optimization to epicsTime

["Hill, Jeff" <johill@lanl.gov>]

Dear All,

I inadvertently committed the attached change to the main trunk on a checkout instead of a branch and therefore bypassed the normal review process. If there is (any) opposition I will back out the change and make a branch. I did run the epicsTime regression test, which didn't detect any issues, before making this commit.

Jeff

Commit message:

After optimizing the epicsTime::addNanoSec function we now have a larger code which uses only unsigned integer arithmetic. This is for the benefit of embedded cpu's lacking a hardware floating point coprocessor at the expense of some additional code to maintain. I measured the cpu load improving from 40% free to 80% free after making this change, on a nios2 soft core processor.

Change:

== modified file src/libCom/osi/epicsTime.cpp
--- src/libCom/osi/epicsTime.cpp	2012-05-03 17:19:34 +0000
+++ src/libCom/osi/epicsTime.cpp	2012-11-14 19:51:30 +0000
@@ -113,8 +113,48 @@
 //
 inline void epicsTime::addNanoSec (long nSecAdj)
 {
-    double secAdj = static_cast <double> (nSecAdj) / nSecPerSec;
-    *this += secAdj;
+    // After optimizing this function we now have a larger
+    // code which uses only unsigned integer arithmetic.
+    // This is for the benefit of embedded cpu's lacking
+    // a hardware floating point coprocessor at the
+    // expense of some additional code to maintain.
+    // joh 14-11-2012
+    if ( nSecAdj >= 0 ) {
+        unsigned long nSecOffsetLong =
+              static_cast < unsigned long > ( nSecAdj );
+        while ( nSecOffsetLong >= nSecPerSec ) {
+            this->secPastEpoch++; // overflow expected
+            nSecOffsetLong -= nSecPerSec;
+        }
+        const epicsUInt32 nSecOffset =
+            static_cast < epicsUInt32 > ( nSecOffsetLong );
+        epicsUInt32 nSecPerSecRemaining = nSecPerSec - nSecOffset;
+        if ( this->nSec >= nSecPerSecRemaining ) {
+            this->secPastEpoch++; // overflow expected
+            this->nSec -= nSecPerSecRemaining;
+        }
+        else {
+            this->nSec += nSecOffset;
+        }
+    }
+    else {
+        unsigned long nSecOffsetLong =
+            static_cast <unsigned long> ( -nSecAdj );
+        while ( nSecOffsetLong >= nSecPerSec ) {
+            this->secPastEpoch--; // underflow expected
+            nSecOffsetLong -= nSecPerSec;
+        }
+        const epicsUInt32 nSecOffset =
+            static_cast < epicsUInt32 > ( nSecOffsetLong );
+        if ( this->nSec >= nSecOffset ) {
+            this->nSec -= nSecOffset;
+        }
+        else {
+            // borrow
+            this->secPastEpoch--; // underflow expected
+            this->nSec += nSecPerSec - nSecOffset;
+        }
+    }
 }
 
 //


Jeff
______________________________________________________
Jeffrey O. Hill           Email        johill@lanl.gov
LANL MS H820              Voice        505 665 1831
Los Alamos NM 87545 USA   FAX          505 665 5107

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