[PATCH v6 08/10] util/bufferiszero: Simplify test_buffer_is_zero_next_accel

[Richard Henderson <richard.henderson@linaro.org>]

Because the three alternatives are monotonic, we don't
need to keep a couple of bitmasks, just identify the
strongest alternative at startup.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 util/bufferiszero.c | 56 ++++++++++++++++++---------------------------
 1 file changed, 22 insertions(+), 34 deletions(-)

diff --git a/util/bufferiszero.c b/util/bufferiszero.c
index eb8030a3f0..ff003dc40e 100644
--- a/util/bufferiszero.c
+++ b/util/bufferiszero.c
@@ -179,51 +179,39 @@ buffer_zero_avx2(const void *buf, size_t len)
 }
 #endif /* CONFIG_AVX2_OPT */
 
-
-
-static unsigned __attribute__((noinline))
-select_accel_cpuinfo(unsigned info)
-{
-    /* Array is sorted in order of algorithm preference. */
-    static const struct {
-        unsigned bit;
-        biz_accel_fn fn;
-    } all[] = {
+static biz_accel_fn const accel_table[] = {
+    buffer_is_zero_int_ge256,
+    buffer_zero_sse2,
 #ifdef CONFIG_AVX2_OPT
-        { CPUINFO_AVX2,    buffer_zero_avx2 },
+    buffer_zero_avx2,
 #endif
-        { CPUINFO_SSE2,    buffer_zero_sse2 },
-        { CPUINFO_ALWAYS,  buffer_is_zero_int_ge256 },
-    };
-
-    for (unsigned i = 0; i < ARRAY_SIZE(all); ++i) {
-        if (info & all[i].bit) {
-            buffer_is_zero_accel = all[i].fn;
-            return all[i].bit;
-        }
-    }
-    return 0;
-}
-
-static unsigned used_accel;
+};
+static unsigned accel_index;
 
 static void __attribute__((constructor)) init_accel(void)
 {
-    used_accel = select_accel_cpuinfo(cpuinfo_init());
+    unsigned info = cpuinfo_init();
+    unsigned index = (info & CPUINFO_SSE2 ? 1 : 0);
+
+#ifdef CONFIG_AVX2_OPT
+    if (info & CPUINFO_AVX2) {
+        index = 2;
+    }
+#endif
+
+    accel_index = index;
+    buffer_is_zero_accel = accel_table[index];
 }
 
 #define INIT_ACCEL NULL
 
 bool test_buffer_is_zero_next_accel(void)
 {
-    /*
-     * Accumulate the accelerators that we've already tested, and
-     * remove them from the set to test this round.  We'll get back
-     * a zero from select_accel_cpuinfo when there are no more.
-     */
-    unsigned used = select_accel_cpuinfo(cpuinfo & ~used_accel);
-    used_accel |= used;
-    return used;
+    if (accel_index != 0) {
+        buffer_is_zero_accel = accel_table[--accel_index];
+        return true;
+    }
+    return false;
 }
 #else
 bool test_buffer_is_zero_next_accel(void)
-- 
2.34.1