[libc] Partially implement 'rand' for the GPU (#66167)

Summary:
This patch partially implements the `rand` function on the GPU. This is
partial because the GPU currently doesn't support thread local storage
or static initializers. To implement this on the GPU. I use 1/8th of the
local / shared memory quota to treak the shared memory as thread local
storage. This is done by simply allocating enough storage for each
thread in the block and indexing into this based off of the thread id.
The downside to this is that it does not initialize `srand` correctly to
be `1` as the standard says, it is also wasteful. In the future we
should figure out a way to support TLS on the GPU so that this can be
completely common and less resource intensive.

1 files changed, 6 insertions, 4 deletions

diff --git a/libc/src/stdlib/rand.cpp b/libc/src/stdlib/rand.cpp
index c5737a8b8454..ad543b4048a9 100644
--- a/libc/src/stdlib/rand.cpp
+++ b/libc/src/stdlib/rand.cpp
@@ -15,10 +15,12 @@ namespace LIBC_NAMESPACE {
 // 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;
+  unsigned long x = rand_next;
+  x ^= x >> 12;
+  x ^= x << 25;
+  x ^= x >> 27;
+  rand_next = x;
+  return static_cast<int>((x * 0x2545F4914F6CDD1Dul) >> 32) & RAND_MAX;
 }
 
 } // namespace LIBC_NAMESPACE