Improve compression speed on small blocks

lib/compress/zstd_compress_internal.h

@@ -557,6 +557,25 @@ MEM_STATIC int ZSTD_cParam_withinBounds(ZSTD_cParameter cParam, int value)
     return 1;
 }
 
+/* ZSTD_selectAddr:
+ * @return index >= lowLimit ? candidate : backup,
+ * tries to force branchless codegen. */
+MEM_STATIC const BYTE*
+ZSTD_selectAddr(U32 index, U32 lowLimit, const BYTE* candidate, const BYTE* backup)
+{
+#if defined(__GNUC__) && defined(__x86_64__)
+    __asm__ (
+        "cmp %1, %2\n"
+        "cmova %3, %0\n"
+        : "+r"(candidate)
+        : "r"(index), "r"(lowLimit), "r"(backup)
+        );
+    return candidate;
+#else
+    return index >= lowLimit ? candidate : backup;
+#endif
+}
+
 /* ZSTD_noCompressBlock() :
  * Writes uncompressed block to dst buffer from given src.
  * Returns the size of the block */

lib/compress/zstd_double_fast.c

@@ -140,11 +140,17 @@ size_t ZSTD_compressBlock_doubleFast_noDict_generic(
     U32 idxl1; /* the long match index for ip1 */
 
     const BYTE* matchl0; /* the long match for ip */
+    const BYTE* matchl0_safe; /* matchl0 or safe address */
     const BYTE* matchs0; /* the short match for ip */
     const BYTE* matchl1; /* the long match for ip1 */
+    const BYTE* matchs0_safe; /* matchs0 or safe address */
 
     const BYTE* ip = istart; /* the current position */
     const BYTE* ip1; /* the next position */
+    /* Array of ~random data, should have low probability of matching data
+     * we load from here instead of from tables, if matchl0/matchl1 are
+     * invalid indices. Used to avoid unpredictable branches. */
+    const BYTE dummy[] = {0x12,0x34,0x56,0x78,0x9a,0xbc,0xde,0xf0,0xe2,0xb4};
 
     DEBUGLOG(5, "ZSTD_compressBlock_doubleFast_noDict_generic");
 
@@ -191,24 +197,29 @@ size_t ZSTD_compressBlock_doubleFast_noDict_generic(
 
             hl1 = ZSTD_hashPtr(ip1, hBitsL, 8);
 
-            if (idxl0 > prefixLowestIndex) {
-                /* check prefix long match */
-                if (MEM_read64(matchl0) == MEM_read64(ip)) {
-                    mLength = ZSTD_count(ip+8, matchl0+8, iend) + 8;
-                    offset = (U32)(ip-matchl0);
-                    while (((ip>anchor) & (matchl0>prefixLowest)) && (ip[-1] == matchl0[-1])) { ip--; matchl0--; mLength++; } /* catch up */
-                    goto _match_found;
-                }
+            /* idxl0 > prefixLowestIndex is a (somewhat) unpredictable branch.
+             * However expression below complies into conditional move. Since
+             * match is unlikely and we only *branch* on idxl0 > prefixLowestIndex
+             * if there is a match, all branches become predictable. */
+            matchl0_safe = ZSTD_selectAddr(prefixLowestIndex, idxl0, &dummy[0], matchl0);
+
+            /* check prefix long match */
+            if (MEM_read64(matchl0_safe) == MEM_read64(ip) && matchl0_safe == matchl0) {
+                mLength = ZSTD_count(ip+8, matchl0+8, iend) + 8;
+                offset = (U32)(ip-matchl0);
+                while (((ip>anchor) & (matchl0>prefixLowest)) && (ip[-1] == matchl0[-1])) { ip--; matchl0--; mLength++; } /* catch up */
+                goto _match_found;
             }
 
             idxl1 = hashLong[hl1];
             matchl1 = base + idxl1;
 
