accel/tcg: Add x86_64 load_atom_extract_al16_or_al8

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

host/include/x86_64/host/load-extract-al16-al8.h

@@ -0,0 +1,50 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ * Atomic extract 64 from 128-bit, x86_64 version.
+ *
+ * Copyright (C) 2023 Linaro, Ltd.
+ */
+
+#ifndef X86_64_LOAD_EXTRACT_AL16_AL8_H
+#define X86_64_LOAD_EXTRACT_AL16_AL8_H
+
+#ifdef CONFIG_INT128_TYPE
+#include "host/cpuinfo.h"
+
+/**
+ * load_atom_extract_al16_or_al8:
+ * @pv: host address
+ * @s: object size in bytes, @s <= 8.
+ *
+ * Load @s bytes from @pv, when pv % s != 0.  If [p, p+s-1] does not
+ * cross an 16-byte boundary then the access must be 16-byte atomic,
+ * otherwise the access must be 8-byte atomic.
+ */
+static inline uint64_t ATTRIBUTE_ATOMIC128_OPT
+load_atom_extract_al16_or_al8(void *pv, int s)
+{
+    uintptr_t pi = (uintptr_t)pv;
+    __int128_t *ptr_align = (__int128_t *)(pi & ~7);
+    int shr = (pi & 7) * 8;
+    Int128Alias r;
+
+    /*
+     * ptr_align % 16 is now only 0 or 8.
+     * If the host supports atomic loads with VMOVDQU, then always use that,
+     * making the branch highly predictable.  Otherwise we must use VMOVDQA
+     * when ptr_align % 16 == 0 for 16-byte atomicity.
+     */
+    if ((cpuinfo & CPUINFO_ATOMIC_VMOVDQU) || (pi & 8)) {
+        asm("vmovdqu %1, %0" : "=x" (r.i) : "m" (*ptr_align));
+    } else {
+        asm("vmovdqa %1, %0" : "=x" (r.i) : "m" (*ptr_align));
+    }
+    return int128_getlo(int128_urshift(r.s, shr));
+}
+#else
+/* Fallback definition that must be optimized away, or error.  */
+uint64_t QEMU_ERROR("unsupported atomic")
+    load_atom_extract_al16_or_al8(void *pv, int s);
+#endif
+
+#endif /* X86_64_LOAD_EXTRACT_AL16_AL8_H */