X-Git-Url: http://repo.macrolet.net/gitweb/?a=blobdiff_plain;f=src%2Fcompiler%2Fx86%2Farith.lisp;h=36b869d71e2bcfd0a114f4c51ad413c651c3ca52;hb=d131dfb49a3e6522d2358d14252f3f52cfcd202a;hp=949d73b337e8fd19f90d1d8e0f1651e1267e0ed4;hpb=09957fcf57b49ed5ae5f05d62ad12d7ddbfd8e1d;p=sbcl.git

diff --git a/src/compiler/x86/arith.lisp b/src/compiler/x86/arith.lisp
index 949d73b..36b869d 100644
--- a/src/compiler/x86/arith.lisp
+++ b/src/compiler/x86/arith.lisp
@@ -184,7 +184,6 @@
   (define-binop logior 2 or)
   (define-binop logxor 2 xor))
 
-
 ;;; Special handling of add on the x86; can use lea to avoid a
 ;;; register load, otherwise it uses add.
 (define-vop (fast-+/fixnum=>fixnum fast-safe-arith-op)
@@ -633,21 +632,20 @@
     ;; The result-type ensures us that this shift will not overflow.
     (inst shl result :cl)))
 
-(define-vop (fast-ash-c)
+(define-vop (fast-ash-c/signed=>signed)
   (:translate ash)
   (:policy :fast-safe)
-  (:args (number :scs (signed-reg unsigned-reg) :target result
-		 :load-if (not (and (sc-is number signed-stack unsigned-stack)
-				    (sc-is result signed-stack unsigned-stack)
+  (:args (number :scs (signed-reg) :target result
+		 :load-if (not (and (sc-is number signed-stack)
+				    (sc-is result signed-stack)
 				    (location= number result)))))
   (:info amount)
-  (:arg-types (:or signed-num unsigned-num) (:constant integer))
-  (:results (result :scs (signed-reg unsigned-reg)
-		    :load-if (not
-			      (and (sc-is number signed-stack unsigned-stack)
-				   (sc-is result signed-stack unsigned-stack)
-				   (location= number result)))))
-  (:result-types (:or signed-num unsigned-num))
+  (:arg-types signed-num (:constant integer))
+  (:results (result :scs (signed-reg)
+		    :load-if (not (and (sc-is number signed-stack)
+				       (sc-is result signed-stack)
+				       (location= number result)))))
+  (:result-types signed-num)
   (:note "inline ASH")
   (:generator 3
     (cond ((and (= amount 1) (not (location= number result)))
@@ -658,54 +656,96 @@
 	   (inst lea result (make-ea :dword :index number :scale 8)))
 	  (t
 	   (move result number)
-	   (cond ((plusp amount)
-		  ;; We don't have to worry about overflow because of the
-		  ;; result type restriction.
-		  (inst shl result amount))
-		 ((sc-is number signed-reg signed-stack)
-		  ;; If the amount is greater than 31, only shift by 31. We
-		  ;; have to do this because the shift instructions only look
-		  ;; at the low five bits of the result.
-		  (inst sar result (min 31 (- amount))))
-		 (t
-		  (inst shr result (min 31 (- amount)))))))))
+	   (cond ((plusp amount) (inst shl result amount))
+		 (t (inst sar result (min 31 (- amount)))))))))
 
