simplify away likelyfaillow

Passed STC non-regressive test on r15:
Total/Win/Draw/Lose: 16548 / 6387 / 3953 / 6208
PTNML: 998 / 1380 / 3408 / 1421 / 1067
WinRate: 50.54%
ELO: 3.22[-1.56, 7.94]
LOS: 90.75
LLR: 2.93[-2.94, 2.94]

bench 4187917f
bench (msvc clang) 1baf495f

Rapfi/search/ab/parameter.h

@@ -114,15 +114,6 @@ constexpr Value failHighMargin(Depth d, int oppo4)
     return Value(FHBias[R] + FHScale[R] * int(d) + FHOppo4[R] * bool(oppo4));
 }
 
-/// Fail low reduction margin
-template <Rule R>
-constexpr Value failLowMargin(Depth d)
-{
-    constexpr int FLBias[RULE_NB]  = {89, 82, 106};
-    constexpr int FLScale[RULE_NB] = {58, 54, 45};
-    return Value(FLBias[R] + int(FLScale[R] * d));
-}
-
 // Lookup tables used for move count based pruning, initialized at startup
 inline const auto FutilityMC = []() {
     std::array<int, MAX_MOVES + 1> MC {0};  // [depth]

Rapfi/search/ab/search.cpp

@@ -925,11 +925,6 @@ Value search(Board &board, SearchStack *ss, Value alpha, Value beta, Depth depth
     // Indicate cutNode that will probably fail high if current eval is far above beta
     bool likelyFailHigh = !PvNode && cutNode && eval >= beta + failHighMargin<Rule>(depth, oppo4);
 
-    // Indicate PvNodes that will probably fail low if node was searched with non-PV search
-    // at depth equal or greater to current depth and result of this search was far below alpha
-    bool likelyFailLow = PvNode && ttHit && (ttBound & BOUND_UPPER) && ttDepth >= depth
-                         && ttValue < alpha + failLowMargin<Rule>(depth);
-
     MovePicker mp(Rule,
                   board,
                   MovePicker::ExtraArgs<MovePicker::MAIN> {
@@ -1119,9 +1114,8 @@ Value search(Board &board, SearchStack *ss, Value alpha, Value beta, Depth depth
             // Dynamic reduction based on complexity
             r += complexity * complexityReduction<Rule>(trivialMove, importantMove, distract);
 
-            // Decrease reduction if position is or has been on the PV and
-            // the node is not likely to fail low.
-            if (ss->ttPv && !likelyFailLow)
+            // Decrease reduction if position is or has been on the PV
+            if (ss->ttPv)
                 r -= 1.0f;
 
             // Increase reduction for nodes that does not improve root alpha