speedup Transliteration_body::transliterateImpl

use alloca in Transliteration_body::transliterateImpl to avoid
over-allocating stack-space (which tends to unnecessarily flush some
cache)

Change-Id: I1843fdcb830a3e948a8bbd0a9c7eb143b21a804c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/122184
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

1 files changed, 5 insertions, 2 deletions

diff --git a/i18npool/source/transliteration/transliteration_body.cxx b/i18npool/source/transliteration/transliteration_body.cxx
index 9fd89df75e42..3581212af8b3 100644
--- a/i18npool/source/transliteration/transliteration_body.cxx
+++ b/i18npool/source/transliteration/transliteration_body.cxx
@@ -102,9 +102,12 @@ Transliteration_body::transliterateImpl(
     // Allocate the max possible buffer. Try to use stack instead of heap,
     // which would have to be reallocated most times anyways.
     constexpr sal_Int32 nLocalBuf = 2048;
-    sal_Unicode aLocalBuf[ nLocalBuf * NMAPPINGMAX ], *out = aLocalBuf;
+    sal_Unicode* out;
     std::unique_ptr<sal_Unicode[]> pHeapBuf;
-    if (nCount > nLocalBuf)
+    size_t nBytes = (nCount + 1) * sizeof(sal_Unicode);
+    if (nBytes <= nLocalBuf * NMAPPINGMAX)
+        out = static_cast<sal_Unicode*>(alloca(nBytes));
+    else
     {
         pHeapBuf.reset(new sal_Unicode[ nCount * NMAPPINGMAX ]);
         out = pHeapBuf.get();