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MINOR: compiler: add ASSUME_NONNULL() to tell the compiler a pointer is valid

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At plenty of places we have ALREADY_CHECKED() or DISGUISE() on a pointer
just to avoid "possibly null-deref" warnings. These ones have the side
effect of weakening optimizations by passing through an assembly step.
Using ASSUME_NONNULL() we can avoid that extra step. And when the
__builtin_unreachable() builtin is not present, we fall back to the old
method using assembly. The macro returns the input value so that it may
be used both as a declarative way to claim non-nullity or directly inside
an expression like DISGUISE().
This commit is contained in:
Willy Tarreau 2024-12-17 10:42:07 +01:00
parent 2ce63b7b17
commit 63798088b3
1 changed files with 6 additions and 0 deletions
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6
include/haproxy/compiler.h
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@ -220,13 +220,19 @@
* with compilers that support it, and we do not want to emit any static code
* for other ones, so we use a construct that the compiler should easily be
* able to optimize away. Clang also has __builtin_assume() since at least 3.x.
* In addition, ASSUME_NONNULL() tells the compiler that the pointer argument
* will never be null. If not supported, it will be disguised via an assembly
* step.
*/
#if __has_builtin(__builtin_assume)
# define ASSUME(expr) __builtin_assume(expr)
# define ASSUME_NONNULL(p) ({ typeof(p) __p = (p); __builtin_assume(__p != NULL); (__p); })
#elif __has_builtin(__builtin_unreachable)
# define ASSUME(expr) do { if (!(expr)) __builtin_unreachable(); } while (0)
# define ASSUME_NONNULL(p) ({ typeof(p) __p = (p); if (__p == NULL) __builtin_unreachable(); (__p); })
#else
# define ASSUME(expr) do { if (!(expr)) break; } while (0)
# define ASSUME_NONNULL(p) ({ typeof(p) __p = (p); asm("" : "=rm"(__p) : "0"(__p)); __p; })
#endif
/* This prevents the compiler from folding multiple identical code paths into a