tdf#92064 sw: Improve large paragraph layout performance

This change includes the following scalability improvements for
documents containing extremely large paragraphs:

- Reduces the size of layout contexts to account for LF control chars.

- Due to typical access patterns while laying out paragraphs, VCL was
making O(n^2) calls to vcl::ScriptRun::next(). VCL now uses an
existing global LRU cache for script runs, avoiding much of this
overhead.

Change-Id: Iee03938683c95776a817d4819fe9a43c65a7c3fc
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/172801
Tested-by: Jenkins
Reviewed-by: Jonathan Clark <jonathan@libreoffice.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/vcl/source/gdi/CommonSalLayout.cxx b/vcl/source/gdi/CommonSalLayout.cxx
index 0602ecca3fc8..a932dd54b16f 100644
--- a/vcl/source/gdi/CommonSalLayout.cxx
+++ b/vcl/source/gdi/CommonSalLayout.cxx
@@ -382,7 +382,7 @@ bool GenericSalLayout::LayoutText(vcl::text::ImplLayoutArgs& rArgs, const SalLay
     const int nLength = rArgs.mrStr.getLength();
     const sal_Unicode *pStr = rArgs.mrStr.getStr();
 
-    std::optional<vcl::text::TextLayoutCache> oNewScriptRun;
+    std::shared_ptr<const vcl::text::TextLayoutCache> pNewScriptRun;
     vcl::text::TextLayoutCache const* pTextLayout;
     if (rArgs.m_pTextLayoutCache)
     {
@@ -390,8 +390,11 @@ bool GenericSalLayout::LayoutText(vcl::text::ImplLayoutArgs& rArgs, const SalLay
     }
     else
     {
-        oNewScriptRun.emplace(pStr, rArgs.mnEndCharPos);
-        pTextLayout = &*oNewScriptRun;
+        // tdf#92064, tdf#162663:
+        // Also use the global LRU cache for full string script runs.
+        // This obviates O(n^2) calls to vcl::ScriptRun::next() when laying out large paragraphs.
+        pNewScriptRun = vcl::text::TextLayoutCache::Create(rArgs.mrStr);
+        pTextLayout = pNewScriptRun.get();
     }
 
     // nBaseOffset is used to align vertical text to the center of rotated