Hi Richard,
The clzsi2/bsch patterns in the MN10300 backend do not work. There
are two problems - firstly the starting bit-search position for the
BSCH instruction is not set.
Yes it is. What do you think that "unused" second operand does?
That actually works ? Gross!
Secondly the BSCH instruction returns
the bit position of the highest set bit, not the number of leading
zeros.
Ah, I do see that. I recommend you put the xor in the clz expander
rather than cluttering up the "bsch" pattern.
OK - revised patch attached. Is this version OK to apply ?
Cheers
Nick
gcc/ChangeLog
2011-06-26 Nick Clifton <ni...@redhat.com>
* mn10300.md (clzsi2): Use XOR after BSCH to convert bit position
of highest bit set into a count of the high zero bits.
Index: gcc/config/mn10300/mn10300.md
===================================================================
--- gcc/config/mn10300/mn10300.md (revision 175395)
+++ gcc/config/mn10300/mn10300.md (working copy)
@@ -1811,10 +1811,24 @@
;; MISCELANEOUS
;; ----------------------------------------------------------------------
+;; Note the use of the (const_int 0) when generating the insn that matches
+;; the bsch pattern. This ensures that the destination register is
+;; initialised with 0 which will make the BSCH instruction set searching
+;; at bit 31.
+;;
+;; The XOR in the instruction sequence below is there because the BSCH
+;; instruction returns the bit number of the highest set bit and we want
+;; the number of zero bits above that bit. The AM33 does not have a
+;; reverse subtraction instruction, but we can use a simple xor instead
+;; since we know that the top 27 bits are clear.
(define_expand "clzsi2"
- [(parallel [(set (match_operand:SI 0 "register_operand" "")
- (unspec:SI [(match_operand:SI 1 "register_operand" "")
+ [(parallel [(set (match_operand:SI 0 "register_operand")
+ (unspec:SI [(match_operand:SI 1 "register_operand")
(const_int 0)] UNSPEC_BSCH))
+ (clobber (reg:CC CC_REG))])
+ (parallel [(set (match_dup 0)
+ (xor:SI (match_dup 0)
+ (const_int 31)))
(clobber (reg:CC CC_REG))])]
"TARGET_AM33"
)
