Zepp-Hanzj opened a new pull request, #19244:
URL: https://github.com/apache/nuttx/pull/19244
## Summary
The existing first-order LCG (seed = 470001 * seed % 999563) has several
quality problems:
- Output range limited to [1, 999562] (~20 bits) instead of full 32-bit
- Lower bits have very short periods (8-bit period = 1, 16-bit = 105)
- Overall period only ~1M, far too short for many applications
- Causes mbedtls_rsa_gen_key to loop forever when rand() consumption aligns
with the cycle length (#16760)
Replace the entire order-based LCG implementation (CONFIG_LIBC_RAND_ORDER
0-3) with Marsaglia's xorshift32:
- Full 32-bit output range
- Period 2^32 - 1 (~4.29 billion)
- Fast: just three XOR/shift operations
- No floating-point math needed
- No CONFIG_LIBC_RAND_ORDER configuration required
Remove the CONFIG_LIBC_RAND_ORDER Kconfig option and clean up all defconfig
references (12 boards) and related comments.
## Verification
Built and tested on sim:nsh. Replaced hello_main.c with a test program that
checks rand() quality, then restored it after testing.
Build command:
make distclean
tools/configure.sh sim:nsh
make -j$(nproc)
Run command:
printf 'hello\npoweroff\n' | timeout 30 ./nuttx 2>/dev/null
Test output (3 consecutive runs, all identical as expected for deterministic
PRNG with same seed):
NuttShell (NSH) NuttX-13.0.0-RC0
nsh> hello
=== rand() quality test ===
Range after 10000 calls: [84907, 2147393180]
Expected: [0, 2147483647]
Full value repeat within 500k: NOT FOUND (period=0)
Expected: NOT FOUND (period should be ~4 billion)
srand(0) first call: 270369 (should be non-zero)
nrand(100) = 89 (should be 0-99)
=== test complete ===
nsh> poweroff
What each check means:
Range [84907, 2147393180] — max value close to INT_MAX (2147483647), old
LCG only reached ~999562
Full value repeat: NOT FOUND — no repeat within 500k calls, old LCG
repeated within ~1M
srand(0) non-zero — zero seed does not degenerate the generator
nrand(100) in [0,99] — range mapping works correctly
Also confirmed via nm that the NuttX xorshift32 is linked into the binary
(not host GLIBC):
$ nm nuttx | grep xorshift32
000000004019d160 b __gcov0.xorshift32
00000000401740a0 d __gcov_.xorshift32
00000000400c4052 t xorshift32
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