The idea of the overflow-arith.h header is to collect overflow checking
functions in one central place.
If gcc compiler supports the __builtin_overflow_* builtins we use them
because they might give better performance, otherwise the code falls
back to normal overflow checking functions.
The builtin_overflow functions are supported by gcc-5 and clang. The
matter of supporting clang is to just provide a corresponding
CC_HAVE_BUILTIN_OVERFLOW, because the specific overflow checking builtins
don't differ between gcc and clang.
I just provide overflow_usub function here as I intend this to get merged
into net, more functions will definitely follow as they are needed.
Signed-off-by: Hannes Frederic Sowa <han...@stressinduktion.org>
---
include/linux/compiler-gcc.h | 4 ++++
include/linux/overflow-arith.h | 18 ++++++++++++++++++
2 files changed, 22 insertions(+)
create mode 100644 include/linux/overflow-arith.h
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index dfaa7b3..82c159e 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -237,6 +237,10 @@
#define KASAN_ABI_VERSION 3
#endif
+#if GCC_VERSION >= 50000
+#define CC_HAVE_BUILTIN_OVERFLOW
+#endif
+
#endif /* gcc version >= 40000 specific checks */
#if !defined(__noclone)
diff --git a/include/linux/overflow-arith.h b/include/linux/overflow-arith.h
new file mode 100644
index 0000000..e12ccf8
--- /dev/null
+++ b/include/linux/overflow-arith.h
@@ -0,0 +1,18 @@
+#pragma once
+
+#include <linux/kernel.h>
+
+#ifdef CC_HAVE_BUILTIN_OVERFLOW
+
+#define overflow_usub __builtin_usub_overflow
+
+#else
+
+static inline bool overflow_usub(unsigned int a, unsigned int b,
+ unsigned int *res)
+{
+ *res = a - b;
+ return *res > a ? true : false;
+}
+
+#endif
--
2.4.3
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