[PATCH 2/6] bithelp: add count trailing zero bits variant for RISC-V

[Jussi Kivilinna]

* cipher/bithelp.h (_gcry_ctz_no_zero): New.
(_gcry_ctz): Use '_gcry_ctz_no_zero'.
* cipher/cipher-internal.h (ocb_get_l): Use '_gcry_ctz_no_zero'.
--

Signed-off-by: Jussi Kivilinna <jussi.kivilinna at iki.fi>
---
 cipher/bithelp.h         | 33 ++++++++++++++++++++++++++++-----
 cipher/cipher-internal.h |  2 +-
 2 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/cipher/bithelp.h b/cipher/bithelp.h
index 7793ce7c..a4faf345 100644
--- a/cipher/bithelp.h
+++ b/cipher/bithelp.h
@@ -19,6 +19,7 @@
 #ifndef GCRYPT_BITHELP_H
 #define GCRYPT_BITHELP_H
 
+#include "config.h"
 #include "types.h"
 
 
@@ -77,13 +78,25 @@ _gcry_bswap64(u64 x)
 
 
 /* Count trailing zero bits in an unsigend int.  We return an int
-   because that is what gcc's builtin does.  Returns the number of
-   bits in X if X is 0. */
+   because that is what gcc's builtin does.  X must not be zero. */
 static inline int
-_gcry_ctz (unsigned int x)
+_gcry_ctz_no_zero (unsigned int x)
 {
-#if defined (HAVE_BUILTIN_CTZ)
-  return x ? __builtin_ctz (x) : 8 * sizeof (x);
+#if defined(__riscv) && \
+    (defined(__riscv_f) && __riscv_f >= 2002000) && \
+    (!defined(__riscv_zbb) || __riscv_zbb < 2002000) && \
+    defined(HAVE_GCC_ATTRIBUTE_MAY_ALIAS)
+  /* Use float cast approach when building for RISC-V without Zbb extension.
+   * Without Zbb, GCC gives us slower generic version for __builtin_ctz().
+   *
+   * See:
+   * http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightFloatCast
+   */
+  float f = (float)(x & -x);
+  typedef u32 __attribute__((may_alias)) may_alias_u32;
+  return ((*(const may_alias_u32 *)&f) >> 23) - 0x7f;
+#elif defined (HAVE_BUILTIN_CTZ)
+  return __builtin_ctz (x);
 #else
   /* See
    * http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightModLookup
@@ -100,6 +113,16 @@ _gcry_ctz (unsigned int x)
 }
 
 
+/* Count trailing zero bits in an unsigend int.  We return an int
+   because that is what gcc's builtin does.  Returns the number of
+   bits in X if X is 0. */
+static inline int
+_gcry_ctz (unsigned int x)
+{
+  return x ? _gcry_ctz_no_zero (x) : 8 * sizeof (x);
+}
+
+
 /* Count trailing zero bits in an u64.  We return an int because that
    is what gcc's builtin does.  Returns the number of bits in X if X
    is 0.  */
diff --git a/cipher/cipher-internal.h b/cipher/cipher-internal.h
index ddf8fbb5..19b3eada 100644
--- a/cipher/cipher-internal.h
+++ b/cipher/cipher-internal.h
@@ -775,7 +775,7 @@ ocb_get_l (gcry_cipher_hd_t c, u64 n)
         : [low] "r" ((unsigned long)n)
         : "cc");
 #else
-  ntz = _gcry_ctz (n);
+  ntz = _gcry_ctz_no_zero (n);
 #endif
 
   return c->u_mode.ocb.L[ntz];
-- 
2.45.2