msvcrt: Improve memset performance using overlapping stores.

For n larger than 16 we store 16 bytes on each end of the buffer, eventually overlapping, and then 16 additional bytes for n > 32. Then we can find a 32-byte aligned range overlapping the remaining part of the destination buffer, which is filled 32 bytes at a time in a loop. Signed-off-by: Rémi Bernon <rbernon@codeweavers.com> Signed-off-by: Piotr Caban <piotr@codeweavers.com> Signed-off-by: Alexandre Julliard <julliard@winehq.org>

msvcrt: Improve memset performance using overlapping stores.
7b17d708 · Rémi Bernon · Alexandre Julliard · e87f3dd5 · 7b17d708
Commit 7b17d708 authored Sep 14, 2021 by Rémi Bernon Committed by Alexandre Julliard Sep 14, 2021
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string.c dlls/msvcrt/string.c +57 -3

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--- a/dlls/msvcrt/string.c
+++ b/dlls/msvcrt/string.c
@@ -2855,13 +2855,67 @@ void * __cdecl memcpy(void *dst, const void *src, size_t n)
    return memmove(dst, src, n);
 }

+static inline void memset_aligned_32(unsigned char *d, uint64_t v, size_t n)
+{
+    while (n >= 32)
+    {
+        *(uint64_t *)(d + n - 32) = v;
+        *(uint64_t *)(d + n - 24) = v;
+        *(uint64_t *)(d + n - 16) = v;
+        *(uint64_t *)(d + n -  8) = v;
+        n -= 32;
+    }
+}
+
 /*********************************************************************
 *		    memset (MSVCRT.@)
 */
-void* __cdecl memset(void *dst, int c, size_t n)
+void *__cdecl memset(void *dst, int c, size_t n)
 {
-    volatile unsigned char *d = dst;  /* avoid gcc optimizations */
-    while (n--) *d++ = c;
+    uint64_t v = 0x101010101010101ull * (unsigned char)c;
+    unsigned char *d = (unsigned char *)dst;
+    size_t a = 0x20 - ((uintptr_t)d & 0x1f);
+
+    if (n >= 16)
+    {
+        *(uint64_t *)(d + 0) = v;
+        *(uint64_t *)(d + 8) = v;
+        *(uint64_t *)(d + n - 16) = v;
+        *(uint64_t *)(d + n - 8) = v;
+        if (n <= 32) return dst;
+        *(uint64_t *)(d + 16) = v;
+        *(uint64_t *)(d + 24) = v;
+        *(uint64_t *)(d + n - 32) = v;
+        *(uint64_t *)(d + n - 24) = v;
+        if (n <= 64) return dst;
+
+        n = (n - a) & ~0x1f;
+        memset_aligned_32(d + a, v, n);
+        return dst;
+    }
+    if (n >= 8)
+    {
+        *(uint64_t *)d = v;
+        *(uint64_t *)(d + n - 8) = v;
+        return dst;
+    }
+    if (n >= 4)
+    {
+        *(uint32_t *)d = v;
+        *(uint32_t *)(d + n - 4) = v;
+        return dst;
+    }
+    if (n >= 2)
+    {
+        *(uint16_t *)d = v;
+        *(uint16_t *)(d + n - 2) = v;
+        return dst;
+    }
+    if (n >= 1)
+    {
+        *(uint8_t *)d = v;
+        return dst;
+    }
    return dst;
 }