-(define-vop (fast-ash-left)
+(define-vop (fast-ash-c/unsigned=>unsigned)
   (:translate ash)
-  (:args (number :scs (signed-reg unsigned-reg) :target result
-		 :load-if (not (and (sc-is number signed-stack unsigned-stack)
-				    (sc-is result signed-stack unsigned-stack)
+  (:policy :fast-safe)
+  (:args (number :scs (unsigned-reg) :target result
+		 :load-if (not (and (sc-is number unsigned-stack)
+				    (sc-is result unsigned-stack)
+				    (location= number result)))))
+  (:info amount)
+  (:arg-types unsigned-num (:constant integer))
+  (:results (result :scs (unsigned-reg)
+		    :load-if (not (and (sc-is number unsigned-stack)
+				       (sc-is result unsigned-stack)
+				       (location= number result)))))
+  (:result-types unsigned-num)
+  (:note "inline ASH")
+  (:generator 3
+    (cond ((and (= amount 1) (not (location= number result)))
+	   (inst lea result (make-ea :dword :index number :scale 2)))
+	  ((and (= amount 2) (not (location= number result)))
+	   (inst lea result (make-ea :dword :index number :scale 4)))
+	  ((and (= amount 3) (not (location= number result)))
+	   (inst lea result (make-ea :dword :index number :scale 8)))
+	  (t
+	   (move result number)
+	   (cond ((< -32 amount 32)
+                  ;; this code is used both in ASH and ASH-MOD32, so
+                  ;; be careful
+                  (if (plusp amount)
+                      (inst shl result amount)
+                      (inst shr result (- amount))))
+		 (t (inst xor result result)))))))
+
+(define-vop (fast-ash-left/signed=>signed)
+  (:translate ash)
+  (:args (number :scs (signed-reg) :target result
+		 :load-if (not (and (sc-is number signed-stack)
+				    (sc-is result signed-stack)
 				    (location= number result))))
 	 (amount :scs (unsigned-reg) :target ecx))
-  (:arg-types (:or signed-num unsigned-num) positive-fixnum)
+  (:arg-types signed-num positive-fixnum)
   (:temporary (:sc unsigned-reg :offset ecx-offset :from (:argument 1)) ecx)
-  (:results (result :scs (signed-reg unsigned-reg) :from (:argument 0)
-		    :load-if (not
-			      (and (sc-is number signed-stack unsigned-stack)
-				   (sc-is result signed-stack unsigned-stack)
-				   (location= number result)))))
-  (:result-types (:or signed-num unsigned-num))
+  (:results (result :scs (signed-reg) :from (:argument 0)
+		    :load-if (not (and (sc-is number signed-stack)
+				       (sc-is result signed-stack)
+				       (location= number result)))))
+  (:result-types signed-num)
+  (:policy :fast-safe)
+  (:note "inline ASH")
+  (:generator 4
+    (move result number)
+    (move ecx amount)
+    (inst shl result :cl)))
+
+(define-vop (fast-ash-left/unsigned=>unsigned)
+  (:translate ash)
+  (:args (number :scs (unsigned-reg) :target result
+		 :load-if (not (and (sc-is number unsigned-stack)
+				    (sc-is result unsigned-stack)
+				    (location= number result))))
+	 (amount :scs (unsigned-reg) :target ecx))
+  (:arg-types unsigned-num positive-fixnum)
+  (:temporary (:sc unsigned-reg :offset ecx-offset :from (:argument 1)) ecx)
+  (:results (result :scs (unsigned-reg) :from (:argument 0)
+		    :load-if (not (and (sc-is number unsigned-stack)
+				       (sc-is result unsigned-stack)
+				       (location= number result)))))
+  (:result-types unsigned-num)
   (:policy :fast-safe)
   (:note "inline ASH")
   (:generator 4
     (move result number)
     (move ecx amount)
-    ;; The result-type ensures us that this shift will not overflow.
     (inst shl result :cl)))
 
