Move EEOP_*_SYSVAR evaluation out of line.

This mainly de-duplicates code. As evaluating a system variable isn't
the hottest path and the current inline implementation ends up calling
out to an external function anyway, this is OK from a performance POV.

The main motivation for de-duplicating is the upcoming slot
abstraction work, after which there's not guaranteed to be a HeapTuple
backing the slot.

Author: Andres Freund, Amit Khandekar
Discussion: https://postgr.es/m/20181105210039.hh4vvi4vwoq5ba2q@alap3.anarazel.de

src/backend/executor/execExprInterp.c

@@ -490,55 +490,19 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 
 		EEO_CASE(EEOP_INNER_SYSVAR)
 		{
-			int			attnum = op->d.var.attnum;
-			Datum		d;
-
-			/* these asserts must match defenses in slot_getattr */
-			Assert(innerslot->tts_tuple != NULL);
-			Assert(innerslot->tts_tuple != &(innerslot->tts_minhdr));
-
-			/* heap_getsysattr has sufficient defenses against bad attnums */
-			d = heap_getsysattr(innerslot->tts_tuple, attnum,
-								innerslot->tts_tupleDescriptor,
-								op->resnull);
-			*op->resvalue = d;
-
+			ExecEvalSysVar(state, op, econtext, innerslot);
 			EEO_NEXT();
 		}
 
 		EEO_CASE(EEOP_OUTER_SYSVAR)
 		{
-			int			attnum = op->d.var.attnum;
-			Datum		d;
-
-			/* these asserts must match defenses in slot_getattr */
-			Assert(outerslot->tts_tuple != NULL);
-			Assert(outerslot->tts_tuple != &(outerslot->tts_minhdr));
-
-			/* heap_getsysattr has sufficient defenses against bad attnums */
-			d = heap_getsysattr(outerslot->tts_tuple, attnum,
-								outerslot->tts_tupleDescriptor,
-								op->resnull);
-			*op->resvalue = d;
-
+			ExecEvalSysVar(state, op, econtext, outerslot);
 			EEO_NEXT();
 		}
 
 		EEO_CASE(EEOP_SCAN_SYSVAR)
 		{
-			int			attnum = op->d.var.attnum;
-			Datum		d;
-
-			/* these asserts must match defenses in slot_getattr */
-			Assert(scanslot->tts_tuple != NULL);
-			Assert(scanslot->tts_tuple != &(scanslot->tts_minhdr));
-
-			/* heap_getsysattr has sufficient defenses against bad attnums */
-			d = heap_getsysattr(scanslot->tts_tuple, attnum,
-								scanslot->tts_tupleDescriptor,
-								op->resnull);
-			*op->resvalue = d;
-
+			ExecEvalSysVar(state, op, econtext, scanslot);
 			EEO_NEXT();
 		}
 
@@ -4006,6 +3970,22 @@ ExecEvalWholeRowVar(ExprState *state, ExprEvalStep *op, ExprContext *econtext)
 	*op->resnull = false;
 }
 
+void
+ExecEvalSysVar(ExprState *state, ExprEvalStep *op, ExprContext *econtext,
+			   TupleTableSlot *slot)
+{
+	bool success;
+
+	/* slot_getsysattr has sufficient defenses against bad attnums */
+	success = slot_getsysattr(slot,
+							  op->d.var.attnum,
+							  op->resvalue,
+							  op->resnull);
+	/* this ought to be unreachable, but it's cheap enough to check */
+	if (unlikely(!success))
+		elog(ERROR, "failed to fetch attribute from slot");
+}
+
 /*
  * Transition value has not been initialized. This is the first non-NULL input
  * value for a group. We use it as the initial value for transValue.

src/backend/jit/llvm/llvmjit.c

@@ -81,9 +81,9 @@ LLVMValueRef FuncStrlen;
 LLVMValueRef FuncVarsizeAny;
 LLVMValueRef FuncSlotGetsomeattrs;
 LLVMValueRef FuncSlotGetmissingattrs;
-LLVMValueRef FuncHeapGetsysattr;
 LLVMValueRef FuncMakeExpandedObjectReadOnlyInternal;
 LLVMValueRef FuncExecEvalArrayRefSubscript;
+LLVMValueRef FuncExecEvalSysVar;
 LLVMValueRef FuncExecAggTransReparent;
 LLVMValueRef FuncExecAggInitGroup;
 
