[Eltwise] use struct member fxn to workaround HLS pipeline style bug

src/finn/custom_op/fpgadataflow/eltwise.py

@@ -61,10 +61,19 @@ class StreamingEltwise(HLSCustomOp):
 
     def get_eltwise_op_lambda(self):
         eltwise_op = self.get_nodeattr("eltwiseOp")
+        idt0 = self.get_input_datatype(0)
+        idt1 = self.get_input_datatype(1)
+        odt = self.get_output_datatype()
+        tin0 = idt0.get_hls_datatype_str()
+        tin1 = idt1.get_hls_datatype_str()
+        tout = odt.get_hls_datatype_str()
         eltwise_ops = {
-            "Add": "[](auto a, auto b) { return  a + b; }",
-            "Sub": "[](auto a, auto b) { return  a - b; }",
-            "AbsDiff": "[](auto a, auto b) { return  a>b? a-b : b-a; }",
+            # "Add": "[](auto a, auto b) { return  a + b; }",
+            # "Sub": "[](auto a, auto b) { return  a - b; }",
+            # "AbsDiff": "[](auto a, auto b) { return  a>b? a-b : b-a; }",
+            "Add": f"add<{tin0}, {tin1}, {tout}>()",
+            "Sub": f"sub<{tin0}, {tin1}, {tout}>()",
+            "AbsDiff": f"absdiff<{tin0}, {tin1}, {tout}>()",
         }
         return eltwise_ops[eltwise_op]
 
@@ -296,6 +305,32 @@ class StreamingEltwise(HLSCustomOp):
             '#include "interpret.hpp"',
         ]
 
+        self.code_gen_dict["$GLOBALS$"].extend(
+            [
+                "template<typename TI1, typename TI2, typename TO>",
+                "struct absdiff {",
+                "TO operator()(TI1 const &a, TI2 const &b) const {",
+                "#pragma HLS inline",
+                "return  a>b? a-b : b-a;",
+                "}",
+                "};",
+                "template<typename TI1, typename TI2, typename TO>",
+                "struct sub {",
+                "TO operator()(TI1 const &a, TI2 const &b) const {",
+                "#pragma HLS inline",
+                "return  a-b;",
+                "}",
+                "};",
+                "template<typename TI1, typename TI2, typename TO>",
+                "struct add {",
+                "TO operator()(TI1 const &a, TI2 const &b) const {",
+                "#pragma HLS inline",
+                "return  a+b;",
+                "}",
+                "};",
+            ]
+        )
+
     def defines(self, var):
         self.code_gen_dict["$DEFINES$"] = []