-            if (idxs0 > prefixLowestIndex) {
-                /* check prefix short match */
-                if (MEM_read32(matchs0) == MEM_read32(ip)) {
-                    goto _search_next_long;
-                }
+            /* Same optimization as matchl0 above */
+            matchs0_safe = ZSTD_selectAddr(prefixLowestIndex, idxs0, &dummy[0], matchs0);
+
+            /* check prefix short match */
+            if(MEM_read32(matchs0_safe) == MEM_read32(ip) && matchs0_safe == matchs0) {
+                  goto _search_next_long;
             }
 
             if (ip1 >= nextStep) {

lib/compress/zstd_fast.c

@@ -45,7 +45,7 @@ void ZSTD_fillHashTableForCDict(ZSTD_matchState_t* ms,
                 size_t const hashAndTag = ZSTD_hashPtr(ip + p, hBits, mls);
                 if (hashTable[hashAndTag >> ZSTD_SHORT_CACHE_TAG_BITS] == 0) {  /* not yet filled */
                     ZSTD_writeTaggedIndex(hashTable, hashAndTag, curr + p);
-                }   }   }   }
+    }   }   }   }
 }
 
 static
@@ -97,6 +97,50 @@ void ZSTD_fillHashTable(ZSTD_matchState_t* ms,
 }
 
 
+typedef int (*ZSTD_match4Found) (const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit);
+
+static int
+ZSTD_match4Found_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit)
+{
+    /* Array of ~random data, should have low probability of matching data.
+     * Load from here if the index is invalid.
+     * Used to avoid unpredictable branches. */
+    static const BYTE dummy[] = {0x12,0x34,0x56,0x78 };
+
+    /* currentIdx >= lowLimit is a (somewhat) unpredictable branch.
+     * However expression below compiles into conditional move.
+     */
+    const BYTE* mvalAddr = ZSTD_selectAddr(matchIdx, idxLowLimit, matchAddress, dummy);
+    /* Note: this used to be written as : return test1 && test2;
+     * Unfortunately, once inlined, these tests become branches,
+     * in which case it becomes critical that they are executed in the right order (test1 then test2).
+     * So we have to write these tests in a specific manner to ensure their ordering.
+     */
+    if (MEM_read32(currentPtr) != MEM_read32(mvalAddr)) return 0;
+    /* force ordering of these tests, which matters once the function is inlined, as they become branches */
+#if defined(__GNUC__)
+    __asm__("");
+#endif
+    return matchIdx >= idxLowLimit;
+}
+
+static int
+ZSTD_match4Found_branch(const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit)
+{
+    /* using a branch instead of a cmov,
+     * because it's faster in scenarios where matchIdx >= idxLowLimit is generally true,
+     * aka almost all candidates are within range */
+    U32 mval;
+    if (matchIdx >= idxLowLimit) {
+        mval = MEM_read32(matchAddress);
+    } else {
+        mval = MEM_read32(currentPtr) ^ 1; /* guaranteed to not match. */
+    }
+
+    return (MEM_read32(currentPtr) == mval);
+}
+
+
 /**
  * If you squint hard enough (and ignore repcodes), the search operation at any
  * given position is broken into 4 stages:
@@ -148,13 +192,12 @@ ZSTD_ALLOW_POINTER_OVERFLOW_ATTR
 size_t ZSTD_compressBlock_fast_noDict_generic(
         ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM],
         void const* src, size_t srcSize,
-        U32 const mls, U32 const hasStep)
+        U32 const mls, int useCmov)
 {
     const ZSTD_compressionParameters* const cParams = &ms->cParams;
     U32* const hashTable = ms->hashTable;
     U32 const hlog = cParams->hashLog;
-    /* support stepSize of 0 */
-    size_t const stepSize = hasStep ? (cParams->targetLength + !(cParams->targetLength) + 1) : 2;
+    size_t const stepSize = cParams->targetLength + !(cParams->targetLength) + 1; /* min 2 */
     const BYTE* const base = ms->window.base;
     const BYTE* const istart = (const BYTE*)src;
     const U32   endIndex = (U32)((size_t)(istart - base) + srcSize);
@@ -176,8 +219,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
 
     size_t hash0; /* hash for ip0 */
     size_t hash1; /* hash for ip1 */
-    U32 idx; /* match idx for ip0 */
-    U32 mval; /* src value at match idx */
+    U32 matchIdx; /* match idx for ip0 */
 
