Confirmed users
753
edits
User:Bjacob/ArithmeticTimingDifferences: Difference between revisions
Jump to navigation
Jump to search
User:Bjacob/ArithmeticTimingDifferences (view source)
Revision as of 20:48, 20 June 2013
, 20 June 2013→Some CPUs have surprising timing differences on 64-bit integer arithmetic using 32-bit integer arithmetic instructions, when DAZ is enabled
| Line 171: | Line 171: | ||
movl %edx, 20(%ebx) | movl %edx, 20(%ebx) | ||
On [http://people.mozilla.org/~bjacob/ta/results/intel-pentium-4-3.2GHz-family-15-model-3-stepping-3-tycho/ one Pentium 4 CPU] that we used, this does not quite run in constant time, with timing differences of up to 25% and | On [http://people.mozilla.org/~bjacob/ta/results/intel-pentium-4-3.2GHz-family-15-model-3-stepping-3-tycho/ one Pentium 4 CPU] that we used, this does not quite run in constant time, with timing differences of up to 25% and about 3 sigma from the overall median; and the weirdest part is that this only happens when DAZ is enabled! This is of course particularly surprising as DAZ is not supposed to have any relevance to integer arithmetic. | ||
Here are the results with DAZ, showing slower-than-normal operation when the right hand side of the subtraction is INT64_MIN, with the slowest case being INT64_MIN - INT64_MIN: | Here are the results with DAZ, showing slower-than-normal operation when the right hand side of the subtraction is INT64_MIN, with the slowest case being INT64_MIN - INT64_MIN: | ||