Initial import of libdpf.

Co-authored-by: Cursor <cursoragent@cursor.com>

thirdparty/portable-snippets/atomic/README.md

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+# Atomic Operations
+
+This header provides atomic operations in a way which is portable to
+most platforms.  Current back-ends include:
+
+ * C11
+ * GCC-style __atomic_* builtins
+ * Old-style GCC sync_* builtins
+ * clang-style __c11_atomic_* builtins
+ * MSVC-style Interlocked* intrinsics
+ * OpenMP (using critical sections; there is no atomic CAS)
+
+This is sufficient to support most modern, actively developed
+compilers.  [PGI C/C++ Compiler](http://www.pgroup.com/), which
+[doesn't currently support
+atomics](http://www.pgroup.com/userforum/viewtopic.php?t=5504) but
+hopefully will soon, in the meantime you can use the OpenMP backend
+(just pass `-mp` to the compiler).
+
+Most things are implemented with the preprocessor, but if they were
+functions the prototypes (the 64-bit versions, just s/64/32/ for the
+32-bit versions) would loo like:
+
+```c
+psnip_int64_t psnip_atomic_int64_load(
+  psnip_atomic_int64* object);
+
+void psnip_atomic_int64_store(
+  psnip_atomic_int64* object,
+  psnip_int64_t desired);
+
+_Bool psnip_atomic_int64_compare_exchange(
+  psnip_atomic_int64* object,
+  psnip_int64_t* expected,
+  psnip_int64_t desired);
+
+psnip_nonatomic_int64 psnip_atomic_int64_add(
+  psnip_atomic_int64* object,
+  psnip_int64_t operand);
+
+psnip_nonatomic_int64 psnip_atomic_int64_sub(
+  psnip_atomic_int64* object,
+  psnip_int64_t operand);
+```
+
+If no atomics are supported, `PSNIP_ATOMIC_NOT_FOUND` will be defined;
+you'll probably have to use locks (if you want a portable API for that
+you may be interested in
+[TinyCThread](https://github.com/tinycthread/tinycthread/)), or you
+could try the [atomic_ops](https://github.com/ivmai/libatomic_ops/)
+package.
+
+## Dependencies
+
+To maximize portability you should #include the exact-int module
+before including atomic.h, but if you don't want to add the extra
+file to your project you can omit it and this module will simply rely
+on <stdint.h>.  As an alternative you may define `psnip_int32_t`, and
+`psnip_int64_t` to appropriate values yourself before including
+atomic.h.