From 61765cacd08103d47ce7c6709135f340d1074568 Mon Sep 17 00:00:00 2001
From: "W. Felix Handte" <w@felixhandte.com>
Date: Mon, 8 Nov 2021 20:03:52 -0500
Subject: [PATCH 1/2] Avoid Reducing Indices to Reserved Values

Previously, if an index was equal to `reducerValue + 1`, it would get remapped
during index reduction to 1 i.e. `ZSTD_DUBT_UNSORTED_MARK`. This can affect the
parsing of the input slightly, by causing tree nodes to be nullified when they
otherwise wouldn't be. This hardly matters from a correctness or efficiency
perspective, but it does impact determinism.

So this commit changes index reduction to avoid mapping indices to collide with
`ZSTD_DUBT_UNSORTED_MARK`.
---
 lib/compress/zstd_compress.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/lib/compress/zstd_compress.c b/lib/compress/zstd_compress.c
index f870bad63..59a216918 100644
--- a/lib/compress/zstd_compress.c
+++ b/lib/compress/zstd_compress.c
@@ -2311,6 +2311,8 @@ ZSTD_reduceTable_internal (U32* const table, U32 const size, U32 const reducerVa
     int const nbRows = (int)size / ZSTD_ROWSIZE;
     int cellNb = 0;
     int rowNb;
+    /* Protect special index values < ZSTD_WINDOW_START_INDEX. */
+    U32 const reducerThreshold = reducerValue + ZSTD_WINDOW_START_INDEX;
     assert((size & (ZSTD_ROWSIZE-1)) == 0);  /* multiple of ZSTD_ROWSIZE */
     assert(size < (1U<<31));   /* can be casted to int */
 
@@ -2330,12 +2332,12 @@ ZSTD_reduceTable_internal (U32* const table, U32 const size, U32 const reducerVa
     for (rowNb=0 ; rowNb < nbRows ; rowNb++) {
         int column;
         for (column=0; column<ZSTD_ROWSIZE; column++) {
-            if (preserveMark) {
-                U32 const adder = (table[cellNb] == ZSTD_DUBT_UNSORTED_MARK) ? reducerValue : 0;
-                table[cellNb] += adder;
+            if (preserveMark && table[cellNb] == ZSTD_DUBT_UNSORTED_MARK) {
+            } else if (table[cellNb] < reducerThreshold) {
+                table[cellNb] = 0;
+            } else {
+                table[cellNb] -= reducerValue;
             }
-            if (table[cellNb] < reducerValue) table[cellNb] = 0;
-            else table[cellNb] -= reducerValue;
             cellNb++;
     }   }
 }

From 48572f52b1c6fe2c7e57206b063ea85980e6c00b Mon Sep 17 00:00:00 2001
From: "W. Felix Handte" <w@felixhandte.com>
Date: Tue, 9 Nov 2021 12:15:18 -0500
Subject: [PATCH 2/2] Rewrite Fix to Still Auto-Vectorize

---
 lib/compress/zstd_compress.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/compress/zstd_compress.c b/lib/compress/zstd_compress.c
index 59a216918..09f18d6d9 100644
--- a/lib/compress/zstd_compress.c
+++ b/lib/compress/zstd_compress.c
@@ -2332,12 +2332,17 @@ ZSTD_reduceTable_internal (U32* const table, U32 const size, U32 const reducerVa
     for (rowNb=0 ; rowNb < nbRows ; rowNb++) {
         int column;
         for (column=0; column<ZSTD_ROWSIZE; column++) {
+            U32 newVal;
             if (preserveMark && table[cellNb] == ZSTD_DUBT_UNSORTED_MARK) {
+                /* This write is pointless, but is required(?) for the compiler
+                 * to auto-vectorize the loop. */
+                newVal = ZSTD_DUBT_UNSORTED_MARK;
             } else if (table[cellNb] < reducerThreshold) {
-                table[cellNb] = 0;
+                newVal = 0;
             } else {
-                table[cellNb] -= reducerValue;
+                newVal = table[cellNb] - reducerValue;
             }
+            table[cellNb] = newVal;
             cellNb++;
     }   }
 }