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MINOR: compiler: add a new "ASSUME" macro to help the compiler

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This macro takes an expression, tests it and calls an unreachable
statement if false. This allows the compiler to know that such a
combination does not happen, and totally eliminate tests that would
be related to this condition. When the statement is not available
in the compiler, we just perform a break from a do {} while loop
so that the expression remains evaluated if needed (e.g. function
call).
This commit is contained in:
Willy Tarreau 2024-11-07 11:09:33 +01:00
parent 41fc18b1d1
commit efc897484b
1 changed files with 15 additions and 0 deletions
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15
include/haproxy/compiler.h
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@ -212,6 +212,21 @@
#endif
#endif
/* By using an unreachable statement, we can tell the compiler that certain
* conditions are not expected to be met and let it arrange as it wants to
* optimize some checks away. The principle is to place a test on the condition
* and call unreachable upon a match. It may also help static code analyzers
* know that some conditions are not supposed to happen. This can only be used
* 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.
*/
#if __has_builtin(__builtin_unreachable)
# define ASSUME(expr) do { if (!(expr)) __builtin_unreachable(); } while (0)
#else
# define ASSUME(expr) do { if (!(expr)) break; } while (0)
#endif
/* This prevents the compiler from folding multiple identical code paths into a
* single one, by adding a dependency on the line number in the path. This may
* typically happen on function tails, or purposely placed abort() before an