diff --git a/.gitignore b/.gitignore
index e7c9a568..0c458153 100644
--- a/.gitignore
+++ b/.gitignore
@@ -40,6 +40,7 @@ projects/cmake/
 
 # Test artefacts
 tmp*
+dictionary
 
 # tmp files
 *.swp
diff --git a/NEWS b/NEWS
index 7ffa4023..d01a3313 100644
--- a/NEWS
+++ b/NEWS
@@ -1,5 +1,6 @@
 v0.8.0
 New : updated compresson format
+Improved : better speed on clang and gcc -O2, thanks to Eric Biggers
 Fixed : legacy mode with ZSTD_HEAPMODE=0, by Christopher Bergqvist
 Fixed : premature end of frame when zero-sized raw block, reported by Eric Biggers
 Fixed : checksum correctly checked in single-pass mode
diff --git a/lib/dictBuilder/zdict.c b/lib/dictBuilder/zdict.c
index f1518555..75a9b1e3 100644
--- a/lib/dictBuilder/zdict.c
+++ b/lib/dictBuilder/zdict.c
@@ -924,7 +924,7 @@ size_t ZDICT_trainFromBuffer_unsafe(
                             const void* samplesBuffer, const size_t* samplesSizes, unsigned nbSamples,
                             ZDICT_params_t params)
 {
-    U32 const dictListSize = MAX( MAX(DICTLISTSIZE, nbSamples), (U32)(maxDictSize/16));
+    U32 const dictListSize = MAX(MAX(DICTLISTSIZE, nbSamples), (U32)(maxDictSize/16));
     dictItem* const dictList = (dictItem*)malloc(dictListSize * sizeof(*dictList));
     unsigned selectivity = params.selectivityLevel;
     size_t const targetDictSize = maxDictSize;
@@ -957,17 +957,25 @@ size_t ZDICT_trainFromBuffer_unsafe(
             DISPLAYLEVEL(3, "\n %u segments found, of total size %u \n", dictList[0].pos, dictContentSize);
             DISPLAYLEVEL(3, "list %u best segments \n", nb);
             for (u=1; u<=nb; u++) {
-                U32 p = dictList[u].pos;
-                U32 l = dictList[u].length;
-                U32 d = MIN(40, l);
+                U32 pos = dictList[u].pos;
+                U32 length = dictList[u].length;
+                U32 printedLength = MIN(40, length);
                 DISPLAYLEVEL(3, "%3u:%3u bytes at pos %8u, savings %7u bytes |",
-                             u, l, p, dictList[u].savings);
-                ZDICT_printHex(3, (const char*)samplesBuffer+p, d);
+                             u, length, pos, dictList[u].savings);
+                ZDICT_printHex(3, (const char*)samplesBuffer+pos, printedLength);
                 DISPLAYLEVEL(3, "| \n");
     }   }   }
 
     /* create dictionary */
     {   U32 dictContentSize = ZDICT_dictSize(dictList);
+        U64 const totalSamplesSize = ZDICT_totalSampleSize(samplesSizes, nbSamples);
+        if (dictContentSize < targetDictSize/2) {
+            DISPLAYLEVEL(2, "!  warning : created dictionary significantly smaller than requested (%u < %u) \n", dictContentSize, (U32)maxDictSize);
+            DISPLAYLEVEL(2, "!  consider increasing selectivity to produce larger dictionary (-s%u) \n", selectivity+1);
+            DISPLAYLEVEL(2, "!  note : larger dictionaries are not necessarily better, test its efficiency on samples \n");
+            if (totalSamplesSize < 10 * targetDictSize)
+                DISPLAYLEVEL(2, "!  consider also increasing the number of samples (total size : %u MB)\n", (U32)(totalSamplesSize>>20));
+        }
 
         /* build dict content */
         {   U32 u;
diff --git a/programs/dibio.c b/programs/dibio.c
index a61ea9cc..cb864ec1 100644
--- a/programs/dibio.c
+++ b/programs/dibio.c
@@ -202,9 +202,16 @@ int DiB_trainFromFiles(const char* dictFileName, unsigned maxDictSize,
 
     /* Checks */
     if ((!fileSizes) || (!srcBuffer) || (!dictBuffer)) EXM_THROW(12, "not enough memory for DiB_trainFiles");   /* should not happen */
+    g_displayLevel = params.notificationLevel;
+    if (nbFiles < 5) {
+        DISPLAYLEVEL(2, "!  Warning : nb of samples too low for proper processing \n");
+        DISPLAYLEVEL(2, "!  Please provide one file per sample \n");
+        DISPLAYLEVEL(2, "!  Avoid concatenating multiple samples into a single file \n");
+        DISPLAYLEVEL(2, "!  otherwise, dictBuilder will be unable to find the beginning of each sample \n");
+        DISPLAYLEVEL(2, "!  resulting in distorted statistics \n");
+    }
 
     /* init */
-    g_displayLevel = params.notificationLevel;
     if (benchedSize < totalSizeToLoad)
         DISPLAYLEVEL(1, "Not enough memory; training on %u MB only...\n", (unsigned)(benchedSize >> 20));