[libc-commits] [libc] [libc] Speed up memmove overlapping check (PR #70017)

[Dmitry Vyukov via libc-commits]

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@@ -89,16 +89,10 @@ template <size_t alignment, typename T> LIBC_INLINE T *assume_aligned(T *ptr) {
 // Returns true iff memory regions [p1, p1 + size] and [p2, p2 + size] are
 // disjoint.
 LIBC_INLINE bool is_disjoint(const void *p1, const void *p2, size_t size) {
-  const char *a = static_cast<const char *>(p1);
-  const char *b = static_cast<const char *>(p2);
-  if (a > b) {
-    // Swap a and b, this compiles down to conditionnal move for aarch64, x86
-    // and RISCV with zbb extension.
-    const char *tmp = a;
-    a = b;
-    b = tmp;
-  }
-  return a + size <= b;
+  const size_t diff =
+      static_cast<const char *>(p1) - static_cast<const char *>(p2);
+  // This is expected to compile to conditional move.
+  return static_cast<ptrdiff_t>(diff) >= 0 ? size <= diff : size <= -diff;
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dvyukov wrote:

I am not sure which of these static_assert's I should use. They seem to be mutually exclusive: if we use bit_cast, we don't need sizeof(a) == sizeof(b); and if we don't use bit_cast, then we don't need -1==~0 (also not sure what casts should be here, if any).

I've changed code to your bit_cast version.
PTAL

https://github.com/llvm/llvm-project/pull/70017