[Qemu-devel] [PATCH v2 8/8] cutils: Rewrite ppc buffer zero checking

[Richard Henderson <rth@twiddle.net>]

GCC versions through 6 do a poor job with the indexed addressing,
and (for ppc64le) issues unnecessary xxswapd insns.

Cc: qemu-ppc@nongnu.org
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 util/bufferiszero.c | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

diff --git a/util/bufferiszero.c b/util/bufferiszero.c
index 28a1419..d580b57 100644
--- a/util/bufferiszero.c
+++ b/util/bufferiszero.c
@@ -119,8 +119,44 @@ static bool select_accel_int(const void *buf, size_t len)
 #undef pixel
 #undef bool
 #define bool _Bool
-#define DO_NONZERO(X)  vec_any_ne(X, (__vector unsigned char){ 0 })
-ACCEL_BUFFER_ZERO(buffer_zero_ppc, 128, __vector unsigned char, DO_NONZERO)
+
+static bool __attribute__((noinline))
+buffer_zero_ppc(const void *buf, size_t len)
+{
+    typedef unsigned char vec __attribute__((vector_size(16)));
+    const vec *p = buf;
+    const vec *end = buf + len;
+    vec t0, t1, t2, t3, zero = (vec){ 0 };
+
+    do {
+        p += 8;
+        __builtin_prefetch(p);
+        barrier();
+        /* ??? GCC6 does poorly with power64le; extra xxswap.  */
+        __asm("lvebx %0,%4,%5\n\t"
+              "lvebx %1,%4,%6\n\t"
+              "lvebx %2,%4,%7\n\t"
+              "lvebx %3,%4,%8\n\t"
+              "vor %0,%0,%1\n\t"
+              "vor %1,%2,%3\n\t"
+              "lvebx %2,%4,%9\n\t"
+              "lvebx %3,%4,%10\n\t"
+              "vor %0,%0,%1\n\t"
+              "vor %1,%2,%3\n\t"
+              "lvebx %2,%4,%11\n\t"
+              "lvebx %3,%4,%12\n\t"
+              "vor %0,%0,%1\n\t"
+              "vor %1,%2,%3\n\t"
+              "vor %0,%0,%1"
+              : "=v"(t0), "=v"(t1), "=v"(t2), "=v"(t3)
+              : "b"(p), "b"(-8 * 16), "b"(-7 * 16), "b"(-6 * 16), "b"(-5 * 16),
+                "b"(-4 * 16), "b"(-3 * 16), "b"(-2 * 16), "b"(-1 * 16));
+        if (unlikely(vec_any_ne(t0, zero))) {
+            return false;
+        }
+    } while (p < end);
+    return true;
+}
 
 static bool select_accel_fn(const void *buf, size_t len)
 {
-- 
2.7.4