[OpenMP] Fixed a potential integer overflow

[tianshilei1992]

Summary

size_t has different width on 32- and 64-bit architecture, but the
computation to floor to power of two assumed it is 64-bit, which can cause an
integer overflow. In this patch, architecture detection is added so that the
operation for 64-bit size_t. Thank Luke for reporting the issue.

[jdoerfert]

LGTM.

Though, now that D89426 landed we could also call the LLVM ADT method for this ;)

[tianshilei1992]

In D89878#2345887, @jdoerfert wrote:

LGTM.

Though, now that D89426 landed we could also call the LLVM ADT method for this ;)

That'll be a good direction in the future.

openmp/libomptarget/src/MemoryManager.cpp

 /// Find the previous number that is power of 2 given a number that is not power
 /// of 2.
 size_t floorToPowerOfTwo(size_t Num) {
   Num |= Num >> 1;
   Num |= Num >> 2;
   Num |= Num >> 4;
   Num |= Num >> 8;
   Num |= Num >> 16;
+#if INTPTR_MAX == INT64_MAX
   Num |= Num >> 32;
+#elif INTPTR_MAX == INT32_MAX
+  // Do nothing with 32-bit
+#else
+#error Unsupported architecture
+#endif
   Num += 1;
   return Num >> 1;
 }
 
 /// Find a suitable bucket
 int findBucket(size_t Size) {
   const size_t F = floorToPowerOfTwo(Size);