Don't generate tuple deforming functions for virtual slots.

Virtual tuple table slots never need tuple deforming. Therefore, if we
know at expression compilation time, that a certain slot will always
be virtual, there's no need to create a tuple deforming routine for
it.

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

src/backend/jit/llvm/llvmjit_deform.c

@@ -31,7 +31,8 @@
  * Create a function that deforms a tuple of type desc up to natts columns.
  */
 LLVMValueRef
-slot_compile_deform(LLVMJitContext *context, TupleDesc desc, int natts)
+slot_compile_deform(LLVMJitContext *context, TupleDesc desc,
+					const TupleTableSlotOps *ops, int natts)
 {
 	char	   *funcname;
 
@@ -88,6 +89,10 @@ slot_compile_deform(LLVMJitContext *context, TupleDesc desc, int natts)
 
 	int			attnum;
 
+	/* virtual tuples never need deforming, so don't generate code */
+	if (ops == &TTSOpsVirtual)
+		return NULL;
+
 	mod = llvm_mutable_module(context);
 
 	funcname = llvm_expand_funcname(context, "deform");

src/backend/jit/llvm/llvmjit_expr.c

@@ -324,6 +324,7 @@ llvm_compile_expr(ExprState *state)
 					{
 						l_jit_deform =
 							slot_compile_deform(context, desc,
+												tts_ops,
 												op->d.fetch.last_var);
 					}
 

src/include/jit/llvmjit.h

@@ -111,7 +111,9 @@ extern void llvm_inline(LLVMModuleRef mod);
  ****************************************************************************
  */
 extern bool llvm_compile_expr(struct ExprState *state);
-extern LLVMValueRef slot_compile_deform(struct LLVMJitContext *context, TupleDesc desc, int natts);
+struct TupleTableSlotOps;
+extern LLVMValueRef slot_compile_deform(struct LLVMJitContext *context, TupleDesc desc,
+										const struct TupleTableSlotOps *ops, int natts);
 
 /*
  ****************************************************************************