests/rseq/compiler.h
@@ -33,4 +33,30 @@
 #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_ */
-- 
2.25.1