     U32 offcode;
     const BYTE* match0;
@@ -190,6 +232,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
     size_t step;
     const BYTE* nextStep;
     const size_t kStepIncr = (1 << (kSearchStrength - 1));
+    const ZSTD_match4Found findMatch = useCmov ? ZSTD_match4Found_cmov : ZSTD_match4Found_branch;
 
     DEBUGLOG(5, "ZSTD_compressBlock_fast_generic");
     ip0 += (ip0 == prefixStart);
@@ -218,7 +261,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
     hash0 = ZSTD_hashPtr(ip0, hlog, mls);
     hash1 = ZSTD_hashPtr(ip1, hlog, mls);
 
-    idx = hashTable[hash0];
+    matchIdx = hashTable[hash0];
 
     do {
         /* load repcode match for ip[2]*/
@@ -238,35 +281,27 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
             offcode = REPCODE1_TO_OFFBASE;
             mLength += 4;
 
-            /* First write next hash table entry; we've already calculated it.
-             * This write is known to be safe because the ip1 is before the
+            /* Write next hash table entry: it's already calculated.
+             * This write is known to be safe because ip1 is before the
              * repcode (ip2). */
             hashTable[hash1] = (U32)(ip1 - base);
 
             goto _match;
         }
 
-        /* load match for ip[0] */
-        if (idx >= prefixStartIndex) {
-            mval = MEM_read32(base + idx);
-        } else {
-            mval = MEM_read32(ip0) ^ 1; /* guaranteed to not match. */
-        }
-
-        /* check match at ip[0] */
-        if (MEM_read32(ip0) == mval) {
+         if (findMatch(ip0, base + matchIdx, matchIdx, prefixStartIndex)) {
             /* found a match! */
 
-            /* First write next hash table entry; we've already calculated it.
-             * This write is known to be safe because the ip1 == ip0 + 1, so
-             * we know we will resume searching after ip1 */
+            /* Write next hash table entry (it's already calculated).
+            * This write is known to be safe because the ip1 == ip0 + 1,
+            * so searching will resume after ip1 */
             hashTable[hash1] = (U32)(ip1 - base);
 
             goto _offset;
         }
 
         /* lookup ip[1] */
-        idx = hashTable[hash1];
+        matchIdx = hashTable[hash1];
 
         /* hash ip[2] */
         hash0 = hash1;
@@ -281,36 +316,22 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
         current0 = (U32)(ip0 - base);
         hashTable[hash0] = current0;
 
-        /* load match for ip[0] */
-        if (idx >= prefixStartIndex) {
-            mval = MEM_read32(base + idx);
-        } else {
-            mval = MEM_read32(ip0) ^ 1; /* guaranteed to not match. */
-        }
-
-        /* check match at ip[0] */
-        if (MEM_read32(ip0) == mval) {
+         if (findMatch(ip0, base + matchIdx, matchIdx, prefixStartIndex)) {
             /* found a match! */
 
-            /* first write next hash table entry; we've already calculated it */
+            /* Write next hash table entry; it's already calculated */
             if (step <= 4) {
-                /* We need to avoid writing an index into the hash table >= the
-                 * position at which we will pick up our searching after we've
-                 * taken this match.
-                 *
-                 * The minimum possible match has length 4, so the earliest ip0
-                 * can be after we take this match will be the current ip0 + 4.
-                 * ip1 is ip0 + step - 1. If ip1 is >= ip0 + 4, we can't safely
-                 * write this position.
-                 */
+                /* Avoid writing an index if it's >= position where search will resume.
+                * The minimum possible match has length 4, so search can resume at ip0 + 4.
+                */
                 hashTable[hash1] = (U32)(ip1 - base);
             }
 
             goto _offset;
         }
 
         /* lookup ip[1] */
-        idx = hashTable[hash1];
+        matchIdx = hashTable[hash1];
 
