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Re: [PATCH] PR libstdc++/71044 optimize std::filesystem::path construction


On 13/12/18 20:34 +0000, Jonathan Wakely wrote:
   Construction and modification of paths is now done more efficiently, by
   splitting the input into a stack-based buffer of string_view objects
   instead of a dynamically-allocated vector containing strings. Once the
   final size is known only a single allocation is needed to reserve space
   for it.  The append and concat operations no longer require constructing
   temporary path objects, nor re-parsing the entire native pathname.

This patch fixes a regression in the append/concat code, introduced by
the changes to avoid constructing  an intermediate path. The problem
wasn't caught by existing tests, so I've improved the tests.

Tested x86_64-linux, committed to trunk.


commit 7583088c09b5dd5d33a98abd1f61b0ccff961d6e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 17 21:42:48 2018 +0000

    PR libstdc++/71044 fix off-by-one errors introduced recently
    
    The recent changes to append/concat directly from strings (without
    constructing paths) introduced regressions where one of the components
    could be omitted from the iteration sequence in the result.
    
            PR libstdc++/71044
            * src/filesystem/std-path.cc (path::_M_append): Fix off-by-one error
            that caused a component to be lost from the iteration sequence.
            (path::_M_concat): Likewise.
            * testsuite/27_io/filesystem/path/append/source.cc: Test appending
            long strings.
            * testsuite/27_io/filesystem/path/concat/strings.cc: Test
            concatenating long strings.
            * testsuite/27_io/filesystem/path/construct/string_view.cc: Test
            construction from long string.

diff --git a/libstdc++-v3/src/filesystem/std-path.cc b/libstdc++-v3/src/filesystem/std-path.cc
index d2520492c03..b5ddbdad149 100644
--- a/libstdc++-v3/src/filesystem/std-path.cc
+++ b/libstdc++-v3/src/filesystem/std-path.cc
@@ -781,10 +781,11 @@ path::_M_append(basic_string_view<value_type> s)
 	      ::new(output++) _Cmpt(c.str, c.type, parser.offset(c));
 	      ++_M_cmpts._M_impl->_M_size;
 	    }
-	  for (auto c = parser.next(); c.valid(); c = parser.next())
+	  while (cmpt.valid())
 	    {
-	      ::new(output++) _Cmpt(c.str, c.type, parser.offset(c));
+	      ::new(output++) _Cmpt(cmpt.str, cmpt.type, parser.offset(cmpt));
 	      ++_M_cmpts._M_impl->_M_size;
+	      cmpt = parser.next();
 	    }
 
 	  if (s.back() == '/')
@@ -1139,7 +1140,7 @@ path::_M_concat(basic_string_view<value_type> s)
 	    }
 #endif
 	}
-      else if (orig_filenamelen == 0)
+      else if (orig_filenamelen == 0 && extra.empty())
 	{
 	  // Replace empty filename at end of original path.
 	  std::prev(output)->_M_pathname = it->str;
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc
index e440ca921c7..21ae6be3d97 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc
@@ -137,6 +137,22 @@ test05()
   s = second->native();
   p3 /= s;
   VERIFY( p3.string() == "0/123456789/a/123456789" );
+  }
+
+void
+test06()
+{
+  const std::string s0 = "a/b/c";
+  path p = s0;
+  std::string s;
+  for (int i = 0; i < 10; ++i)
+    s += "0/1/2/3/4/5/6/7/8/9/";
+  // append a long string with many components
+  test(p, s.c_str());
+
+  // Same again but with a trailing slash on the left operand:
+  path p2 = s0 + '/';
+  test(p2, s.c_str());
 }
 
 int
@@ -147,4 +163,5 @@ main()
   test03();
   test04();
   test05();
+  test06();
 }
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/concat/strings.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/concat/strings.cc
index eea9b6dc69b..316f7c3d7bd 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/concat/strings.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/concat/strings.cc
@@ -24,8 +24,10 @@
 #include <filesystem>
 #include <testsuite_hooks.h>
 #include <testsuite_iterators.h>
+#include <testsuite_fs.h>
 
 using std::filesystem::path;
+using __gnu_test::compare_paths;
 
 void
 test01()
@@ -60,7 +62,7 @@ test01()
 void
 test02()
 {
-  std::basic_string_view<path::value_type> s;
+  std::basic_string_view<path::value_type> s, expected;
 
   path p = "0/1/2/3/4/5/6";
   // The string_view aliases the path's internal string:
@@ -68,25 +70,54 @@ test02()
   // Append that string_view, which must work correctly even though the
   // internal string will be reallocated during the operation:
   p += s;
-  VERIFY( p.string() == "0/1/2/3/4/5/60/1/2/3/4/5/6" );
+  compare_paths(p, "0/1/2/3/4/5/60/1/2/3/4/5/6");
 
   // Same again with a trailing slash:
   path p2 = "0/1/2/3/4/5/";
   s = p2.native();
   p2 += s;
-  VERIFY( p2.string() == "0/1/2/3/4/5/0/1/2/3/4/5/" );
+  compare_paths(p2, "0/1/2/3/4/5/0/1/2/3/4/5/");
 
   // And aliasing one of the components of the path:
   path p3 = "0/123456789";
   path::iterator second = std::next(p3.begin());
   s = second->native();
   p3 += s;
-  VERIFY( p3.string() == "0/123456789123456789" );
+  compare_paths(p3, "0/123456789123456789" );
 }
 
+void
+test03()
+{
+  const std::string s0 = "a/b/c";
+  path p = s0;
+  std::string s;
+  for (int i = 0; i < 10; ++i)
+    s += "0/1/2/3/4/5/6/7/8/9/";
+  // concat a long string with many components:
+  p += s;
+  compare_paths(p, path(s0+s));
+
+  // Same again but with a trailing slash on the left operand:
+  path p2 = s0 + '/';
+  p2 += s;
+  compare_paths(p2, path(s0+'/'+s));
+
+  // And again but with a leading slash on the right operand:
+  path p3 = s0;
+  s.insert(0, 1, '/');
+  p3 += s;
+  compare_paths(p2, path(s0+s));
+
+  // And again but with a slash on both operands:
+  path p4 = s0 + '/';
+  p4 += s;
+  compare_paths(p4, path(s0+'/'+s));
+}
 int
 main()
 {
   test01();
   test02();
+  test03();
 }
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/construct/string_view.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/construct/string_view.cc
index 0339df73b30..a6130d389bf 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/construct/string_view.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/construct/string_view.cc
@@ -25,6 +25,7 @@
 #include <string_view>
 #include <string>
 #include <testsuite_fs.h>
+#include <testsuite_hooks.h>
 
 using std::filesystem::path;
 using __gnu_test::compare_paths;
@@ -49,8 +50,31 @@ test01()
   }
 }
 
+void
+test02()
+{
+  std::string s;
+  for (int i = 0; i < 10; ++i)
+    s += "0/1/2/3/4/5/6/7/8/9/";
+  // Construct a path with a large number of components:
+  path p = s;
+  auto iter = p.begin();
+  for (int i = 0; i < 100; ++i)
+  {
+    char c = '0' + i % 10;
+    std::string_view sv(&c, 1);
+    VERIFY( iter != p.end() );
+    compare_paths( *iter, sv );
+    ++iter;
+  }
+  VERIFY( iter != p.end() );
+  VERIFY( iter->native().empty() );
+  VERIFY( ++iter == p.end() );
+}
+
 int
 main()
 {
   test01();
+  test02();
 }

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