here's the C without the `lookup` helper:


/* myloop123 in base64e/scheme in k203 in k199 in k196 in k193 in k190 */
static C_word f_220(C_word t0,C_word t1,C_word t2){
C_word tmp;
C_word t3;
C_word t4;
C_word t5;
C_word t6;
C_word t7;
C_word t8;
C_word t9;
C_word t10;
C_word t11;
C_word t12;
C_word t13;
C_word t14;
C_word t15;
C_word t16;
C_word t17;
C_word t18;
C_word t19;
C_word t20;
C_word t21;
C_word t22;
C_word t23;
C_word t24;
C_word t25;
C_word t26;
C_word t27;
C_word t28;
C_word t29;
C_word t30;
C_word t31;
C_word t32;
C_word t33;
C_word t34;
C_word t35;
C_word t36;
loop:{}
if(C_truep(C_fixnum_less_or_equal_p(t1,((C_word*)t0)[2]))){
t3=C_u_fixnum_plus(t1,C_fix(0));
t4=C_u_i_u8vector_ref(((C_word*)t0)[3],t3);
t5=C_fixnum_shift_left(t4,C_fix(16));
t6=C_u_fixnum_plus(t1,C_fix(1));
t7=C_u_i_u8vector_ref(((C_word*)t0)[3],t6);
t8=C_fixnum_shift_left(t7,C_fix(8));
t9=C_u_fixnum_plus(t1,C_fix(2));
t10=C_u_i_u8vector_ref(((C_word*)t0)[3],t9);
t11=C_fixnum_shift_left(t10,C_fix(0));
t12=C_u_fixnum_plus(t8,t11);
t13=C_u_fixnum_plus(t5,t12);
t14=C_u_fixnum_plus(t2,C_fix(0));
t15=C_fixnum_shift_right(t13,C_fix(-18));
t16=C_u_fixnum_and(t15,C_fix(63));
t17=C_u_i_u8vector_ref(((C_word*)t0)[4],t16);
t18=C_u_i_u8vector_set(((C_word*)t0)[5],t14,t17);
t19=C_u_fixnum_plus(t2,C_fix(1));
t20=C_fixnum_shift_right(t13,C_fix(-12));
t21=C_u_fixnum_and(t20,C_fix(63));
t22=C_u_i_u8vector_ref(((C_word*)t0)[4],t21);
t23=C_u_i_u8vector_set(((C_word*)t0)[5],t19,t22);
t24=C_u_fixnum_plus(t2,C_fix(2));
t25=C_fixnum_shift_right(t13,C_fix(-6));
t26=C_u_fixnum_and(t25,C_fix(63));
t27=C_u_i_u8vector_ref(((C_word*)t0)[4],t26);
t28=C_u_i_u8vector_set(((C_word*)t0)[5],t24,t27);
t29=C_u_fixnum_plus(t2,C_fix(3));
t30=C_fixnum_shift_right(t13,C_fix(0));
t31=C_u_fixnum_and(t30,C_fix(63));
t32=C_u_i_u8vector_ref(((C_word*)t0)[4],t31);
t33=C_u_i_u8vector_set(((C_word*)t0)[5],t29,t32);
t35=C_u_fixnum_plus(t1,C_fix(3));
t36=C_u_fixnum_plus(t2,C_fix(4));
t1=t35;
t2=t36;
goto loop;}
else{
return(t2);}}


nice and tight. with the lookup, an extra C call is introduced: 

static C_word f_220(C_word t0,C_word t1,C_word t2){
C_word tmp;
C_word t3;
C_word t4;
C_word t5;
C_word t6;
C_word t7;
C_word t8;
C_word t9;
C_word t10;
C_word t11;
C_word t12;
C_word t13;
C_word t14;
C_word t15;
C_word t16;
C_word t17;
C_word t18;
C_word t19;
C_word t20;
C_word t21;
C_word t22;
C_word t23;
C_word t24;
C_word t25;
C_word t26;
C_word t27;
C_word t28;
C_word t29;
C_word t30;
C_word t31;
C_word t32;
loop:{}
if(C_truep(C_fixnum_less_or_equal_p(t1,((C_word*)t0)[2]))){
t3=C_u_fixnum_plus(t1,C_fix(0));
t4=C_u_i_u8vector_ref(((C_word*)t0)[3],t3);
t5=C_fixnum_shift_left(t4,C_fix(16));
t6=C_u_fixnum_plus(t1,C_fix(1));
t7=C_u_i_u8vector_ref(((C_word*)t0)[3],t6);
t8=C_fixnum_shift_left(t7,C_fix(8));
t9=C_u_fixnum_plus(t1,C_fix(2));
t10=C_u_i_u8vector_ref(((C_word*)t0)[3],t9);
t11=C_fixnum_shift_left(t10,C_fix(0));
t12=C_u_fixnum_plus(t8,t11);
t13=C_u_fixnum_plus(t5,t12);
t14=C_u_fixnum_plus(t2,C_fix(0));
t15=C_fixnum_shift_right(t13,C_fix(-18));
t16=(
/* /home/klm/prj/chicken-base64/bench64.scm:22: lookup */
  f_206(((C_word*)t0)[4],C_u_fixnum_and(t15,C_fix(63)))
);
t17=C_u_i_u8vector_set(((C_word*)t0)[5],t14,t16);
t18=C_u_fixnum_plus(t2,C_fix(1));
t19=C_fixnum_shift_right(t13,C_fix(-12));
t20=(
/* /home/klm/prj/chicken-base64/bench64.scm:23: lookup */
  f_206(((C_word*)t0)[4],C_u_fixnum_and(t19,C_fix(63)))
);
t21=C_u_i_u8vector_set(((C_word*)t0)[5],t18,t20);
t22=C_u_fixnum_plus(t2,C_fix(2));
t23=C_fixnum_shift_right(t13,C_fix(-6));
t24=(
/* /home/klm/prj/chicken-base64/bench64.scm:24: lookup */
  f_206(((C_word*)t0)[4],C_u_fixnum_and(t23,C_fix(63)))
);
t25=C_u_i_u8vector_set(((C_word*)t0)[5],t22,t24);
t26=C_u_fixnum_plus(t2,C_fix(3));
t27=C_fixnum_shift_right(t13,C_fix(0));
t28=(
/* /home/klm/prj/chicken-base64/bench64.scm:25: lookup */
  f_206(((C_word*)t0)[4],C_u_fixnum_and(t27,C_fix(63)))
);
t29=C_u_i_u8vector_set(((C_word*)t0)[5],t26,t28);
t31=C_u_fixnum_plus(t1,C_fix(3));
t32=C_u_fixnum_plus(t2,C_fix(4));
t1=t31;
t2=t32;
goto loop;}
else{
return(t2);}}

where 


/* lookup in k203 in k199 in k196 in k193 in k190 */
static C_word f_206(C_word t0,C_word t1){
C_word tmp;
C_word t2;{}
return(C_u_i_u8vector_ref(((C_word*)t0)[2],t1));}

this probably causes the 10-30% performance degrade. why isn't the C compiler able to inline this, though? and why isn't CHICKEN?