Use new 16-bit unsigned multiply routine that complies with C standards.

This changes unsigned 16-bit multiplies to use the new ~CUMul2 routine in ORCALib, rather than ~UMul2 in SysLib. They differ in that ~CUMul2 gives the low-order 16 bits of the true result in case of overflow. The C standards require this behavior for arithmetic on unsigned types.

Gen.pas

@@ -6457,12 +6457,13 @@ procedure GenTree {op: icptr};
       end {if}
    else
       LoadX(op^.right);
-   if op^.opcode = pc_mpi then
-      GenCall(28)
+   if op^.opcode = pc_mpi then begin
+      GenCall(28);      
+      if rangeCheck then
+         GenCall(25);
+      end {if}
    else {pc_umi}
-      GenCall(39);
-   if rangeCheck then
-      GenCall(25);
+      GenCall(94);
    end; {GenMpi}
 
 

Native.pas

@@ -2049,6 +2049,7 @@ case callNum of
    91: sp := @'~SINGLEPRECISION';
    92: sp := @'~DOUBLEPRECISION';
    93: sp := @'~COMPPRECISION';
+   94: sp := @'~CUMUL2';
    otherwise:
       Error(cge1);
    end; {case}

cc.notes

@@ -262,7 +262,7 @@ ORCA/C now supports additional forms of expressions known as generic selection e
 
 p. 311
 
-If integer overflow occurs during signed integer multiplication, the resulting value is not predictable.  Contrary to what the description in the manual implies, it will not necessarily be the low-order bits from the true product of the operands.
+If integer overflow occurs during signed integer multiplication, the resulting value is not predictable.  Contrary to what the description in the manual implies, it will not necessarily be the low-order bits from the true product of the operands.  (Unsigned integer multiplication does give the low-order bits from the true product.)
 
 p. 321
 
@@ -1861,6 +1861,8 @@ int foo(int[42]);
 
 199. If a file is open for reading and writing, it should be possible to read from it until end-of-file is encountered and then immediately write to it, but an error would be reported when the write was attempted.
 
+200. Arithmetic operations on unsigned integer types should give the low-order bits of the true mathematical result in the event of overflow, but 16-bit unsigned multiplication did not always do this.
+
 -- Bugs from C 2.1.0 that have been fixed -----------------------------------
 
 1.  In some situations, fread() reread the first 1K or so of the file.