[PATCH] annotate vector::_M_default_append fo better codegen (PR 83229)

[Jonathan Wakely]

On 10/12/17 22:13 +0100, François Dumont wrote:
>On 06/12/2017 04:44, Martin Sebor wrote:
>>
>>Since it's possible that there may be other opportunities for
>>similar annotation in libstdc++, if changes along these lines
>>become more than isolated instances, it may be worth considering
>>adding a macro to make the conditions more readable.  E.g., to
>>follow the example of __glibcxx_assert, something like:
>>
>>Â  #define __glibcxx_assume(expr) \
>>Â Â Â  ((expr) ? (void)0 : __builtin_unreachable())
>>
>
>Or perhaps do something like that:
>
>@@ -485,7 +485,9 @@ namespace std
>Â #if defined(_GLIBCXX_ASSERTIONS)
>Â # define __glibcxx_assert(_Condition) __glibcxx_assert_impl(_Condition)
>Â #else
>-# define __glibcxx_assert(_Condition)
>+# define __glibcxx_assert(_Condition)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  \
>+Â Â Â  ((_Condition) ? (void)0 : __builtin_unreachable())
>Â #endif
>
>So that assertions are automatically transform into optimizer hints on 
>release builds.

That's an interesting idea, but quite scary to make that change at
this point in Stage 3, as it would affect dozens of places in the
code. We should consider it in Stage 1 though.

This might be a slightly better way to do it:

--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -458,6 +458,9 @@ namespace std
 
 #if defined(_GLIBCXX_ASSERTIONS)
 # define __glibcxx_assert(_Condition) __glibcxx_assert_impl(_Condition)
+#elif defined(__OPTIMIZE__)
+# define __glibcxx_assert(_Condition) \
+  do { if (! (_Condition)) __builtin_unreachable(); } while (false)
 #else
 # define __glibcxx_assert(_Condition)
 #endif

Since the __builtin_unreachable won't be used when not optimizing, so
there's no point evaluating the condition.

All our tests pass with this change.

>As the problem is that gcc doesn't remember anything about pointers 
>then those assertions at the begining of the method could acheive the 
>same result:
>
>__glibcxx_assert(this->_M_impl._M_start <= this->_M_impl._M_finish);
>__glibcxx_assert(this->_M_impl._M_finish < this->_M_impl._M_end_of_storage);