The hash() function must be bijective for the skewed caches to work,
however when the hashing is done on top of a one-bit address, the
MSB and LSB refer to the same bit, and therefore their xor will
always be zero.
This patch adds a fatal error to not allow the user to set an invalid
value for the number of sets that would generate that bug.
As a side note, the missing header for the bitfields functions has
been added.
Change-Id: I35a03ac5fdc4debb091f7f2db5db33568d0b0021
Reviewed-on: https://gem5-review.googlesource.com/12724
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
#include <vector>
+#include "base/bitfield.hh"
#include "base/logging.hh"
SkewedAssoc::SkewedAssoc(const Params *p)
// skewing functions accordingly to make good use of the hashing function
panic_if(setShift + 2 * (msbShift + 1) > 64, "Unsuported number of bits " \
"for the skewing functions.");
+
+ // We must have more than two sets, otherwise the MSB and LSB are the same
+ // bit, and the xor of them will always be 0
+ fatal_if(numSets <= 2, "The number of sets must be greater than 2");
}
Addr
* applies an XOR to the MSB and LSB, shifts all bits one bit to the right,
* and set the result of the XOR as the new MSB.
*
+ * This function is not bijective if the address has only 1 bit, as the MSB
+ * and LSB will be the same, and therefore the xor will always be 0.
+ *
* @param addr The address to be hashed.
* @param The hashed address.
*/