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Fix a few issues with $RANDOM seeding in subshells (#339)

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This commit fixes an issue I found in the subshell $RANDOM
reseeding code.

The main issue is a performance regression in the shbench fibonacci
benchmark, introduced in commit af6a32d1. Performance dropped in
this benchmark because $RANDOM is always reseeded and restored,
even when it's never used in a subshell. Performance results from
before and after this performance fix (results are on Linux with
CC=gcc and CCFLAGS='-O2 -D_std_malloc'):

  $ ./shbench -b bench/fibonacci.ksh -l 100 ./ksh-0f06a2e ./ksh-af6a32d ./ksh-f31e368 ./ksh-randfix

  benchmarking ./ksh-0f06a2e, ./ksh-af6a32d, ./ksh-f31e368, ./ksh-randfix ...
  *** fibonacci.ksh ***
  # ./ksh-0f06a2e  # Recent version of ksh93u+m
  # ./ksh-af6a32d  # Commit that introduced the regression
  # ./ksh-f31e368  # Commit without the regression
  # ./ksh-randfix  # Ksh93u+m with this patch applied

  -------------------------------------------------------------------------------------------------
  name           ./ksh-0f06a2e        ./ksh-af6a32d        ./ksh-f31e368        ./ksh-randfix
  -------------------------------------------------------------------------------------------------
  fibonacci.ksh  0.481 [0.459-0.515]  0.472 [0.455-0.504]  0.396 [0.380-0.442]  0.407 [0.385-0.439]
  -------------------------------------------------------------------------------------------------

src/cmd/ksh93/include/variables.h,
src/cmd/ksh93/sh/{init,subshell}.c:
- Rather than reseed $RANDOM every time a subshell is created, add
  a sh_save_rand_seed() function that does this only when the
  $RANDOM variable is used in a subshell. This function is called
  by the $RANDOM discipline functions nget_rand() and put_rand().
  As a minor optimization, sh_save_rand_seed doesn't reseed if it's
  called from put_rand().
- Because $RANDOM may have a seed of zero (i.e., RANDOM=0),
  sp->rand_seed isn't enough to tell if $RANDOM has been reseeded.
  Add sp->rand_state for this purpose.
- sh_subshell(): Only restore the former $RANDOM seed and state if
  it is necessary to prevent a subshell leak.

src/cmd/ksh93/tests/variables.sh:
- Add two regression tests for bugs I ran into while making this
  patch.
This commit is contained in:
Johnothan King 2021-11-18 23:17:25 -08:00 • committed by Martijn Dekker
parent 745ffd366d
commit 396b388e1f
5 changed files with 59 additions and 12 deletions
Download patch file Download diff file Expand all files Collapse all files

5
src/cmd/ksh93/sh/init.c
View file

@ -645,12 +645,13 @@ static Sfdouble_t nget_seconds(register Namval_t* np, Namfun_t *fp)
}
/*
* These three functions are used to get and set the RANDOM variable
* These four functions are used to get and set the RANDOM variable
*/
static void put_rand(register Namval_t* np,const char *val,int flags,Namfun_t *fp)
{
struct rand *rp = (struct rand*)fp;
register long n;
sh_save_rand_seed(rp, 0);
if(!val)
{
fp = nv_stack(np, NIL(Namfun_t*));
@ -677,7 +678,7 @@ static Sfdouble_t nget_rand(register Namval_t* np, Namfun_t *fp)
{
struct rand *rp = (struct rand*)fp;
register long cur, last= *np->nvalue.lp;
NOT_USED(fp);
sh_save_rand_seed(rp, 1);
do
cur = (rand_r(&rp->rand_seed)>>rand_shift)&RANDMASK;
while(cur==last);