@@ -822,9 +822,9 @@ llvm_create_types(void)
 	FuncVarsizeAny = LLVMGetNamedFunction(mod, "varsize_any");
 	FuncSlotGetsomeattrs = LLVMGetNamedFunction(mod, "slot_getsomeattrs");
 	FuncSlotGetmissingattrs = LLVMGetNamedFunction(mod, "slot_getmissingattrs");
-	FuncHeapGetsysattr = LLVMGetNamedFunction(mod, "heap_getsysattr");
 	FuncMakeExpandedObjectReadOnlyInternal = LLVMGetNamedFunction(mod, "MakeExpandedObjectReadOnlyInternal");
 	FuncExecEvalArrayRefSubscript = LLVMGetNamedFunction(mod, "ExecEvalArrayRefSubscript");
+	FuncExecEvalSysVar = LLVMGetNamedFunction(mod, "ExecEvalSysVar");
 	FuncExecAggTransReparent = LLVMGetNamedFunction(mod, "ExecAggTransReparent");
 	FuncExecAggInitGroup = LLVMGetNamedFunction(mod, "ExecAggInitGroup");
 

src/backend/jit/llvm/llvmjit_expr.c

@@ -406,13 +406,8 @@ llvm_compile_expr(ExprState *state)
 			case EEOP_OUTER_SYSVAR:
 			case EEOP_SCAN_SYSVAR:
 				{
-					int			attnum = op->d.var.attnum;
-					LLVMValueRef v_attnum;
-					LLVMValueRef v_tuple;
-					LLVMValueRef v_tupleDescriptor;
-					LLVMValueRef v_params[4];
-					LLVMValueRef v_syscol;
 					LLVMValueRef v_slot;
+					LLVMValueRef v_params[4];
 
 					if (opcode == EEOP_INNER_SYSVAR)
 						v_slot = v_innerslot;
@@ -421,31 +416,14 @@ llvm_compile_expr(ExprState *state)
 					else
 						v_slot = v_scanslot;
 
-					Assert(op->d.var.attnum < 0);
-
-					v_tuple = l_load_struct_gep(b, v_slot,
-												FIELDNO_TUPLETABLESLOT_TUPLE,
-												"v.tuple");
+					v_params[0] = v_state;
+					v_params[1] = l_ptr_const(op, l_ptr(StructExprEvalStep));
+					v_params[2] = v_econtext;
+					v_params[3] = v_slot;
 
-					/*
-					 * Could optimize this a bit for fixed descriptors, but
-					 * this shouldn't be that critical a path.
-					 */
-					v_tupleDescriptor =
-						l_load_struct_gep(b, v_slot,
-										  FIELDNO_TUPLETABLESLOT_TUPLEDESCRIPTOR,
-										  "v.tupledesc");
-					v_attnum = l_int32_const(attnum);
-
-					v_params[0] = v_tuple;
-					v_params[1] = v_attnum;
-					v_params[2] = v_tupleDescriptor;
-					v_params[3] = v_resnullp;
-					v_syscol = LLVMBuildCall(b,
-											 llvm_get_decl(mod, FuncHeapGetsysattr),
-											 v_params, lengthof(v_params),
-											 "");
-					LLVMBuildStore(b, v_syscol, v_resvaluep);
+					LLVMBuildCall(b,
+								  FuncExecEvalSysVar,
+								  v_params, lengthof(v_params), "");
 
 					LLVMBuildBr(b, opblocks[i + 1]);
 					break;

src/backend/jit/llvm/llvmjit_types.c

@@ -99,9 +99,9 @@ void	   *referenced_functions[] =
 	varsize_any,
 	slot_getsomeattrs,
 	slot_getmissingattrs,
-	heap_getsysattr,
 	MakeExpandedObjectReadOnlyInternal,
 	ExecEvalArrayRefSubscript,
+	ExecEvalSysVar,
 	ExecAggTransReparent,
 	ExecAggInitGroup
 };

src/include/executor/execExpr.h

@@ -734,6 +734,8 @@ extern void ExecEvalAlternativeSubPlan(ExprState *state, ExprEvalStep *op,
 						   ExprContext *econtext);
 extern void ExecEvalWholeRowVar(ExprState *state, ExprEvalStep *op,
 					ExprContext *econtext);
+extern void ExecEvalSysVar(ExprState *state, ExprEvalStep *op,
+			   ExprContext *econtext, TupleTableSlot *slot);
 
 extern void ExecAggInitGroup(AggState *aggstate, AggStatePerTrans pertrans, AggStatePerGroup pergroup);
 extern Datum ExecAggTransReparent(AggState *aggstate, AggStatePerTrans pertrans,

src/include/jit/llvmjit.h

@@ -79,9 +79,9 @@ extern LLVMValueRef FuncStrlen;
 extern LLVMValueRef FuncVarsizeAny;
 extern LLVMValueRef FuncSlotGetsomeattrs;
 extern LLVMValueRef FuncSlotGetmissingattrs;
-extern LLVMValueRef FuncHeapGetsysattr;
 extern LLVMValueRef FuncMakeExpandedObjectReadOnlyInternal;
 extern LLVMValueRef FuncExecEvalArrayRefSubscript;
+extern LLVMValueRef FuncExecEvalSysVar;
 extern LLVMValueRef FuncExecAggTransReparent;
 extern LLVMValueRef FuncExecAggInitGroup;