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[libc] Improve the implementation of the rand() function (#66131)

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Summary:
This patch improves the implementation of the standard `rand()` function
by implementing it in terms of the xorshift64star pRNG as described in
https://en.wikipedia.org/wiki/Xorshift#xorshift*. This is a good,
general purpose random number generator that is sufficient for most
applications that do not require an extremely long period. This patch
also correctly initializes the seed to be `1` as described by the
standard. We also increase the `RAND_MAX` value to be `INT_MAX` as the
standard only specifies that it can be larger than 32768.
This commit is contained in:
Joseph Huber 2023-09-12 16:52:20 -05:00 committed by GitHub
parent d671126ad0
commit ef169f5707
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GPG Key ID: 4AEE18F83AFDEB23
4 changed files with 21 additions and 7 deletions
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2
libc/include/llvm-libc-macros/stdlib-macros.h
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@ -17,6 +17,6 @@
#define EXIT_SUCCESS 0
#define EXIT_FAILURE 1
#define RAND_MAX 32767
#define RAND_MAX 2147483647
#endif // __LLVM_LIBC_MACROS_STDLIB_MACROS_H

12
libc/src/stdlib/rand.cpp
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@ -12,11 +12,13 @@
namespace __llvm_libc {
// This rand function is the example implementation from the C standard. It is
// not cryptographically secure.
LLVM_LIBC_FUNCTION(int, rand, (void)) { // RAND_MAX is assumed to be 32767
rand_next = rand_next * 1103515245 + 12345;
return static_cast<unsigned int>((rand_next / 65536) % 32768);
// An implementation of the xorshift64star pseudo random number generator. This
// is a good general purpose generator for most non-cryptographics applications.
LLVM_LIBC_FUNCTION(int, rand, (void)) {
rand_next ^= rand_next >> 12;
rand_next ^= rand_next << 25;
rand_next ^= rand_next >> 27;
return static_cast<int>((rand_next * 0x2545F4914F6CDD1Dul) >> 32) & RAND_MAX;
}
} // namespace __llvm_libc

4
libc/src/stdlib/rand_util.cpp
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@ -11,6 +11,8 @@
namespace __llvm_libc {
LIBC_THREAD_LOCAL unsigned long rand_next;
// C standard 7.10p2: If 'rand' is called before 'srand' it is to proceed as if
// the 'srand' function was called with a value of '1'.
LIBC_THREAD_LOCAL unsigned long rand_next = 1;
} // namespace __llvm_libc

10
libc/test/src/stdlib/rand_test.cpp
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@ -14,11 +14,21 @@
#include <stdlib.h>
TEST(LlvmLibcRandTest, UnsetSeed) {
static int vals[1000];
for (size_t i = 0; i < 1000; ++i) {
int val = __llvm_libc::rand();
ASSERT_GE(val, 0);
ASSERT_LE(val, RAND_MAX);
vals[i] = val;
}
// The C standard specifies that if 'srand' is never called it should behave
// as if 'srand' was called with a value of 1. If we seed the value with 1 we
// should get the same sequence as the unseeded version.
__llvm_libc::srand(1);
for (size_t i = 0; i < 1000; ++i)
ASSERT_EQ(__llvm_libc::rand(), vals[i]);
}
TEST(LlvmLibcRandTest, SetSeed) {