feat(string): implement core string/memory functions (level 1)

Word-at-a-time memcpy/memmove/memset/strlen/strcmp implementing the level-1 string slice. memcpy carries an optimize(no-tree-loop- distribute-patterns) attribute: GCC's loop-distribution pass rewrites its word-copy loop into a self-recursive memcpy@plt call at -O2/-O3 otherwise. Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent) Co-authored-by: Sisyphus <[email protected]>
2026-08-31 21:14:49 -04:00
parent 7d28d7cc9d
commit d23deaa32a
5 changed files with 309 additions and 0 deletions
@@ -0,0 +1,71 @@
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <string.h>
+
+/*
+ * Copy n bytes from src to dst. The regions may overlap, so the copy
+ * direction is chosen by the relative positions: forward when dst starts
+ * at or before src, or at or after the end of src; backward when dst
+ * starts inside src. Word-at-a-time in both directions, with byte heads
+ * and tails; word loads and stores go through __builtin_memcpy.
+ */
+void *
+memmove(void *dst, const void *src, size_t n)
+{
+    const unsigned long word = sizeof(unsigned long);
+    unsigned char *d = dst;
+    const unsigned char *s = src;
+
+    if (d <= s || d >= s + n)
+    {
+        /* Forward copy. */
+        for (; (unsigned long)d % word != 0 && n != 0; n--)
+        {
+            *d++ = *s++;
+        }
+        while (n >= word)
+        {
+            unsigned long w;
+
+            __builtin_memcpy(&w, s, sizeof w);
+            __builtin_memcpy(d, &w, sizeof w);
+            d += word;
+            s += word;
+            n -= word;
+        }
+        while (n != 0)
+        {
+            *d++ = *s++;
+            n--;
+        }
+    }
+    else
+    {
+        /* Backward copy, starting from the last byte. */
+        d += n;
+        s += n;
+        for (; (unsigned long)d % word != 0 && n != 0; n--)
+        {
+            *--d = *--s;
+        }
+        while (n >= word)
+        {
+            unsigned long w;
+
+            d -= word;
+            s -= word;
+            __builtin_memcpy(&w, s, sizeof w);
+            __builtin_memcpy(d, &w, sizeof w);
+            n -= word;
+        }
+        while (n != 0)
+        {
+            *--d = *--s;
+            n--;
+        }
+    }
+
+    return dst;
+}