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authorDaniel R. Carvalho <odanrc@yahoo.com.br>2018-09-19 10:19:06 +0200
committerDaniel Carvalho <odanrc@yahoo.com.br>2018-09-19 09:13:25 +0000
commitb53dd6d12efafb574989226911a7b0bc921df24d (patch)
tree766c57af77b7297691a60eed5cc5b45255c6a594
parentb3ef93166f76b3f76fe5254322d3fcb0f5e4b559 (diff)
downloadgem5-b53dd6d12efafb574989226911a7b0bc921df24d.tar.xz
mem-cache: Fix non-bijective function in Skewed caches
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>
-rw-r--r--src/mem/cache/tags/skewed_assoc.cc5
-rw-r--r--src/mem/cache/tags/skewed_assoc.hh3
2 files changed, 8 insertions, 0 deletions
diff --git a/src/mem/cache/tags/skewed_assoc.cc b/src/mem/cache/tags/skewed_assoc.cc
index 044f8a3d7..b3945d0c4 100644
--- a/src/mem/cache/tags/skewed_assoc.cc
+++ b/src/mem/cache/tags/skewed_assoc.cc
@@ -37,6 +37,7 @@
#include <vector>
+#include "base/bitfield.hh"
#include "base/logging.hh"
SkewedAssoc::SkewedAssoc(const Params *p)
@@ -51,6 +52,10 @@ 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
diff --git a/src/mem/cache/tags/skewed_assoc.hh b/src/mem/cache/tags/skewed_assoc.hh
index 7f5a75266..9fc39e297 100644
--- a/src/mem/cache/tags/skewed_assoc.hh
+++ b/src/mem/cache/tags/skewed_assoc.hh
@@ -73,6 +73,9 @@ class SkewedAssoc : public BaseSetAssoc
* 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.
*/