-(define-vop (fast-ash)
+(define-vop (fast-ash/signed=>signed)
   (:translate ash)
   (:policy :fast-safe)
-  (:args (number :scs (signed-reg unsigned-reg) :target result)
+  (:args (number :scs (signed-reg) :target result)
 	 (amount :scs (signed-reg) :target ecx))
-  (:arg-types (:or signed-num unsigned-num) signed-num)
-  (:results (result :scs (signed-reg unsigned-reg) :from (:argument 0)))
-  (:result-types (:or signed-num unsigned-num))
+  (:arg-types signed-num signed-num)
+  (:results (result :scs (signed-reg) :from (:argument 0)))
+  (:result-types signed-num)
   (:temporary (:sc signed-reg :offset ecx-offset :from (:argument 1)) ecx)
   (:note "inline ASH")
   (:generator 5
     (move result number)
-     (move ecx amount)
+    (move ecx amount)
     (inst or ecx ecx)
     (inst jmp :ns positive)
     (inst neg ecx)
@@ -713,9 +753,7 @@
     (inst jmp :be okay)
     (inst mov ecx 31)
     OKAY
-    (sc-case number
-      (signed-reg (inst sar result :cl))
-      (unsigned-reg (inst shr result :cl)))
+    (inst sar result :cl)
     (inst jmp done)
 
     POSITIVE
@@ -723,6 +761,70 @@
     (inst shl result :cl)
 
     DONE))
+
+(define-vop (fast-ash/unsigned=>unsigned)
+  (:translate ash)
+  (:policy :fast-safe)
+  (:args (number :scs (unsigned-reg) :target result)
+	 (amount :scs (signed-reg) :target ecx))
+  (:arg-types unsigned-num signed-num)
+  (:results (result :scs (unsigned-reg) :from (:argument 0)))
+  (:result-types unsigned-num)
+  (:temporary (:sc signed-reg :offset ecx-offset :from (:argument 1)) ecx)
+  (:note "inline ASH")
+  (:generator 5
+    (move result number)
+    (move ecx amount)
+    (inst or ecx ecx)
+    (inst jmp :ns positive)
+    (inst neg ecx)
+    (inst cmp ecx 31)
+    (inst jmp :be okay)
+    (inst xor result result)
+    (inst jmp done)
+    OKAY
+    (inst shr result :cl)
+    (inst jmp done)
+
+    POSITIVE
+    ;; The result-type ensures us that this shift will not overflow.
+    (inst shl result :cl)
+
+    DONE))
+
+;;; FIXME: before making knowledge of this too public, it needs to be
+;;; fixed so that it's actually _faster_ than the non-CMOV version; at
+;;; least on my Celeron-XXX laptop, this version is marginally slower
+;;; than the above version with branches.  -- CSR, 2003-09-04
+(define-vop (fast-cmov-ash/unsigned=>unsigned)
+  (:translate ash)
+  (:policy :fast-safe)
+  (:args (number :scs (unsigned-reg) :target result)
+	 (amount :scs (signed-reg) :target ecx))
+  (:arg-types unsigned-num signed-num)
+  (:results (result :scs (unsigned-reg) :from (:argument 0)))
+  (:result-types unsigned-num)
+  (:temporary (:sc signed-reg :offset ecx-offset :from (:argument 1)) ecx)
+  (:temporary (:sc any-reg :from (:eval 0) :to (:eval 1)) zero)
+  (:note "inline ASH")
+  (:guard (member :cmov *backend-subfeatures*))
+  (:generator 4
+    (move result number)
+    (move ecx amount)
+    (inst or ecx ecx)
+    (inst jmp :ns positive)
+    (inst neg ecx)
+    (inst xor zero zero)
+    (inst shr result :cl)
+    (inst cmp ecx 31)
+    (inst cmov :nbe result zero)
+    (inst jmp done)
+    
+    POSITIVE
+    ;; The result-type ensures us that this shift will not overflow.
+    (inst shl result :cl)
+
+    DONE))
 
