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ARM7TDMI: SUBS vs SUB + CMP

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

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Peter Harris over 12 years ago +1 verified

Note: This was originally posted on 6th April 2009 at http://forums.arm.com I'd highly recommend the ARM System Developer's Guide ( http://www.arm.com/documentation/books/4975.html ) - it covers lots of...

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+1 Peter Harris over 12 years ago

Note: This was originally posted on 6th April 2009 at http://forums.arm.com

I'd highly recommend the ARM System Developer's Guide (http://www.arm.com/documentation/books/4975.html) - it covers lots of useful stuff from algorithms to operating system techniques.

It is slightly out of date now - it mostly covers ARMv5T(E) which is ARM, Thumb and ARM 'E' DSP instructions. This is all relevant to newer processors too - but if you expect to be working on ARM11 (ARMv6) or any of the Cortex (ARMv7)processors, expect minor differences and additions (such as the Thumb2 instruction set).
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+1 Peter Harris over 12 years ago

Note: This was originally posted on 6th April 2009 at http://forums.arm.com

I'd highly recommend the ARM System Developer's Guide (http://www.arm.com/documentation/books/4975.html) - it covers lots of useful stuff from algorithms to operating system techniques.

It is slightly out of date now - it mostly covers ARMv5T(E) which is ARM, Thumb and ARM 'E' DSP instructions. This is all relevant to newer processors too - but if you expect to be working on ARM11 (ARMv6) or any of the Cortex (ARMv7)processors, expect minor differences and additions (such as the Thumb2 instruction set).
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