Avoid unpredictable branch. Use conditional move to generate the address
that is guaranteed to be safe and compare unconditionally.
Instead of

if (idx < limit && x[idx] == val ) // mispredicted idx < limit branch

Do

addr = cmov(safe,x+idx)
if (*addr == val && idx < limit) // almost always false so well predicted

Using microbenchmarks from https://github.com/google/fleetbench,
I get about ~10% speed-up:

name                                                                                          old cpu/op   new cpu/op    delta
BM_ZSTD_COMPRESS_Fleet/compression_level:-7/window_log:15                                     1.46ns ± 3%   1.31ns ± 7%   -9.88%  (p=0.000 n=35+38)
BM_ZSTD_COMPRESS_Fleet/compression_level:-7/window_log:16                                     1.41ns ± 3%   1.28ns ± 3%   -9.56%  (p=0.000 n=36+39)
BM_ZSTD_COMPRESS_Fleet/compression_level:-5/window_log:15                                     1.61ns ± 1%   1.43ns ± 3%  -10.70%  (p=0.000 n=30+39)
BM_ZSTD_COMPRESS_Fleet/compression_level:-5/window_log:16                                     1.54ns ± 2%   1.39ns ± 3%   -9.21%  (p=0.000 n=37+39)
BM_ZSTD_COMPRESS_Fleet/compression_level:-3/window_log:15                                     1.82ns ± 2%   1.61ns ± 3%  -11.31%  (p=0.000 n=37+40)
BM_ZSTD_COMPRESS_Fleet/compression_level:-3/window_log:16                                     1.73ns ± 3%   1.56ns ± 3%   -9.50%  (p=0.000 n=38+39)
BM_ZSTD_COMPRESS_Fleet/compression_level:-1/window_log:15                                     2.12ns ± 2%   1.79ns ± 3%  -15.55%  (p=0.000 n=34+39)
BM_ZSTD_COMPRESS_Fleet/compression_level:-1/window_log:16                                     1.99ns ± 3%   1.72ns ± 3%  -13.70%  (p=0.000 n=38+38)
BM_ZSTD_COMPRESS_Fleet/compression_level:0/window_log:15                                      3.22ns ± 3%   2.94ns ± 3%   -8.67%  (p=0.000 n=38+40)
BM_ZSTD_COMPRESS_Fleet/compression_level:0/window_log:16                                      3.19ns ± 4%   2.86ns ± 4%  -10.55%  (p=0.000 n=40+38)
BM_ZSTD_COMPRESS_Fleet/compression_level:1/window_log:15                                      2.60ns ± 3%   2.22ns ± 3%  -14.53%  (p=0.000 n=40+39)
BM_ZSTD_COMPRESS_Fleet/compression_level:1/window_log:16                                      2.46ns ± 3%   2.13ns ± 2%  -13.67%  (p=0.000 n=39+36)
BM_ZSTD_COMPRESS_Fleet/compression_level:2/window_log:15                                      2.69ns ± 3%   2.46ns ± 3%   -8.63%  (p=0.000 n=37+39)
BM_ZSTD_COMPRESS_Fleet/compression_level:2/window_log:16                                      2.63ns ± 3%   2.36ns ± 3%  -10.47%  (p=0.000 n=40+40)
BM_ZSTD_COMPRESS_Fleet/compression_level:3/window_log:15                                      3.20ns ± 2%   2.95ns ± 3%   -7.94%  (p=0.000 n=35+40)
BM_ZSTD_COMPRESS_Fleet/compression_level:3/window_log:16                                      3.20ns ± 4%   2.87ns ± 4%  -10.33%  (p=0.000 n=40+40)

I've also measured the impact on internal workloads and saw similar
~10% improvement in performance, measured by cpu usage/byte of data.

make hot variables more local

for easier swapping with a parameter

between cmov and branch
and use a simple heuristic based on wlog to select between them.

note: performance is not good on clang (yet)

suggested by @terrelln

feels more logical, better contained

noticed by @terrelln

findMatch -> matchFound
since it's a test, as opposed to an active search operation.
suggested by @terrelln