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* [committed] libstdc++: Avoid try-catch and O(N) size in std::list::merge for old ABI
@ 2022-08-03 12:31 Jonathan Wakely
  0 siblings, 0 replies; only message in thread
From: Jonathan Wakely @ 2022-08-03 12:31 UTC (permalink / raw)
  To: libstdc++, gcc-patches

Tested x86_64-linux, pushed to gcc-10 branch.

-- >8 --

The current std::list::merge code calls size() before starting to merge
any elements, so that the _M_size members can be updated after the merge
finishes. The work is done in a try-block so that the sizes can still be
updated in an exception handler if any element comparison throws.

The _M_size members only exist for the cxx11 ABI, so the initial call to
size() and the try-catch are only needed for that ABI. For the old ABI
the size() call performs an O(N) list traversal to get a value that
isn't even used, and catching exceptions just to rethrow them isn't
needed either.

In r11-10123 this code was refactored to use an RAII guard type, but for
the gcc-10 branch a less invasive change using preprocessor conditionals
seems more appropriate.


	* include/bits/list.tcc (list::merge) [!USE_CXX11_ABI]: Remove
	call to size() and try-catch block.
 libstdc++-v3/include/bits/list.tcc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libstdc++-v3/include/bits/list.tcc b/libstdc++-v3/include/bits/list.tcc
index ce9e983c539..1f27d8b786a 100644
--- a/libstdc++-v3/include/bits/list.tcc
+++ b/libstdc++-v3/include/bits/list.tcc
 	  iterator __last1 = end();
 	  iterator __first2 = __x.begin();
 	  iterator __last2 = __x.end();
 	  const size_t __orig_size = __x.size();
 	  __try {
 	    while (__first1 != __last1 && __first2 != __last2)
 	      if (*__first2 < *__first1)

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2022-08-03 12:31 [committed] libstdc++: Avoid try-catch and O(N) size in std::list::merge for old ABI Jonathan Wakely

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