random: statically compute poolbitshift, poolbytes, poolbits

Use a macro to statically compute poolbitshift (will be used in a
subsequent patch), poolbytes, and poolbits.  On virtually all
architectures the cost of a memory load with an offset is the same as
the one of a memory load.

It is still possible for this to generate worse code since the C
compiler doesn't know the fixed relationship between these fields, but
that is somewhat unlikely.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 19 insertions, 20 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 54d020815b4e..20651a2fd8a7 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -309,46 +309,45 @@ static DEFINE_PER_CPU(int, trickle_count);
  * scaled squared error sum) except for the last tap, which is 1 to
  * get the twisting happening as fast as possible.
  */
+
 static struct poolinfo {
-        int poolwords;
+        int poolbitshift, poolwords, poolbytes, poolbits;
+#define S(x) ilog2(x)+5, (x), (x)*4, (x)*32
         int tap1, tap2, tap3, tap4, tap5;
 } poolinfo_table[] = {
         /* x^128 + x^103 + x^76 + x^51 +x^25 + x + 1 -- 105 */
-        { 128,  103,    76,     51,     25,     1 },
+        { S(128),       103,    76,     51,     25,     1 },
         /* x^32 + x^26 + x^20 + x^14 + x^7 + x + 1 -- 15 */
-        { 32,   26,     20,     14,     7,      1 },
+        { S(32),        26,     20,     14,     7,      1 },
 #if 0
         /* x^2048 + x^1638 + x^1231 + x^819 + x^411 + x + 1  -- 115 */
-        { 2048, 1638,   1231,   819,    411,    1 },
+        { S(2048),      1638,   1231,   819,    411,    1 },
 
         /* x^1024 + x^817 + x^615 + x^412 + x^204 + x + 1 -- 290 */
-        { 1024, 817,    615,    412,    204,    1 },
+        { S(1024),      817,    615,    412,    204,    1 },
 
         /* x^1024 + x^819 + x^616 + x^410 + x^207 + x^2 + 1 -- 115 */
-        { 1024, 819,    616,    410,    207,    2 },
+        { S(1024),      819,    616,    410,    207,    2 },
 
         /* x^512 + x^411 + x^308 + x^208 + x^104 + x + 1 -- 225 */
-        { 512,  411,    308,    208,    104,    1 },
+        { S(512),       411,    308,    208,    104,    1 },
 
         /* x^512 + x^409 + x^307 + x^206 + x^102 + x^2 + 1 -- 95 */
-        { 512,  409,    307,    206,    102,    2 },
+        { S(512),       409,    307,    206,    102,    2 },
         /* x^512 + x^409 + x^309 + x^205 + x^103 + x^2 + 1 -- 95 */
-        { 512,  409,    309,    205,    103,    2 },
+        { S(512),       409,    309,    205,    103,    2 },
 
         /* x^256 + x^205 + x^155 + x^101 + x^52 + x + 1 -- 125 */
-        { 256,  205,    155,    101,    52,     1 },
+        { S(256),       205,    155,    101,    52,     1 },
 
         /* x^128 + x^103 + x^78 + x^51 + x^27 + x^2 + 1 -- 70 */
-        { 128,  103,    78,     51,     27,     2 },
+        { S(128),       103,    78,     51,     27,     2 },
 
         /* x^64 + x^52 + x^39 + x^26 + x^14 + x + 1 -- 15 */
-        { 64,   52,     39,     26,     14,     1 },
+        { S(64),        52,     39,     26,     14,     1 },
 #endif
 };
 
-#define POOLBITS        poolwords*32
-#define POOLBYTES       poolwords*4
-
 /*
  * For the purposes of better mixing, we use the CRC-32 polynomial as
  * well to make a twisted Generalized Feedback Shift Reigster
@@ -599,8 +598,8 @@ retry:
         if (entropy_count < 0) {
                 DEBUG_ENT("negative entropy/overflow\n");
                 entropy_count = 0;
-        } else if (entropy_count > r->poolinfo->POOLBITS)
-                entropy_count = r->poolinfo->POOLBITS;
+        } else if (entropy_count > r->poolinfo->poolbits)
+                entropy_count = r->poolinfo->poolbits;
         if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
                 goto retry;
 
@@ -815,7 +814,7 @@ static void xfer_secondary_pool(struct entropy_store *r, size_t nbytes)
         __u32   tmp[OUTPUT_POOL_WORDS];
 
         if (r->pull && r->entropy_count < nbytes * 8 &&
-            r->entropy_count < r->poolinfo->POOLBITS) {
+            r->entropy_count < r->poolinfo->poolbits) {
                 /* If we're limited, always leave two wakeup worth's BITS */
                 int rsvd = r->limit ? 0 : random_read_wakeup_thresh/4;
                 int bytes = nbytes;
@@ -857,7 +856,7 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
         /* Hold lock while accounting */
         spin_lock_irqsave(&r->lock, flags);
 
-        BUG_ON(r->entropy_count > r->poolinfo->POOLBITS);
+        BUG_ON(r->entropy_count > r->poolinfo->poolbits);
         DEBUG_ENT("trying to extract %zu bits from %s\n",
                   nbytes * 8, r->name);
 
@@ -1112,7 +1111,7 @@ static void init_std_data(struct entropy_store *r)
         r->entropy_total = 0;
         r->last_data_init = false;
         mix_pool_bytes(r, &now, sizeof(now), NULL);
-        for (i = r->poolinfo->POOLBYTES; i > 0; i -= sizeof(rv)) {
+        for (i = r->poolinfo->poolbytes; i > 0; i -= sizeof(rv)) {
                 if (!arch_get_random_long(&rv))
                         break;
                 mix_pool_bytes(r, &rv, sizeof(rv), NULL);