[libc] Efficiently implement 'realloc' for AMDGPU devices (#145960)

Summary:
Now that we have `malloc` we can implement `realloc` efficiently. This
uses the known chunk sizes to avoid unnecessary allocations. We just
return nullptr for NVPTX. I'd remove the list for the entrypoint but
then the libc++ code would stop working. When someone writes the NVPTX
support this will be trivial.

libc/test/integration/src/stdlib/gpu/CMakeLists.txt

@@ -17,6 +17,21 @@ if(NOT LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
       --blocks 1024
   )
 
+  add_integration_test(
+    realloc
+    SUITE
+      stdlib-gpu-integration-tests
+    SRCS
+      realloc.cpp
+    DEPENDS
+      libc.src.stdlib.malloc
+      libc.src.stdlib.free
+      libc.src.stdlib.realloc
+    LOADER_ARGS
+      --threads 256
+      --blocks 1024
+  )
+
   add_integration_test(
     malloc_stress
     SUITE

libc/test/integration/src/stdlib/gpu/realloc.cpp

@@ -0,0 +1,44 @@
+#include "test/IntegrationTest/test.h"
+
+#include "src/__support/GPU/utils.h"
+#include "src/stdlib/free.h"
+#include "src/stdlib/malloc.h"
+#include "src/stdlib/realloc.h"
+
+using namespace LIBC_NAMESPACE;
+
+TEST_MAIN(int, char **, char **) {
+  // realloc(nullptr, size) is equivalent to malloc.
+  int *alloc = reinterpret_cast<int *>(LIBC_NAMESPACE::realloc(nullptr, 32));
+  EXPECT_NE(alloc, nullptr);
+  *alloc = 42;
+  EXPECT_EQ(*alloc, 42);
+
+  // realloc to same size returns the same pointer.
+  void *same = LIBC_NAMESPACE::realloc(alloc, 32);
+  EXPECT_EQ(same, alloc);
+  EXPECT_EQ(reinterpret_cast<int *>(same)[0], 42);
+
+  // realloc to smaller size returns same pointer.
+  void *smaller = LIBC_NAMESPACE::realloc(same, 16);
+  EXPECT_EQ(smaller, alloc);
+  EXPECT_EQ(reinterpret_cast<int *>(smaller)[0], 42);
+
+  // realloc to larger size returns new pointer and preserves contents.
+  int *larger = reinterpret_cast<int *>(LIBC_NAMESPACE::realloc(smaller, 128));
+  EXPECT_NE(larger, nullptr);
+  EXPECT_EQ(larger[0], 42);
+
+  // realloc works when called with a divergent size.
+  int *div = reinterpret_cast<int *>(
+      LIBC_NAMESPACE::malloc((gpu::get_thread_id() + 1) * 16));
+  EXPECT_NE(div, nullptr);
+  div[0] = static_cast<int>(gpu::get_thread_id());
+  int *div_realloc = reinterpret_cast<int *>(
+      LIBC_NAMESPACE::realloc(div, ((gpu::get_thread_id() + 1) * 32)));
+  EXPECT_NE(div_realloc, nullptr);
+  EXPECT_EQ(div_realloc[0], static_cast<int>(gpu::get_thread_id()));
+  LIBC_NAMESPACE::free(div_realloc);
+
+  return 0;
+}