[compiler-rt] [AArch64][compiler-rt] Add memcpy, memset, memmove, memchr simple imp… (PR #77496)

[Fangrui Song via llvm-commits]

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@@ -0,0 +1,90 @@
+#include <stdlib.h>
+
+// WARNING: When building the scalar versions of these functions you need to
+// use the compiler flag "-mllvm -disable-loop-idiom-all" to prevent clang
+// from recognising a loop idiom and planting calls to memcpy!
+
+static void *__arm_sc_memcpy_fwd(void *dest, const void *src,
+                                 size_t n) __arm_streaming_compatible {
+  unsigned char *destp = (unsigned char *)dest;
+  const unsigned char *srcp = (const unsigned char *)src;
+  for (size_t i = 0; i < n; ++i)
+    destp[i] = srcp[i];
+
+  return dest;
+}
+
+// If dest and src overlap then behaviour is undefined, hence we can add the
+// restrict keywords here. This also matches the definition of the libc memcpy
+// according to the man page.
+void *__arm_sc_memcpy(void *__restrict__ dest, const void *__restrict__ src,
+                      size_t n) __arm_streaming_compatible {
+  return __arm_sc_memcpy_fwd(dest, src, n);
+}
+
+void *__arm_sc_memset(void *dest, int c, size_t n) __arm_streaming_compatible {
+  unsigned char *destp = (unsigned char *)dest;
+  unsigned char c8 = (unsigned char)c;
+  for (size_t i = 0; i < n; ++i)
+    destp[i] = c8;
+
+  return dest;
+}
+
+static void *__arm_sc_memcpy_rev(void *dest, const void *src,
+                                 size_t n) __arm_streaming_compatible {
+  unsigned char *destp = (unsigned char *)dest;
+  const unsigned char *srcp = (const unsigned char *)src;
+  // TODO: Improve performance by copying larger chunks in reverse, or by
+  // using SVE.
+  while (n > 0) {
+    --n;
+    destp[n] = srcp[n];
+  }
+  return dest;
+}
+
+// Semantically a memmove is equivalent to the following:
+//   1. Copy the entire contents of src to a temporary array that does not
+//      overlap with src or dest.
+//   2. Copy the contents of the temporary array into dest.
+void *__arm_sc_memmove(void *dest, const void *src,
+                       size_t n) __arm_streaming_compatible {
+  unsigned char *destp = (unsigned char *)dest;
+  const unsigned char *srcp = (const unsigned char *)src;
+  // If src and dest are identical there is nothing to do!
+  if ((destp == srcp) || (n == 0))
----------------
MaskRay wrote:

I wonder whether the two conditions `(destp == srcp) || (n == 0)` are useful.

A lot of libc implementations don't check `destp == srcp`. Some don't check `n==0`

https://github.com/llvm/llvm-project/pull/77496