 ;;; Note: documentation for this function is wrong - rtfm
 (define-vop (signed-byte-32-len)
@@ -731,9 +833,9 @@
   (:policy :fast-safe)
   (:args (arg :scs (signed-reg) :target res))
   (:arg-types signed-num)
-  (:results (res :scs (any-reg)))
-  (:result-types positive-fixnum)
-  (:generator 30
+  (:results (res :scs (unsigned-reg)))
+  (:result-types unsigned-num)
+  (:generator 28
     (move res arg)
     (inst cmp res 0)
     (inst jmp :ge POS)
@@ -742,7 +844,23 @@
     (inst bsr res res)
     (inst jmp :z zero)
     (inst inc res)
-    (inst shl res 2)
+    (inst jmp done)
+    ZERO
+    (inst xor res res)
+    DONE))
+
+(define-vop (unsigned-byte-32-len)
+  (:translate integer-length)
+  (:note "inline (unsigned-byte 32) integer-length")
+  (:policy :fast-safe)
+  (:args (arg :scs (unsigned-reg)))
+  (:arg-types unsigned-num)
+  (:results (res :scs (unsigned-reg)))
+  (:result-types unsigned-num)
+  (:generator 26
+    (inst bsr res arg)
+    (inst jmp :z zero)
+    (inst inc res)
     (inst jmp done)
     ZERO
     (inst xor res res)
@@ -962,65 +1080,23 @@
     (move result prev)
     (inst shrd result next :cl)))
 
-(define-vop (32bit-logical)
-  (:args (x :scs (unsigned-reg) :target r
-	    :load-if (not (and (sc-is x unsigned-stack)
-			       (sc-is r unsigned-stack)
-			       (location= x r))))
-	 (y :scs (unsigned-reg)
-	    :load-if (or (not (sc-is y unsigned-stack))
-			 (and (sc-is x unsigned-stack)
-			      (sc-is y unsigned-stack)
-			      (location= x r)))))
-  (:arg-types unsigned-num unsigned-num)
-  (:results (r :scs (unsigned-reg)
-	       :from (:argument 0)
-	       :load-if (not (and (sc-is x unsigned-stack)
-				  (sc-is r unsigned-stack)
-				  (location= x r)))))
-  (:result-types unsigned-num)
-  (:policy :fast-safe))
-
-(define-vop (32bit-logical-not)
-  (:translate 32bit-logical-not)
-  (:args (x :scs (unsigned-reg) :target r
-	    :load-if (not (and (sc-is x unsigned-stack)
-			       (sc-is r unsigned-stack)
-			       (location= x r)))))
-  (:arg-types unsigned-num)
-  (:results (r :scs (unsigned-reg)
-	       :load-if (not (and (sc-is x unsigned-stack)
-				  (sc-is r unsigned-stack)
-				  (location= x r)))))
-  (:result-types unsigned-num)
-  (:policy :fast-safe)
-  (:generator 1
-    (move r x)
-    (inst not r)))
+(define-source-transform 32bit-logical-not (x)
+  `(logand (lognot (the (unsigned-byte 32) ,x)) #.(1- (ash 1 32))))
 
-(define-vop (32bit-logical-and 32bit-logical)
-  (:translate 32bit-logical-and)
-  (:generator 1
-    (move r x)
-    (inst and r y)))
+(deftransform 32bit-logical-and ((x y))
+  '(logand x y))
 
 (define-source-transform 32bit-logical-nand (x y)
   `(32bit-logical-not (32bit-logical-and ,x ,y)))
 
-(define-vop (32bit-logical-or 32bit-logical)
-  (:translate 32bit-logical-or)
-  (:generator 1
-    (move r x)
-    (inst or r y)))
+(deftransform 32bit-logical-or ((x y))
+  '(logior x y))
 
 (define-source-transform 32bit-logical-nor (x y)
   `(32bit-logical-not (32bit-logical-or ,x ,y)))
 
-(define-vop (32bit-logical-xor 32bit-logical)
-  (:translate 32bit-logical-xor)
-  (:generator 1
-    (move r x)
-    (inst xor r y)))
+(deftransform 32bit-logical-xor ((x y))
+  '(logxor x y))
 
 (define-source-transform 32bit-logical-eqv (x y)
   `(32bit-logical-not (32bit-logical-xor ,x ,y)))
@@ -1063,6 +1139,67 @@
     (move ecx amount)
     (inst shl r :cl)))
 
