refactor search into an inline function

for easier swapping with a parameter

lib/compress/zstd_compress_internal.h

@@ -561,14 +561,14 @@ MEM_STATIC int ZSTD_cParam_withinBounds(ZSTD_cParameter cParam, int value)
  * @return a >= b ? trueAddr : falseAddr,
  * tries to force branchless codegen. */
 MEM_STATIC const BYTE*
-ZSTD_selectAddr(U32 a, U32 b, const BYTE* trueAddr, const BYTE* falseAddr)
+ZSTD_selectAddr(U32 index, U32 lowLimit, const BYTE* trueAddr, const BYTE* falseAddr)
 {
 #if defined(__GNUC__) && defined(__x86_64__)
     __asm__ (
         "cmp %1, %2\n"
         "cmova %3, %0\n"
         : "+r"(trueAddr)
-        : "r"(a), "r"(b), "r"(falseAddr)
+        : "r"(index), "r"(lowLimit), "r"(falseAddr)
         );
     return trueAddr;
 #else

lib/compress/zstd_fast.c

@@ -97,6 +97,18 @@ void ZSTD_fillHashTable(ZSTD_matchState_t* ms,
 }
 
 
+static int
+ZSTD_findMatch_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 currentIdx, U32 lowLimit,  const BYTE* fakeAddress)
+{
+    /* idx >= prefixStartIndex 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. */
+    const BYTE* mvalAddr = ZSTD_selectAddr(currentIdx, lowLimit, matchAddress, fakeAddress);
+    return ((MEM_read32(currentPtr) == MEM_read32(mvalAddr)) & (currentIdx >= lowLimit));
+}
+
+
 /**
  * If you squint hard enough (and ignore repcodes), the search operation at any
  * given position is broken into 4 stages:
@@ -148,13 +160,12 @@ 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, U32 const unpredictable)
 {
     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 = cParams->targetLength + !(cParams->targetLength) + 1; /* min 2 */
-    size_t const stepSize = hasStep ? (cParams->targetLength + !(cParams->targetLength) + 1) : 2;
     const BYTE* const base = ms->window.base;
     const BYTE* const istart = (const BYTE*)src;
     const U32   endIndex = (U32)((size_t)(istart - base) + srcSize);
@@ -193,6 +204,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
     size_t step;
     const BYTE* nextStep;
     const size_t kStepIncr = (1 << (kSearchStrength - 1));
+    (void)unpredictable;
 
     DEBUGLOG(5, "ZSTD_compressBlock_fast_generic");
     ip0 += (ip0 == prefixStart);
@@ -249,16 +261,7 @@ _start: /* Requires: ip0 */
             goto _match;
         }
 
-        /* idx >= prefixStartIndex is a (somewhat) unpredictable branch.
+         if (ZSTD_findMatch_cmov(ip0, base + idx, idx, prefixStartIndex, dummy)) {
-         * 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. */
-        {   const BYTE* mvalAddr = ZSTD_selectAddr(idx, prefixStartIndex, base + idx, &dummy[0]);
-            /* load match for ip[0] */
-            U32 const mval = MEM_read32(mvalAddr);
-
-            /* check match at ip[0] */
-            if (MEM_read32(ip0) == mval && idx >= prefixStartIndex) {
             /* found a match! */
 
             /* Write next hash table entry (it's already calculated).
@@ -268,7 +271,6 @@ _start: /* Requires: ip0 */
 
             goto _offset;
         }
-        }
 
         /* lookup ip[1] */
         idx = hashTable[hash1];
@@ -286,15 +288,10 @@ _start: /* Requires: ip0 */
         current0 = (U32)(ip0 - base);
         hashTable[hash0] = current0;
 
-        {   const BYTE* mvalAddr = ZSTD_selectAddr(idx, prefixStartIndex, base + idx, &dummy[0]);
+         if (ZSTD_findMatch_cmov(ip0, base + idx, idx, prefixStartIndex, dummy)) {
-            /* load match for ip[0] */
-            U32 const mval = MEM_read32(mvalAddr);
-
-            /* check match at ip[0] */
-            if (MEM_read32(ip0) == mval && idx >= prefixStartIndex) {
             /* found a match! */
 
-                /* first write next hash table entry; we've already calculated it */
+            /* first 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
@@ -310,7 +307,6 @@ _start: /* Requires: ip0 */
 
             goto _offset;
         }
-        }
 
         /* lookup ip[1] */
         idx = hashTable[hash1];
@@ -409,12 +405,12 @@ _match: /* Requires: ip0, match0, offcode */
     goto _start;
 }
 
-#define ZSTD_GEN_FAST_FN(dictMode, mls, step)                                                            \
+#define ZSTD_GEN_FAST_FN(dictMode, mls, cmov)                                                       \
-    static size_t ZSTD_compressBlock_fast_##dictMode##_##mls##_##step(                                      \
+    static size_t ZSTD_compressBlock_fast_##dictMode##_##mls##_##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, mls, cmov); \
     }
 
 ZSTD_GEN_FAST_FN(noDict, 4, 1)
@@ -432,8 +428,10 @@ size_t ZSTD_compressBlock_fast(
         void const* src, size_t srcSize)
 {
     U32 const mls = ms->cParams.minMatch;
+    /* use cmov instead of branch when the branch is likely unpredictable */
+    int const useCmov = 1;
     assert(ms->dictMatchState == NULL);
-    if (ms->cParams.targetLength > 1) {
+    if (useCmov) {
         switch(mls)
         {
         default: /* includes case 3 */
@@ -447,6 +445,7 @@ size_t ZSTD_compressBlock_fast(
             return ZSTD_compressBlock_fast_noDict_7_1(ms, seqStore, rep, src, srcSize);
         }
     } else {
+        /* use a branch instead */
         switch(mls)
         {
         default: /* includes case 3 */