A road named NoNameUno

Producer consumer approach is very popular in network driver design. Transmit and Receive descriptors are usually laid out in physical pages for shared memory between host and device. And there is a producer index and a consumer index. To give it a ring structure, these indices are incremented modulo the size of the total memory each of those descriptor table takes. So indices wrap around. Depending on the device memory available, and the size of the descriptor structure we can probably use the native types like unsigned int_16 or something so that every time we increase the indices we don't need to check for wrapping around. But 16bit is 64K, so if the structure size is say 64 bytes, we need about 1K of descriptor before the indices wrap around. But if the device provides, say, 16KB of memory for this, then one option is to check after each increment of an index for possible wrap around. Well, this seems like an extra burden in high performance interrupt path. So the alternative would be to define a bit field that would wrap automatically after 16KB of addresses. So we can avoid the check for wrap around, and might have a performance boost. But if we look thru the microscope, we might have different conclusion. In the test and set case, there is hardly any branch hazard if you worry about it. Think about it! By looking at the code generated by gcc on an example case, I was surprised to see that the bit field approach is indeed slower. Take a look!

GAS LISTING /tmp/ccF3PvQX.s page 1


   1              .file "sundry.c"
   4              .text
   5              Ltext0:
 211              .section .rdata,"dr"
 212              LC0:
 213 0000 6D796269 .ascii "mybitfield.mytestfield is:%d\0"
 213      74666965
 213      6C642E6D
 213      79746573
 213      74666965
 214 001d 000000   .text
 216              .globl _bitfield_0
 218              _bitfield_0:
   1:sundry.c      **** /*
   2:sundry.c      ****  * sundry.c
   3:sundry.c      ****  *
   4:sundry.c      ****  *  Created on: Sep 15, 2010
   5:sundry.c      ****  *      Author: Prokash Sinha
   6:sundry.c      ****  */
   7:sundry.c      **** #include <stdio.h>
   8:sundry.c      ****
   9:sundry.c      **** void bitfield_0()
  10:sundry.c      **** {
 220              LM1:
 221 0000 55       pushl %ebp
 222 0001 89E5     movl %esp, %ebp
 223 0003 83EC18   subl $24, %esp
  11:sundry.c      ****
  12:sundry.c      ****         unsigned int uintvar =0;
 225              LM2:
 226 0006 C745FC00 movl $0, -4(%ebp)
 226      000000
  13:sundry.c      ****
  14:sundry.c      **** struct {
  15:sundry.c      **** unsigned int : 0;
  16:sundry.c      **** unsigned int mytestfield: 4;
  17:sundry.c      **** unsigned int pad: 28;
  18:sundry.c      **** }mybitfield;
  19:sundry.c      ****
  20:sundry.c      **** mybitfield.mytestfield = 7;
 228              LM3:
 229 000d 8B45F8   movl -8(%ebp), %eax
 230 0010 83E0F0   andl $-16, %eax
 231 0013 83C807   orl $7, %eax
 232 0016 8945F8   movl %eax, -8(%ebp)
  21:sundry.c      **** printf("mybitfield.mytestfield is:%d", mybitfield.mytestfield);
 234              LM4:
 235 0019 0FB645F8 movzbl -8(%ebp), %eax
 236 001d 83E00F   andl $15, %eax
 237 0020 89442404 movl %eax, 4(%esp)
 238 0024 C7042400 movl $LC0, (%esp)
 238      000000
 239 002b E8000000 call _printf
 239      00
  22:sundry.c      ****
  23:sundry.c      ****         /*show that inc beyond the width makes it zero */
  24:sundry.c      **** mybitfield.mytestfield = 16;

GAS LISTING /tmp/ccF3PvQX.s page 2


 241              LM5:
 242 0030 8B45F8   movl -8(%ebp), %eax
 243 0033 83E0F0   andl $-16, %eax
 244 0036 8945F8   movl %eax, -8(%ebp)
  25:sundry.c      **** printf("mybitfield.mytestfield is:%d", mybitfield.mytestfield);
 246              LM6:
 247 0039 0FB645F8 movzbl -8(%ebp), %eax
 248 003d 83E00F   andl $15, %eax
 249 0040 89442404 movl %eax, 4(%esp)
 250 0044 C7042400 movl $LC0, (%esp)
 250      000000
 251 004b E8000000 call _printf
 251      00
  26:sundry.c      ****
  27:sundry.c      ****         /*show just the instructions to inc w/o worry for wrapping */
  28:sundry.c      **** mybitfield.mytestfield++ ;
 253              LM7:
 254 0050 0FB645F8 movzbl -8(%ebp), %eax
 255 0054 83E00F   andl $15, %eax
 256 0057 8D5001   leal 1(%eax), %edx
 257 005a 8B45F8   movl -8(%ebp), %eax
 258 005d 83E20F   andl $15, %edx
 259 0060 83E0F0   andl $-16, %eax
 260 0063 09D0     orl %edx, %eax
 261 0065 8945F8   movl %eax, -8(%ebp)
  29:sundry.c      ****
  30:sundry.c      ****         /* show how many instruction it generates */
  31:sundry.c      ****         uintvar++;
 263              LM8:
 264 0068 8D45FC   leal -4(%ebp), %eax
 265 006b FF00     incl (%eax)
  32:sundry.c      ****         if ( uintvar > 4000000)
 267              LM9:
 268 006d 817DFC00 cmpl $4000000, -4(%ebp)
 268      093D00
 269 0074 7607     jbe L1
  33:sundry.c      ****              uintvar = 0;
 271              LM10:
 272 0076 C745FC00 movl $0, -4(%ebp)
 272      000000
 273              L1:
  34:sundry.c      ****
  35:sundry.c      **** }
 275              LM11:
 276 007d C9       leave
 277 007e C3       ret
 282              Lscope0:
 286              .globl _main
 288              _main:
  36:sundry.c      ****
  37:sundry.c      **** void main (void)
  38:sundry.c      **** {
 290              LM12:
 291 007f 55       pushl %ebp
 292 0080 89E5     movl %esp, %ebp
 293 0082 83EC08   subl $8, %esp
 294 0085 83E4F0   andl $-16, %esp

GAS LISTING /tmp/ccF3PvQX.s page 3


 295 0088 B8000000 movl $0, %eax
 295      00
 296 008d 83C00F   addl $15, %eax
 297 0090 83C00F   addl $15, %eax
 298 0093 C1E804   shrl $4, %eax
 299 0096 C1E004   sall $4, %eax
 300 0099 8945FC   movl %eax, -4(%ebp)
 301 009c 8B45FC   movl -4(%ebp), %eax
 302 009f E8000000 call __alloca
 302      00
 304              LM13:
 305 00a4 E8000000 call ___main
 305      00
  39:sundry.c      **** bitfield_0();
 307              LM14:
 308 00a9 E852FFFF call _bitfield_0
 308      FF
  40:sundry.c      **** }
 310              LM15:
 311 00ae C9       leave
 312 00af C3       ret
 313              Lscope1:
 315              .text
 317              Letext:

GAS LISTING /tmp/ccF3PvQX.s page 4


DEFINED SYMBOLS
                            *ABS*:00000000 sundry.c
     /tmp/ccF3PvQX.s:218    .text:00000000 _bitfield_0
     /tmp/ccF3PvQX.s:288    .text:0000007f _main

UNDEFINED SYMBOLS
___main
__alloca
_printf