Reduce default MaxRegs setting to avoid wraparound.

Fix wraparound bug in XLS that was reducing available registers.

Reviewed By: @bertmaher

Differential Revision: D1142120

hphp/runtime/base/runtime-option.h

@@ -409,7 +409,7 @@ public:
   F(bool, HHIRRefcountOpts,            true)                            \
   F(bool, HHIRRefcountOptsAlwaysSink,  false)                           \
   F(bool, HHIRExtraOptPass,            true)                            \
-  F(uint32_t, HHIRNumFreeRegs,         -1)                              \
+  F(uint32_t, HHIRNumFreeRegs,         64)                              \
   F(bool, HHIREnableGenTimeInlining,   true)                            \
   F(uint32_t, HHIRInliningMaxCost,     13)                              \
   F(uint32_t, HHIRAlwaysInlineMaxCost, 10)                              \

hphp/runtime/vm/jit/xls.cpp

@@ -534,6 +534,7 @@ void XLS::buildIntervals() {
   }
   // Implement stress mode by blocking more registers.
   stress += RuntimeOption::EvalHHIRNumFreeRegs;
+  assert(stress >= RuntimeOption::EvalHHIRNumFreeRegs); // no wraparound.
   for (auto r : m_blocked) {
     auto& blocked = m_blocked[r];
     if (blocked.empty() || blocked.start() == 0) continue;