diff --git a/lib/common/debug.h b/lib/common/debug.h
index 0c04ad2c..b4fc89d4 100644
--- a/lib/common/debug.h
+++ b/lib/common/debug.h
@@ -57,9 +57,9 @@ extern "C" {
 #endif
 
 
-/* static assert is triggered at compile time, leaving no runtime artefact,
- * but can only work with compile-time constants.
- * This variant can only be used inside a function. */
+/* static assert is triggered at compile time, leaving no runtime artefact.
+ * static assert only works with compile-time constants.
+ * Also, this variant can only be used inside a function. */
 #define DEBUG_STATIC_ASSERT(c) (void)sizeof(char[(c) ? 1 : -1])
 
 
@@ -70,9 +70,19 @@ extern "C" {
 #  define DEBUGLEVEL 0
 #endif
 
+
+/* DEBUGFILE can be defined externally,
+ * typically through compiler command line.
+ * note : currently useless.
+ * Value must be stderr or stdout */
+#ifndef DEBUGFILE
+#  define DEBUGFILE stderr
+#endif
+
+
 /* recommended values for DEBUGLEVEL :
- * 0 : no debug, all run-time functions disabled
- * 1 : no display, enables assert() only
+ * 0 : release mode, no debug, all run-time checks disabled
+ * 1 : enables assert() only, no display
  * 2 : reserved, for currently active debug path
  * 3 : events once per object lifetime (CCtx, CDict, etc.)
  * 4 : events once per frame
@@ -81,7 +91,7 @@ extern "C" {
  * 7+: events at every position (*very* verbose)
  *
  * It's generally inconvenient to output traces > 5.
- * In which case, it's possible to selectively enable higher verbosity levels
+ * In which case, it's possible to selectively trigger high verbosity levels
  * by modifying g_debug_level.
  */
 
@@ -95,11 +105,12 @@ extern "C" {
 
 #if (DEBUGLEVEL>=2)
 #  include <stdio.h>
-extern int g_debuglevel; /* here, this variable is only declared,
-                           it actually lives in debug.c,
-                           and is shared by the whole process.
-                           It's typically used to enable very verbose levels
-                           on selective conditions (such as position in src) */
+extern int g_debuglevel; /* the variable is only declared,
+                            it actually lives in debug.c,
+                            and is shared by the whole process.
+                            It's not thread-safe.
+                            It's useful when enabling very verbose levels
+                            on selective conditions (such as position in src) */
 
 #  define RAWLOG(l, ...) {                                      \
                 if (l<=g_debuglevel) {                          \
diff --git a/lib/compress/zstd_opt.c b/lib/compress/zstd_opt.c
index dcca6ed5..ce233f47 100644
--- a/lib/compress/zstd_opt.c
+++ b/lib/compress/zstd_opt.c
@@ -67,20 +67,31 @@ static void ZSTD_setBasePrices(optState_t* optPtr, int optLevel)
 }
 
 
+/* ZSTD_downscaleStat() :
+ * reduce all elements in table by a factor 2^(ZSTD_FREQ_DIV+malus)
+ * return the resulting sum of elements */
 static U32 ZSTD_downscaleStat(U32* table, U32 lastEltIndex, int malus)
 {
     U32 s, sum=0;
+    DEBUGLOG(2, "ZSTD_downscaleStat (nbElts=%u)", lastEltIndex+1);
     assert(ZSTD_FREQ_DIV+malus > 0 && ZSTD_FREQ_DIV+malus < 31);
-    for (s=0; s<=lastEltIndex; s++) {
+    for (s=0; s<lastEltIndex+1; s++) {
         table[s] = 1 + (table[s] >> (ZSTD_FREQ_DIV+malus));
         sum += table[s];
     }
     return sum;
 }
 
-static void ZSTD_rescaleFreqs(optState_t* const optPtr,
-                              const BYTE* const src, size_t const srcSize,
-                              int optLevel)
+/* ZSTD_rescaleFreqs() :
+ * if first block (detected by optPtr->litLengthSum == 0) : init statistics
+ *    take hints from dictionary if there is one
+ *    or init from zero, using src for literals stats, or flat 1 for match symbols
+ * otherwise downscale existing stats, to be used as seed for next block.
+ */
+static void
+ZSTD_rescaleFreqs(optState_t* const optPtr,
+                  const BYTE* const src, size_t const srcSize,
+                  int optLevel)
 {
     optPtr->priceType = zop_dynamic;
 
@@ -778,6 +789,20 @@ static U32 ZSTD_totalLen(ZSTD_optimal_t sol)
     return sol.litlen + sol.mlen;
 }
 
+
+static void
+listStats(const U32* table, int lastEltID)
+{
+    int const nbElts = lastEltID + 1;
+    int enb;
+    for (enb=0; enb < nbElts; enb++) {
+        (void)table;
+        //RAWLOG(2, "%3i:%3i,  ", enb, table[enb]);
+        RAWLOG(2, "%4i,", table[enb]);
+    }
+    RAWLOG(2, " \n");
+}
+
 FORCE_INLINE_TEMPLATE size_t
 ZSTD_compressBlock_opt_generic(ZSTD_matchState_t* ms,
                                seqStore_t* seqStore,
@@ -809,6 +834,12 @@ ZSTD_compressBlock_opt_generic(ZSTD_matchState_t* ms,
     ZSTD_rescaleFreqs(optStatePtr, (const BYTE*)src, srcSize, optLevel);
     ip += (ip==prefixStart);
 
+    DEBUGLOG(2, "OffCode table on entry : ");
+    listStats(optStatePtr->offCodeFreq, MaxOff);
+
+    DEBUGLOG(2, "Literals table on entry : ");
+    listStats(optStatePtr->litFreq, MaxLit);
+
     /* Match Loop */
     while (ip < ilimit) {
         U32 cur, last_pos = 0;
@@ -1036,6 +1067,12 @@ _shortestPath:   /* cur, last_pos, best_mlen, best_off have to be set */
 
