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Added a DML_site_rotl32() function to eliminate Undefined Behaviour #11

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Added a DML_site_rotl32() function to eliminate Undefined Behaviour #11

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32 changes: 30 additions & 2 deletions lib/dml/DML_utils.c
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,8 @@
#include <sys/time.h>
//#include <time.h>
#include <qmp.h>
#include <stdint.h>
#include <limits.h>

#undef DML_DEBUG

Expand Down Expand Up @@ -993,6 +995,28 @@ void DML_destroy_subset_rank(DML_SiteList *sites){

/*------------------------------------------------------------------*/
/* Checksum "class" */

/* A safe and nearly constant time bitwise rotate which
* avoids Undefined Behaviour.
*
* This implementation follows this Wikipedia entry:
* https://en.wikipedia.org/wiki/Circular_shift#Implementing_circular_shifts
*
* and is based on the work of:
* John Regehr https://blog.regehr.org/archives/1063
* and
* Peter Cordes in his stack overflow answers:
*
* https://stackoverflow.com/questions/776508/best-practices-for-circular-shift-rotate-operations-in-c/776523#776523
* https://stackoverflow.com/questions/31387778/near-constant-time-rotate-that-does-not-violate-the-standards/31488147#31488147
*/
uint32_t DML_rank_rotl32(uint32_t work, DML_SiteRank rank)
{
const unsigned int mask = CHAR_BIT * sizeof(work) - 1;
rank &= mask;
return (work << rank) | (work >> (-rank & mask));
}

/* We do a crc32 sum on the site data -- then do two lexicographic-
rank-based bit rotations and XORs on the resulting crc32
checksum */
Expand All @@ -1013,8 +1037,12 @@ void DML_checksum_accum(DML_Checksum *checksum, DML_SiteRank rank,

rank29 %= 29; rank31 %= 31;

checksum->suma ^= work<<rank29 | work>>(32-rank29);
checksum->sumb ^= work<<rank31 | work>>(32-rank31);
/*
checksum->suma ^= work<<rank29 | work>>(32-rank29);
checksum->sumb ^= work<<rank31 | work>>(32-rank31);
*/
checksum->suma ^= DML_rank_rotl32(work, rank29);
checksum->sumb ^= DML_rank_rotl32(work, rank31);
}

/* Combine checksums over all nodes */
Expand Down