bjit: keep track which vregs are known to be non null

this transform some xdecrefs to decrefs
and removes some calls of assertNameDefinedHelper

src/asm_writing/rewriter.h

@@ -202,7 +202,8 @@ public:
 
     template <typename Src, typename Dst> inline RewriterVar* getAttrCast(int offset, Location loc = Location::any());
 
-    bool isConstant() { return is_constant; }
+    bool isConstant() const { return is_constant; }
+    bool isContantNull() const { return isConstant() && constant_value == 0; }
 
 protected:
     void incref();

src/codegen/baseline_jit.cpp

@@ -412,8 +412,14 @@ RewriterVar* JitFragmentWriter::emitGetLocal(InternedString s, int vreg) {
     assert(vreg >= 0);
     // TODO Can we use BORROWED here? Not sure if there are cases when we can't rely on borrowing the ref
     // from the vregs array.  Safer like this.
-    RewriterVar* val_var = vregs_array->getAttr(vreg * 8)->setType(RefType::OWNED);
-    addAction([=]() { _emitGetLocal(val_var, s.c_str()); }, { val_var }, ActionType::NORMAL);
+    RewriterVar* val_var = vregs_array->getAttr(vreg * 8);
+    if (known_non_null_vregs.count(vreg) == 0) {
+        addAction([=]() { _emitGetLocal(val_var, s.c_str()); }, { val_var }, ActionType::NORMAL);
+        known_non_null_vregs.insert(vreg);
+    } else {
+        val_var->incref();
+    }
+    val_var->setType(RefType::OWNED);
     if (LOG_BJIT_ASSEMBLY)
         comment("BJIT: emitGetLocal end");
     return val_var;
@@ -696,10 +702,14 @@ void JitFragmentWriter::emitSetLocal(InternedString s, int vreg, bool set_closur
         // The issue is that definedness analysis is somewhat expensive to compute, so we don't compute it
         // for the bjit.  We could try calculating it (which would require some re-plumbing), which might help
         // but I suspect is not that big a deal as long as the llvm jit implements this kind of optimization.
-        bool prev_nullable = true;
+        bool prev_nullable = known_non_null_vregs.count(vreg) == 0;
 
         assert(!block->cfg->getVRegInfo().isBlockLocalVReg(vreg));
         vregs_array->replaceAttr(8 * vreg, v, prev_nullable);
+        if (v->isContantNull())
+            known_non_null_vregs.erase(vreg);
+        else
+            known_non_null_vregs.insert(vreg);
     }
     if (LOG_BJIT_ASSEMBLY)
         comment("BJIT: emitSetLocal() end");

src/codegen/baseline_jit.h

@@ -207,6 +207,9 @@ private:
     RewriterVar* interp;
     RewriterVar* vregs_array;
     llvm::DenseMap<InternedString, RewriterVar*> local_syms;
+    // keeps track which non block local vregs are known to have a non NULL value
+    // TODO: in the future we could reuse this information between different basic blocks
+    llvm::DenseSet<int> known_non_null_vregs;
     std::unique_ptr<ICInfo> ic_info;
 
     // Optional points to a CFGBlock and a patch location which should get patched to a direct jump if