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unnecessary code generation

Heinz Saathoff

Hello,

I'm relatively new in using Keil C166 development system. First tests are successfully so far using the Keil MCB167NET board. As a long time C programmer I'm interested in the quality of generated code. I noticed that the generated code sometimes has unnecessary code lines that I thought an optimizer should be able to find. Here is a example:

#include <C167CS.H>
#include "comtype.h"
#include "timer.h"
void timer_init()
{  struct T01_CON {
      uint  T0L :3;
      uint  T0M :1;
      uint  un00:2;
      uint  T0R :1;
      uint  un01:1;
      uint  T1L :3;
      uint  T1M :1;
      uint  un10:2;
      uint  T1R :1;
      uint  un11:1;
   };
   union {
      struct T01_CON tcon;
      uint   reg;
   } u;
   u.reg = T01CON;
   u.tcon.T0L = 2;    // FCPU / 2^(3+T0L) = 2 µs at 16MHz
   u.tcon.T0M = 0;    // Timer Mode
   u.tcon.T0R = 1;    // run Timer
   T01CON = u.reg;
}//timer_init
The genearated code looks for speed optimizer, Level 7 is:
   ; FUNCTION timer_init (BEGIN  RMASK = @0x4030)
0000 2802          SUB       R0,#02H
0002 F2F550FF      MOV       R5,T01CON
0006 B850          MOV       [R0],R5       ; u
0008 A840          MOV       R4,[R0]       ; u
000A 0AF54F42      BFLDL     R5,#04FH,#042H
000E B850          MOV       [R0],R5       ; u
0010 A840          MOV       R4,[R0]       ; u
0012 F6F550FF      MOV       T01CON,R5
0016 0802          ADD       R0,#02H
0018 CB00          RET
   ; FUNCTION timer_init (END    RMASK = @0x4030)

At first I see that at Offset 0008 and 0010 the mov to R4 is unnecessary because R4 is never read again. Removing this moves a second look noticed that also the moves at offset 0006 and 000E are unnecessary.

Should this kind of optimization be doable by a peephole optimizer or by data flow analysis?

NB: This is not critical, just as a hint for future improvements.
The most important thing with optimizers is still the correctness of generated code.

- Heinz (from Delmenhorst, Germany)

Parents
  • 0 in reply to Jon Ward

    A question that needs to be answered is, how often do developers need a dividend and remainder from the same quotient and divisor?

    My two cents: pretty often!

    For one example, it's very common for me to run into a situation where I have some sort of table in a series of registers, where the individual items are less than the full register width. Let's say you have 16 items, two bits wide, 4 items per byte, thus taking up 4 bytes of address space in total, thus:

    
addr  item num
----  --------
0000: 3 2 1 0
0001: 7 6 5 4
0002: b a 9 8
0003: f d e c


    To access item n, you need code along the lines of:

    Item GetItem (int n)
{
index = n / ItemsPerWord;
offset = n % ItemsPerWord;

return (*(base + index) >> offset) & mask;
}

    With most designs, the div and mod should strength-reduce to shifts and masks because they're powers of two.

    I also find it moderately common any time I need a division to also need the remainder for any sort of fraction where I want to avoid floating point.

Reply
  • 0 in reply to Jon Ward

    A question that needs to be answered is, how often do developers need a dividend and remainder from the same quotient and divisor?

    My two cents: pretty often!

    For one example, it's very common for me to run into a situation where I have some sort of table in a series of registers, where the individual items are less than the full register width. Let's say you have 16 items, two bits wide, 4 items per byte, thus taking up 4 bytes of address space in total, thus:

    
addr  item num
----  --------
0000: 3 2 1 0
0001: 7 6 5 4
0002: b a 9 8
0003: f d e c


    To access item n, you need code along the lines of:

    Item GetItem (int n)
{
index = n / ItemsPerWord;
offset = n % ItemsPerWord;

return (*(base + index) >> offset) & mask;
}

    With most designs, the div and mod should strength-reduce to shifts and masks because they're powers of two.

    I also find it moderately common any time I need a division to also need the remainder for any sort of fraction where I want to avoid floating point.

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