commit cb5d7e047598bff6d0f1d707a70d9fb1a1c7f0e2

Author: Julian Seward <jseward@acm.org>

Date:   Tue Nov 20 11:46:55 2018 +0100

    VEX/priv/ir_opt.c

    fold_Expr: transform PopCount64(And64(Add64(x,-1),Not64(x))) into CtzNat64(x).

    This is part of the fix for bug 386945.

diff --git a/VEX/priv/ir_opt.c b/VEX/priv/ir_opt.c

index f40870b..23964be 100644

--- a/VEX/priv/ir_opt.c

+++ b/VEX/priv/ir_opt.c

@@ -1377,6 +1377,8 @@ static IRExpr* fold_Expr ( IRExpr** env, IRExpr* e )

    case Iex_Unop:

       /* UNARY ops */

       if (e->Iex.Unop.arg->tag == Iex_Const) {

+

+         /* cases where the arg is a const */

          switch (e->Iex.Unop.op) {

          case Iop_1Uto8:

             e2 = IRExpr_Const(IRConst_U8(toUChar(

@@ -1690,8 +1692,56 @@ static IRExpr* fold_Expr ( IRExpr** env, IRExpr* e )

          default: 

             goto unhandled;

-      }

-      }

+         } // switch (e->Iex.Unop.op)

+

+      } else {

+

+         /* other cases (identities, etc) */

+         switch (e->Iex.Unop.op) {

+         case Iop_PopCount64: {

+            // PopCount64( And64( Add64(x,-1), Not64(x) ) ) ==> CtzNat64(x)

+            // bindings:

+            //   a1:And64( a11:Add64(a111:x,a112:-1), a12:Not64(a121:x) )

+            IRExpr* a1 = chase(env, e->Iex.Unop.arg);

+            if (!a1)

+               goto nomatch;

+            if (a1->tag != Iex_Binop || a1->Iex.Binop.op != Iop_And64)

+               goto nomatch;

+            // a1 is established

+            IRExpr* a11 = chase(env, a1->Iex.Binop.arg1);

+            if (!a11)

+               goto nomatch;

+            if (a11->tag != Iex_Binop || a11->Iex.Binop.op != Iop_Add64)

+               goto nomatch;

+            // a11 is established

+            IRExpr* a12 = chase(env, a1->Iex.Binop.arg2);

+            if (!a12)

+               goto nomatch;

+            if (a12->tag != Iex_Unop || a12->Iex.Unop.op != Iop_Not64)

+               goto nomatch;

+            // a12 is established

+            IRExpr* a111 = a11->Iex.Binop.arg1;

+            IRExpr* a112 = chase(env, a11->Iex.Binop.arg2);

+            IRExpr* a121 = a12->Iex.Unop.arg;

+            if (!a111 || !a112 || !a121)

+               goto nomatch;

+            // a111 and a121 need to be the same temp.

+            if (!eqIRAtom(a111, a121))

+               goto nomatch;

+            // Finally, a112 must be a 64-bit version of -1.

+            if (!isOnesU(a112))

+               goto nomatch;

+            // Match established.  Transform.

+            e2 = IRExpr_Unop(Iop_CtzNat64, a111);

+            break;

+           nomatch:

+            break;

+         }

+         default:

+            break;

+         } // switch (e->Iex.Unop.op)

+

+      } // if (e->Iex.Unop.arg->tag == Iex_Const)

       break;

    case Iex_Binop: