From 715672c643443225d21008b177b558504b6284e8 Mon Sep 17 00:00:00 2001 From: Jordan Rose Date: Fri, 26 Oct 2012 22:08:46 +0000 Subject: [PATCH 1/1] Suggest llvm_unreachable over assert(0). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166821 91177308-0d34-0410-b5e6-96231b3b80d8 --- docs/CodingStandards.rst | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-) diff --git a/docs/CodingStandards.rst b/docs/CodingStandards.rst index 418e3f05a36..90835307b15 100644 --- a/docs/CodingStandards.rst +++ b/docs/CodingStandards.rst @@ -862,23 +862,28 @@ Here are more examples: You get the idea. -Please be aware that, when adding assert statements, not all compilers are aware -of the semantics of the assert. In some places, asserts are used to indicate a -piece of code that should not be reached. These are typically of the form: +In the past, asserts were used to indicate a piece of code that should not be +reached. These were typically of the form: .. code-block:: c++ - assert(0 && "Some helpful error message"); + assert(0 && "Invalid radix for integer literal"); -When used in a function that returns a value, they should be followed with a -return statement and a comment indicating that this line is never reached. This -will prevent a compiler which is unable to deduce that the assert statement -never returns from generating a warning. +This has a few issues, the main one being that some compilers might not +understand the assertion, or warn about a missing return in builds where +assertions are compiled out. + +Today, we have something much better: ``llvm_unreachable``: .. code-block:: c++ - assert(0 && "Some helpful error message"); - return 0; + llvm_unreachable("Invalid radix for integer literal"); + +When assertions are enabled, this will print the message if it's ever reached +and then exit the program. When assertions are disabled (i.e. in release +builds), ``llvm_unreachable`` becomes a hint to compilers to skip generating +code for this branch. If the compiler does not support this, it will fall back +to the "abort" implementation. Another issue is that values used only by assertions will produce an "unused value" warning when assertions are disabled. For example, this code will warn: -- 2.34.1