+;;;; Modular functions
+
+(define-modular-fun +-mod32 (x y) + 32)
+(define-vop (fast-+-mod32/unsigned=>unsigned fast-+/unsigned=>unsigned)
+  (:translate +-mod32))
+(define-vop (fast-+-mod32-c/unsigned=>unsigned fast-+-c/unsigned=>unsigned)
+  (:translate +-mod32))
+(define-modular-fun --mod32 (x y) - 32)
+(define-vop (fast---mod32/unsigned=>unsigned fast--/unsigned=>unsigned)
+  (:translate --mod32))
+(define-vop (fast---mod32-c/unsigned=>unsigned fast---c/unsigned=>unsigned)
+  (:translate --mod32))
+
+(define-vop (fast-ash-left-mod32-c/unsigned=>unsigned
+             fast-ash-c/unsigned=>unsigned)
+  (:translate ash-left-mod32))
+
+;;; logical operations
+(define-modular-fun lognot-mod32 (x) lognot 32)
+(define-vop (lognot-mod32/unsigned=>unsigned)
+  (:translate lognot-mod32)
+  (:args (x :scs (unsigned-reg unsigned-stack) :target r
+	    :load-if (not (and (sc-is x unsigned-stack)
+			       (sc-is r unsigned-stack)
+			       (location= x r)))))
+  (:arg-types unsigned-num)
+  (:results (r :scs (unsigned-reg)
+	       :load-if (not (and (sc-is x unsigned-stack)
+				  (sc-is r unsigned-stack)
+				  (location= x r)))))
+  (:result-types unsigned-num)
+  (:policy :fast-safe)
+  (:generator 1
+    (move r x)
+    (inst not r)))
+
+(define-modular-fun logxor-mod32 (x y) logxor 32)
+(define-vop (fast-logxor-mod32/unsigned=>unsigned
+             fast-logxor/unsigned=>unsigned)
+  (:translate logxor-mod32))
+(define-vop (fast-logxor-mod32-c/unsigned=>unsigned
+             fast-logxor-c/unsigned=>unsigned)
+  (:translate logxor-mod32))
+
+(define-source-transform logeqv (&rest args)
+  (if (oddp (length args))
+      `(logxor ,@args)
+      `(lognot (logxor ,@args))))
+(define-source-transform logandc1 (x y)
+  `(logand (lognot ,x) ,y))
+(define-source-transform logandc2 (x y)
+  `(logand ,x (lognot ,y)))
+(define-source-transform logorc1 (x y)
+  `(logior (lognot ,x) ,y))
+(define-source-transform logorc2 (x y)
+  `(logior ,x (lognot ,y)))
+(define-source-transform lognor (x y)
+  `(lognot (logior ,x ,y)))
+(define-source-transform lognand (x y)
+  `(lognot (logand ,x ,y)))
+
 ;;;; bignum stuff
 
 (define-vop (bignum-length get-header-data)
@@ -1202,17 +1339,8 @@
     (move hi edx)
     (move lo eax)))
 
-(define-vop (bignum-lognot)
-  (:translate sb!bignum::%lognot)
-  (:policy :fast-safe)
-  (:args (x :scs (unsigned-reg unsigned-stack) :target r))
-  (:arg-types unsigned-num)
-  (:results (r :scs (unsigned-reg)
-	       :load-if (not (location= x r))))
-  (:result-types unsigned-num)
-  (:generator 1
-    (move r x)
-    (inst not r)))
+(define-vop (bignum-lognot lognot-mod32/unsigned=>unsigned)
+  (:translate sb!bignum::%lognot))
 
