• State Accepted Answer
  • +1 person also asked this people also asked this
  • Locked Locked
  • Replies 7 replies
  • Subscribers 350 subscribers
  • Views 19846 views
  • Users 0 members are here
2025 survey

We are running a survey to help us improve the experience for all of our members. If you see the survey appear, please take the time to tell us about your experience if you can.

Options
Related
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

ARM7TDMI: SUBS vs SUB + CMP

Greger Greger
Note: This was originally posted on 2nd April 2009 at http://forums.arm.com

Hi,

I have a question regarding the SUBS instruction and how it compares to SUB and CMP (due to unexpected behavior in a C-program).

The original C-code goes like this (all variables are 32 bit signed integers):

    t = a*b - c*c;
    if (t > 0) d = t;

In a particular case (a = 0x80, b = 0x106DCD9, c = 0x4E352501) I get an overflow in a*b as well as in the subtraction. What is puzzling to me is that there's a difference between the the following two assembler versions in a certain simulated environment - and I'm trying to figure out if this is expected or a bug in the simulator:

MUL temp1, c, c
MUL temp2, a, b
SUBS t, temp2, temp1
MOVGT d, t

and

MUL temp1, c, c
MUL temp2, a, b
SUB t, temp2, temp1
CMP t, #0
MOVGT d, t

(These are genereated depending on different compiler settings.)

Any help is greatly appreciated!

Greger
Parents
  • 0 Arm Employee Badge
    Note: This was originally posted on 2nd April 2009 at http://forums.arm.com

    To merge the comparison into the SUBS flag setting would require the ability to perform a condition check for N clear and Z clear, i.e a positive non-zero value; however, no such condition check exists.
    The closest is GT, with V clear, i.e. Z clear and N==V.

    Whether or not this is a compiler bug is a different question; the compiler choosing GT rathen than NE, indicates that "t" is a "signed int", and this may be why this apparently erroneous behaviour is legal -- the C standard has a number of caveats with respect to arithmetic operations exceeding the precision of "signed" integer types and allowing implemenation defined results.

    hth
    s.
Reply
  • 0 Arm Employee Badge
    Note: This was originally posted on 2nd April 2009 at http://forums.arm.com

    To merge the comparison into the SUBS flag setting would require the ability to perform a condition check for N clear and Z clear, i.e a positive non-zero value; however, no such condition check exists.
    The closest is GT, with V clear, i.e. Z clear and N==V.

    Whether or not this is a compiler bug is a different question; the compiler choosing GT rathen than NE, indicates that "t" is a "signed int", and this may be why this apparently erroneous behaviour is legal -- the C standard has a number of caveats with respect to arithmetic operations exceeding the precision of "signed" integer types and allowing implemenation defined results.

    hth
    s.
Children
No data