add hal_extend_int function

helper function to deal with wrap around and extension of lower-width counters to 64bit ints.

Author: Robert Schöftner

src/hal/hal.h

@@ -692,6 +692,18 @@ extern int hal_get_signal_value_by_name(
 extern int hal_get_param_value_by_name(
     const char *name, hal_type_t *type, hal_data_u **data);
 
+/***********************************************************************
+*                    MISC HELPER FUNCTIONS                             *
+************************************************************************/
+
+/** hal_extend_int() extends a counter value with nbits to 64 bits.
+    Increments between updates need to be less than 2**(nbits-1).
+    Call with current 64bit counter value to be updated, new
+    lower-bit counter value and number of bits of counter.
+    @returns new counter value.
+    Code by Jeff Epler. */
+extern rtapi_s64 hal_extend_int(rtapi_s64 old, int newlow, int nbits);
+
 /***********************************************************************
 *                   EXECUTION RELATED FUNCTIONS                        *
 ************************************************************************/

src/hal/hal_lib.c

@@ -1872,6 +1872,33 @@ int hal_get_param_value_by_name(
 }
 
 
+/***********************************************************************
+*                    MISC HELPER FUNCTIONS                             *
+************************************************************************/
+
+rtapi_s64 hal_extend_int(rtapi_s64 old, rtapi_s64 newlow, int nbits)
+{
+    /* see https://github.com/LinuxCNC/linuxcnc/pull/3932#issuecomment-4239206615
+       This code avoids branches and undefined behaviour due to signed overflow
+       Idea from MicroPython.
+
+       The shift left operations get the sign bit in the right place, the diff
+       is calculated and shifted back as a signed quantity. That assures the sign
+       is extended properly. Addition as unsigned avoids signed overflow, which
+       would be undefined behaviour.
+
+       Attention has to be paied that the magnitude of increments / decrements
+       of the counter stay within 1<<(nshift-1) between calls to this function,
+       else the wrap around will be counted in the wrong direction.
+    */
+    int nshift = 64 - nbits;
+    rtapi_u64 oldlow_shifted = ((rtapi_u64)old << nshift);
+    rtapi_u64 newlow_shifted = ((rtapi_u64)newlow << nshift);
+    rtapi_s64 diff_shifted = newlow_shifted - oldlow_shifted;
+    return (rtapi_u64)old + (diff_shifted >> nshift);
+}
+
+
 /***********************************************************************
 *                   EXECUTION RELATED FUNCTIONS                        *
 ************************************************************************/