 (define-vop (fixnum-to-digit)
   (:translate sb!bignum::%fixnum-to-digit)
@@ -1373,3 +1501,178 @@
     (inst mov tmp y)
     (inst shr tmp 18)
     (inst xor y tmp)))
+
+(in-package "SB!C")
+
+(defknown %lea ((or (signed-byte 32) (unsigned-byte 32))
+		(or (signed-byte 32) (unsigned-byte 32))
+		(member 1 2 4 8) (signed-byte 32))
+  (or (signed-byte 32) (unsigned-byte 32))
+  (foldable flushable))
+
+(defoptimizer (%lea derive-type) ((base index scale disp))
+  (when (and (constant-lvar-p scale)
+	     (constant-lvar-p disp))
+    (let ((scale (lvar-value scale))
+	  (disp (lvar-value disp))
+	  (base-type (lvar-type base))
+	  (index-type (lvar-type index)))
+      (when (and (numeric-type-p base-type)
+		 (numeric-type-p index-type))
+	(let ((base-lo (numeric-type-low base-type))
+	      (base-hi (numeric-type-high base-type))
+	      (index-lo (numeric-type-low index-type))
+	      (index-hi (numeric-type-high index-type)))
+	  (make-numeric-type :class 'integer
+			     :complexp :real
+			     :low (when (and base-lo index-lo)
+				    (+ base-lo (* index-lo scale) disp))
+			     :high (when (and base-hi index-hi)
+				     (+ base-hi (* index-hi scale) disp))))))))
+
+(defun %lea (base index scale disp)
+  (+ base (* index scale) disp))
+
+(in-package "SB!VM")
+
+(define-vop (%lea/unsigned=>unsigned)
+  (:translate %lea)
+  (:policy :fast-safe)
+  (:args (base :scs (unsigned-reg))
+	 (index :scs (unsigned-reg)))
+  (:info scale disp)
+  (:arg-types unsigned-num unsigned-num
+	      (:constant (member 1 2 4 8))
+	      (:constant (signed-byte 32)))
+  (:results (r :scs (unsigned-reg)))
+  (:result-types unsigned-num)
+  (:generator 5
+    (inst lea r (make-ea :dword :base base :index index
+			 :scale scale :disp disp))))
+
+(define-vop (%lea/signed=>signed)
+  (:translate %lea)
+  (:policy :fast-safe)
+  (:args (base :scs (signed-reg))
+	 (index :scs (signed-reg)))
+  (:info scale disp)
+  (:arg-types signed-num signed-num
+	      (:constant (member 1 2 4 8))
+	      (:constant (signed-byte 32)))
+  (:results (r :scs (signed-reg)))
+  (:result-types signed-num)
+  (:generator 4
+    (inst lea r (make-ea :dword :base base :index index
+			 :scale scale :disp disp))))
+
+(define-vop (%lea/fixnum=>fixnum)
+  (:translate %lea)
+  (:policy :fast-safe)
+  (:args (base :scs (any-reg))
+	 (index :scs (any-reg)))
+  (:info scale disp)
+  (:arg-types tagged-num tagged-num
+	      (:constant (member 1 2 4 8))
+	      (:constant (signed-byte 32)))
+  (:results (r :scs (any-reg)))
+  (:result-types tagged-num)
+  (:generator 3
+    (inst lea r (make-ea :dword :base base :index index
+			 :scale scale :disp disp))))
+
+(in-package "SB!C")
+
+;;; This is essentially a straight implementation of the algorithm in
+;;; "Strength Reduction of Multiplications by Integer Constants",
+;;; Youfeng Wu, ACM SIGPLAN Notices, Vol. 30, No.2, February 1995.
+(defun basic-decompose-multiplication (arg num n-bits condensed)
+  (case (aref condensed 0)
+    (0
+     (let ((tmp (min 3 (aref condensed 1))))
+       (decf (aref condensed 1) tmp)
