reimplements clz using the branchless/loopless algorithm (#1423)

Inspired by Benjamin Mourad (@bmourad01) implementation in #1416.

It is not only efficient but also helps with bap-veri-fication, as the
latter do not support while loops (which is used in the previous
implementation).

lib/arm/arm_lifter.ml

@@ -335,18 +335,6 @@ let lift_bits mem ops (insn : bits_insn ) =
                    extract 7 0 s ^
                    extract 15 8 s) in
     exec [assn (Env.of_reg dest) rev] cond
-  | `CLZ, [|`Reg dest; src; cond; _|] ->
-    let shift = tmp ~name:"shift" reg32_t in
-    let accum = tmp ~name:"accum" reg32_t in
-    Bil.(exec [
-        shift := exp_of_op src;
-        accum := int32 32;
-        while_ (var shift <> int32 0) [
-          shift := var shift lsr int32 1;
-          accum := var accum - int32 1;
-        ];
-        Env.of_reg dest := var accum;
-      ]) cond
   | insn,ops ->
     fail [%here] "ops %s doesn't match bits insn %s"
       (string_of_ops ops) (Arm_insn.to_string (insn :> insn))

lib/arm/arm_types.ml

@@ -159,7 +159,6 @@ type bits_insn = [
   | `UXTH
   | `REV
   | `REV16
-  | `CLZ
 ] [@@deriving bin_io, compare, sexp, enumerate]
 
 type mult_insn = [

plugins/arm/semantics/arm.lisp

@@ -0,0 +1,9 @@
+(declare  (context (target arm)))
+(defpackage llvm-armv7 (:use arm))
+(in-package arm)
+
+(require bits)
+
+(defun CLZ (rd rn pre _)
+  (when (condition-holds pre)
+    (set$ rd (clz32 rn))))

plugins/primus_lisp/semantics/bits.lisp

@@ -21,3 +21,61 @@
    in two's complement overflow."
   (logor (logand (msb rn) (msb rm) (lnot (msb rd)))
          (logand (lnot (msb rn)) (lnot (msb rm)) (msb rd))))
+
+(defmacro popcount/helper (x sh m1 m2 m4 h01)
+  (prog
+     (set x (- x (logand (rshift x 1) m1)))
+     (set x (+ (logand x m2) (logand (rshift x 2) m2)))
+     (set x (logand (+ x (rshift x 4)) m4))
+     (rshift (* x h01) sh)))
+
+(defmacro popcount16 (x)
+  (popcount/helper x 8
+                   0x5555
+                   0x3333
+                   0x0f0f
+                   0x0101))
+
+(defmacro popcount32 (x)
+  (popcount/helper x 24
+                   0x55555555
+                   0x33333333
+                   0x0f0f0f0f
+                   0x01010101))
+
+(defmacro popcount64 (x)
+  (popcount/helper x 56
+                   0x5555555555555555
+                   0x3333333333333333
+                   0x0f0f0f0f0f0f0f0f
+                   0x0101010101010101))
+
+(defun clz16 (r)
+  (let ((x r))
+    (set x (logor x (rshift x 1)))
+    (set x (logor x (rshift x 2)))
+    (set x (logor x (rshift x 4)))
+    (set x (logor x (rshift x 8)))
+    (set x (lnot x))
+    (popcount16 x)))
+
+(defun clz32 (x)
+  (let ((x x))
+    (set x (logor x (rshift x 1)))
+    (set x (logor x (rshift x 2)))
+    (set x (logor x (rshift x 4)))
+    (set x (logor x (rshift x 8)))
+    (set x (logor x (rshift x 16)))
+    (set x (lnot x))
+    (popcount32 x)))
+
+(defun clz64 (x)
+  (let ((x x))
+    (set x (logor x (rshift x 1)))
+    (set x (logor x (rshift x 2)))
+    (set x (logor x (rshift x 4)))
+    (set x (logor x (rshift x 8)))
+    (set x (logor x (rshift x 16)))
+    (set x (logor x (rshift x 32)))
+    (set x (lnot x))
+    (popcount64 x)))