[WIP] Adds optimized emit for int64 constants (#3620)

* Adds optimized emit for int64 constants.

* Adds comment linking to the changing PR.

Thanks for this PR.

Kevin

src/fsharp/IlxGen.fs

@@ -2072,7 +2072,14 @@ and GenConstant cenv cgbuf eenv (c,m,ty) sequel =
           | Const.SByte i -> CG.EmitInstr cgbuf (pop 0) (Push [ilTy]) (mkLdcInt32 (int32 i))
           | Const.Int16 i -> CG.EmitInstr cgbuf (pop 0) (Push [ilTy]) (mkLdcInt32 (int32 i))
           | Const.Int32 i -> CG.EmitInstr cgbuf (pop 0) (Push [ilTy]) (mkLdcInt32 i)
-          | Const.Int64 i -> CG.EmitInstr cgbuf (pop 0) (Push [ilTy]) (iLdcInt64 i)
+          | Const.Int64 i -> 
+            // see https://github.com/Microsoft/visualfsharp/pull/3620 
+            if i >= int64 System.Int32.MinValue && i <= int64 System.Int32.MaxValue then
+                CG.EmitInstrs cgbuf (pop 0) (Push [ilTy]) [ mkLdcInt32 (int32 i); AI_conv DT_I8 ]
+            elif i >= int64 System.UInt32.MinValue && i <= int64 System.UInt32.MaxValue then
+                CG.EmitInstrs cgbuf (pop 0) (Push [ilTy]) [ mkLdcInt32 (int32 i); AI_conv DT_U8 ]
+            else
+                CG.EmitInstr cgbuf (pop 0) (Push [ilTy]) (iLdcInt64 i)
           | Const.IntPtr i -> CG.EmitInstrs cgbuf (pop 0) (Push [ilTy]) [iLdcInt64 i; AI_conv DT_I ]
           | Const.Byte i -> CG.EmitInstr cgbuf (pop 0) (Push [ilTy]) (mkLdcInt32 (int32 i))
           | Const.UInt16 i -> CG.EmitInstr cgbuf (pop 0) (Push [ilTy]) (mkLdcInt32 (int32 i))