         /* hash ip[2] */
         hash0 = hash1;
@@ -332,7 +353,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
     } while (ip3 < ilimit);
 
 _cleanup:
-    /* Note that there are probably still a couple positions we could search.
+    /* Note that there are probably still a couple positions one could search.
      * However, it seems to be a meaningful performance hit to try to search
      * them. So let's not. */
 
@@ -361,7 +382,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
 _offset: /* Requires: ip0, idx */
 
     /* Compute the offset code. */
-    match0 = base + idx;
+    match0 = base + matchIdx;
     rep_offset2 = rep_offset1;
     rep_offset1 = (U32)(ip0-match0);
     offcode = OFFSET_TO_OFFBASE(rep_offset1);
@@ -406,12 +427,12 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
     goto _start;
 }
 
-#define ZSTD_GEN_FAST_FN(dictMode, mls, step)                                                            \
-    static size_t ZSTD_compressBlock_fast_##dictMode##_##mls##_##step(                                      \
+#define ZSTD_GEN_FAST_FN(dictMode, mml, cmov)                                                       \
+    static size_t ZSTD_compressBlock_fast_##dictMode##_##mml##_##cmov(                              \
             ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM],                    \
             void const* src, size_t srcSize)                                                       \
     {                                                                                              \
-        return ZSTD_compressBlock_fast_##dictMode##_generic(ms, seqStore, rep, src, srcSize, mls, step); \
+        return ZSTD_compressBlock_fast_##dictMode##_generic(ms, seqStore, rep, src, srcSize, mml, cmov); \
     }
 
 ZSTD_GEN_FAST_FN(noDict, 4, 1)
@@ -428,10 +449,12 @@ size_t ZSTD_compressBlock_fast(
         ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM],
         void const* src, size_t srcSize)
 {
-    U32 const mls = ms->cParams.minMatch;
+    U32 const mml = ms->cParams.minMatch;
+    /* use cmov when "candidate in range" branch is likely unpredictable */
+    int const useCmov = ms->cParams.windowLog < 19;
     assert(ms->dictMatchState == NULL);
-    if (ms->cParams.targetLength > 1) {
-        switch(mls)
+    if (useCmov) {
+        switch(mml)
         {
         default: /* includes case 3 */
         case 4 :
@@ -444,7 +467,8 @@ size_t ZSTD_compressBlock_fast(
             return ZSTD_compressBlock_fast_noDict_7_1(ms, seqStore, rep, src, srcSize);
         }
     } else {
-        switch(mls)
+        /* use a branch instead */
+        switch(mml)
         {
         default: /* includes case 3 */
         case 4 :
@@ -456,7 +480,6 @@ size_t ZSTD_compressBlock_fast(
         case 7 :
             return ZSTD_compressBlock_fast_noDict_7_0(ms, seqStore, rep, src, srcSize);
         }
-
     }
 }
 
@@ -546,7 +569,7 @@ size_t ZSTD_compressBlock_fast_dictMatchState_generic(
             size_t const dictHashAndTag1 = ZSTD_hashPtr(ip1, dictHBits, mls);
             hashTable[hash0] = curr;   /* update hash table */
 
-            if ((ZSTD_index_overlap_check(prefixStartIndex, repIndex)) 
+            if ((ZSTD_index_overlap_check(prefixStartIndex, repIndex))
                 && (MEM_read32(repMatch) == MEM_read32(ip0 + 1))) {
                 const BYTE* const repMatchEnd = repIndex < prefixStartIndex ? dictEnd : iend;
                 mLength = ZSTD_count_2segments(ip0 + 1 + 4, repMatch + 4, iend, repMatchEnd, prefixStart) + 4;