base: Fix undefined behavior in mask generation

When generating a mask, if the number of bits is greater than
the maximum shift distance (63), the shift will have undefined
behavior. Previously the branch was taking care of a single
trespassing case, and it has been fixed to cover the remaining.

Issue-on: https://gem5.atlassian.net/browse/GEM5-205

Change-Id: Ib5a00917c8d2b23ffdb710c2f9673d956cd9f43e
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27104
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

diff --git a/src/base/bitfield.hh b/src/base/bitfield.hh
index 9a252a406f60a35f9e8dc40a61dc30870b8be2a7..c2ed72be20e8d736d19b6ba0c71512e6247e5349 100644 (file)
--- a/src/base/bitfield.hh
+++ b/src/base/bitfield.hh
@@ -50,12 +50,15 @@
 extern const uint8_t reverseLookUpTable[];
 
 /**
- * Generate a 64-bit mask of 'nbits' 1s, right justified.
+ * Generate a 64-bit mask of 'nbits' 1s, right justified. If a number of bits
+ * greater than 64 is given, it is truncated to 64.
+ *
+ * @param nbits The number of bits set in the mask.
  */
 inline uint64_t
 mask(int nbits)
 {
-    return (nbits == 64) ? (uint64_t)-1LL : (1ULL << nbits) - 1;
+    return (nbits >= 64) ? (uint64_t)-1LL : (1ULL << nbits) - 1;
 }
 
 /**