I installed the attached patches into grep master. These fix the performance 
regressions noted at the start of Bug#22357. I see that the related performance 
problems noted in Bug#21763 seem to be fixed too, I expect because of Norihiro 
Tanaka's recent changes, so I'll boldly close both bug reports.

To some extent the attached patches restore the old behavior for grep -F, when 
grep is given two or more patterns. The patch doesn't change the underlying 
algorithms; it merely uses a different heuristic to decide whether to use the -F 
matcher. Although I wouldn't be surprised if the attached patches hurt 
performance in some cases, I didn't uncover any such cases in my performance 
testing, which I admit mostly consisted of running the examples in the 
abovementioned bug reports.

I'll leave Bug#22239 open, as I get the following performance figures 
(user+system CPU time) for the Bug#22239 benchmark, where list.txt is created by 
"aspell dump master | head -n 100000 >list.txt", and the grep commands all use 
the operands "-F -f list.txt /etc/passwd" in the en_US.utf8 locale on Fedora 24 
x86-64.

   no -i       -i       grep version
    0.25      0.33      2.16
    0.26     10.95      2.21
    0.11      2.90*     current master (including attached patches)

In the C locale, the current grep master is always significantly faster than 
grep 2.16 or 2.21 on the benchmark, so the only significant problem is the 
number marked "*". I ran the benchmarks on an AMD Phenom II X4 910e.