Optional SIMD memcmp

libc-top-half/musl/src/string/memcmp.c

@@ -1,7 +1,42 @@
 #include <string.h>
 
+#ifdef __wasm_simd128__
+#include <wasm_simd128.h>
+#endif
+
 int memcmp(const void *vl, const void *vr, size_t n)
 {
+#if defined(__wasm_simd128__) && defined(__wasilibc_simd_string)
+	if (n >= sizeof(v128_t)) {
+		// memcmp is allowed to read up to n bytes from each object.
+		// Find the first different character in the objects.
+		// Unaligned loads handle the case where the objects
+		// have mismatching alignments.
+		const v128_t *v1 = (v128_t *)vl;
+		const v128_t *v2 = (v128_t *)vr;
+		while (n) {
+			const v128_t cmp = wasm_i8x16_eq(wasm_v128_load(v1), wasm_v128_load(v2));
+			// Bitmask is slow on AArch64, all_true is much faster.
+			if (!wasm_i8x16_all_true(cmp)) {
+				// Find the offset of the first zero bit (little-endian).
+				size_t ctz = __builtin_ctz(~wasm_i8x16_bitmask(cmp));
+				const unsigned char *u1 = (unsigned char *)v1 + ctz;
+				const unsigned char *u2 = (unsigned char *)v2 + ctz;
+				// This may help the compiler if the function is inlined.
+				__builtin_assume(*u1 - *u2 != 0);
+				return *u1 - *u2;
+			}
+			// This makes n a multiple of sizeof(v128_t)
+			// for every iteration except the first.
+			size_t align = (n - 1) % sizeof(v128_t) + 1;
+			v1 = (v128_t *)((char *)v1 + align);
+			v2 = (v128_t *)((char *)v2 + align);
+			n -= align;
+		}
+		return 0;
+	}
+#endif
+
 	const unsigned char *l=vl, *r=vr;
 	for (; n && *l == *r; n--, l++, r++);
 	return n ? *l-*r : 0;

test/src/misc/memcmp.c

@@ -0,0 +1,50 @@
+//! add-flags.py(LDFLAGS): -Wl,--stack-first -Wl,--initial-memory=327680
+
+#include <__macro_PAGESIZE.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <string.h>
+
+int sign(int val) {
+    return (0 < val) - (val < 0);
+}
+void test(char *ptr1, char *ptr2, size_t length, int want) {
+  int got = memcmp(ptr1, ptr2, length);
+  if (sign(got) != sign(want)) {
+    printf("memcmp(%p, %p, %lu) = %d, want %d\n", ptr1, ptr2, length, got,
+           want);
+  }
+}
+
+int main(void) {
+  char *const LIMIT = (char *)(__builtin_wasm_memory_size(0) * PAGESIZE);
+
+  for (ptrdiff_t length = 0; length < 64; length++) {
+    for (ptrdiff_t alignment = 0; alignment < 24; alignment++) {
+      for (ptrdiff_t pos = -2; pos < length + 2; pos++) {
+        // Create a buffer with the given length, at a pointer with the given
+        // alignment. Using the offset LIMIT - PAGESIZE - 8 means many buffers
+        // will straddle a (Wasm, and likely OS) page boundary.
+        // The second buffer has a fixed address, which means it won't
+        // always share alignment with first buffer.
+        // Place the difference to find at every position in the buffers,
+        // including just prior to it and after its end.
+        char *ptr1 = LIMIT - PAGESIZE - 8 + alignment;
+        char *ptr2 = LIMIT - PAGESIZE / 2;
+        memset(LIMIT - 2 * PAGESIZE, 0, 2 * PAGESIZE);
+        memset(ptr1, 5, length);
+        memset(ptr2, 5, length);
+
+        ptr1[pos] = 7;
+        ptr2[pos] = 3;
+
+        test(ptr1, ptr2, length,
+             0 <= pos && pos < length ? ptr1[pos] - ptr2[pos] : 0);
+        test(ptr2, ptr1, length,
+             0 <= pos && pos < length ? ptr2[pos] - ptr1[pos] : 0);
+      }
+    }
+  }
+
+  return 0;
+}