     }   /* while (ip < ilimit) */
 
+    DEBUGLOG(2, "OffCode table on exit : ");
+    listStats(optStatePtr->offCodeFreq, MaxOff);
+
+    DEBUGLOG(2, "Literals table on exit : ");
+    listStats(optStatePtr->litFreq, MaxLit);
+
     /* Return the last literals size */
     return iend - anchor;
 }
@@ -1054,8 +1091,8 @@ size_t ZSTD_compressBlock_btopt(
 static U32 ZSTD_upscaleStat(U32* table, U32 lastEltIndex, int bonus)
 {
     U32 s, sum=0;
-    assert(ZSTD_FREQ_DIV+bonus > 0);
-    for (s=0; s<=lastEltIndex; s++) {
+    assert(ZSTD_FREQ_DIV+bonus >= 0);
+    for (s=0; s<lastEltIndex+1; s++) {
         table[s] <<= ZSTD_FREQ_DIV+bonus;
         table[s]--;
         sum += table[s];
@@ -1071,24 +1108,28 @@ MEM_STATIC void ZSTD_upscaleStats(optState_t* optPtr)
     optPtr->matchLengthSum = ZSTD_upscaleStat(optPtr->matchLengthFreq, MaxML, 1);
     optPtr->offCodeSum = ZSTD_upscaleStat(optPtr->offCodeFreq, MaxOff, 1);
 }
+
 /* ZSTD_initStats_ultra():
  * make a first compression pass, just to seed stats with more accurate starting values.
  * only works on first block, with no dictionary and no ldm.
- * this function must not fail, hence its usage conditions must be respected.
+ * this function cannot error, hence its constract must be respected.
  */
-static void ZSTD_initStats_ultra(
-        ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM],
-        const void* src, size_t srcSize)
+static void
+ZSTD_initStats_ultra(ZSTD_matchState_t* ms,
+                     seqStore_t* seqStore,
+                     U32 rep[ZSTD_REP_NUM],
+               const void* src, size_t srcSize)
 {
     U32 tmpRep[ZSTD_REP_NUM];  /* updated rep codes will sink here */
+    memcpy(tmpRep, rep, sizeof(tmpRep));
 
-    DEBUGLOG(5, "ZSTD_initStats_ultra (srcSize=%zu)", srcSize);
+    DEBUGLOG(2, "ZSTD_initStats_ultra (srcSize=%zu)", srcSize);
+    DEBUGLOG(2, "repCodes: %u, %u, %u", tmpRep[0], tmpRep[1], tmpRep[2]);
     assert(ms->opt.litLengthSum == 0);    /* first block */
     assert(seqStore->sequences == seqStore->sequencesStart);   /* no ldm */
     assert(ms->window.dictLimit == ms->window.lowLimit);   /* no dictionary */
     assert(ms->window.dictLimit - ms->nextToUpdate <= 1);  /* no prefix (note: intentional overflow, defined as 2-complement) */
 
-    memcpy(tmpRep, rep, sizeof(tmpRep));
     ZSTD_compressBlock_opt_generic(ms, seqStore, tmpRep, src, srcSize, 2 /*optLevel*/, ZSTD_noDict);   /* generate stats into ms->opt*/
 
     /* invalidate first scan from history */
@@ -1107,7 +1148,7 @@ size_t ZSTD_compressBlock_btultra(
         ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM],
         const void* src, size_t srcSize)
 {
-    DEBUGLOG(5, "ZSTD_compressBlock_btultra (srcSize=%zu)", srcSize);
+    DEBUGLOG(2, "ZSTD_compressBlock_btultra (srcSize=%zu)", srcSize);
     return ZSTD_compressBlock_opt_generic(ms, seqStore, rep, src, srcSize, 2 /*optLevel*/, ZSTD_noDict);
 }
 
@@ -1115,9 +1156,9 @@ size_t ZSTD_compressBlock_btultra2(
         ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM],
         const void* src, size_t srcSize)
 {
-    DEBUGLOG(5, "ZSTD_compressBlock_btultra2 (srcSize=%zu)", srcSize);
+    DEBUGLOG(2, "ZSTD_compressBlock_btultra2 (srcSize=%zu)", srcSize);
 
-    /* 2-pass strategy
+    /* 2-pass strategy:
      * this strategy makes a first pass over first block to collect statistics
      * and seed next round's statistics with it.
      * After 1st pass, function forgets everything, and starts a new block.
diff --git a/programs/benchzstd.c b/programs/benchzstd.c
index 30864f47..90357b5b 100644
--- a/programs/benchzstd.c
+++ b/programs/benchzstd.c
@@ -359,7 +359,8 @@ BMK_benchMemAdvancedNoAlloc(
                 srcPtrs[nbBlocks] = srcPtr;
                 srcSizes[nbBlocks] = thisBlockSize;
                 cPtrs[nbBlocks] = cPtr;
-                cCapacities[nbBlocks] = (adv->mode == BMK_decodeOnly) ? thisBlockSize : ZSTD_compressBound(thisBlockSize);
+#warning force streaming mode
+                cCapacities[nbBlocks] = (adv->mode == BMK_decodeOnly) ? thisBlockSize : ZSTD_compressBound(thisBlockSize) - 1;
                 resPtrs[nbBlocks] = resPtr;
                 resSizes[nbBlocks] = (adv->mode == BMK_decodeOnly) ? (size_t) ZSTD_findDecompressedSize(srcPtr, thisBlockSize) : thisBlockSize;
                 srcPtr += thisBlockSize;