+       `(logand #xffffffff
+	 (%lea ,arg
+	       ,(decompose-multiplication
+		 arg (ash (1- num) (- tmp)) (1- n-bits) (subseq condensed 1))
+	       ,(ash 1 tmp) 0))))
+    ((1 2 3)
+     (let ((r0 (aref condensed 0)))
+       (incf (aref condensed 1) r0)
+       `(logand #xffffffff
+	 (%lea ,(decompose-multiplication
+		 arg (- num (ash 1 r0)) (1- n-bits) (subseq condensed 1))
+	       ,arg
+	       ,(ash 1 r0) 0))))
+    (t (let ((r0 (aref condensed 0)))
+	 (setf (aref condensed 0) 0)
+	 `(logand #xffffffff
+	   (ash ,(decompose-multiplication
+		  arg (ash num (- r0)) n-bits condensed)
+	        ,r0))))))
+
+(defun decompose-multiplication (arg num n-bits condensed)
+  (cond
+    ((= n-bits 0) 0)
+    ((= num 1) arg)
+    ((= n-bits 1)
+     `(logand #xffffffff (ash ,arg ,(1- (integer-length num)))))
+    ((let ((max 0) (end 0))
+       (loop for i from 2 to (length condensed)
+	     for j = (reduce #'+ (subseq condensed 0 i))
+	     when (and (> (- (* 2 i) 3 j) max)
+		       (< (+ (ash 1 (1+ j))
+			     (ash (ldb (byte (- 32 (1+ j)) (1+ j)) num)
+				  (1+ j)))
+			  (ash 1 32)))
+	       do (setq max (- (* 2 i) 3 j)
+			end i))
+       (when (> max 0)
+	 (let ((j (reduce #'+ (subseq condensed 0 end))))
+	   (let ((n2 (+ (ash 1 (1+ j))
+			(ash (ldb (byte (- 32 (1+ j)) (1+ j)) num) (1+ j))))
+		 (n1 (1+ (ldb (byte (1+ j) 0) (lognot num)))))
+	   `(logand #xffffffff
+	     (- ,(optimize-multiply arg n2) ,(optimize-multiply arg n1))))))))
+    ((dolist (i '(9 5 3))
+       (when (integerp (/ num i))
+	 (when (< (logcount (/ num i)) (logcount num))
+	   (let ((x (gensym)))
+	     (return `(let ((,x ,(optimize-multiply arg (/ num i))))
+		       (logand #xffffffff
+			(%lea ,x ,x (1- ,i) 0)))))))))
+    (t (basic-decompose-multiplication arg num n-bits condensed))))
+	   
+(defun optimize-multiply (arg x)
+  (let* ((n-bits (logcount x))
+	 (condensed (make-array n-bits)))
+    (let ((count 0) (bit 0))
+      (dotimes (i 32)
+	(cond ((logbitp i x)
+	       (setf (aref condensed bit) count)
+	       (setf count 1)
+	       (incf bit))
+	      (t (incf count)))))
+    (decompose-multiplication arg x n-bits condensed)))
+
+(deftransform * ((x y)
+		 ((unsigned-byte 32) (constant-arg (unsigned-byte 32)))
+		 (unsigned-byte 32))
+  "recode as leas, shifts and adds"
+  (let ((y (lvar-value y)))
+    (cond
+      ((= y (ash 1 (integer-length y)))
+       ;; there's a generic transform for y = 2^k
+       (give-up-ir1-transform))
+      ((member y '(3 5 9))
+       ;; we can do these multiplications directly using LEA
+       `(%lea x x ,(1- y) 0))
+      ((member :pentium4 *backend-subfeatures*)
+       ;; the pentium4's multiply unit is reportedly very good
+       (give-up-ir1-transform))
+      ;; FIXME: should make this more fine-grained.  If nothing else,
+      ;; there should probably be a cutoff of about 9 instructions on
+      ;; pentium-class machines.
+      (t (optimize-multiply 'x y)))))
+
+;;; FIXME: we should also be able to write an optimizer or two to
+;;; convert (+ (* x 2) 17), (- (* x 9) 5) to a %LEA.