How to find the absolute value of an integer in Two's complement in ARM
2
If I have -2 (11111111111111111111111111111110), is there a neat ARM instruction or a series of such that will make it (00000000000000000000000000000010). OR or XOR would not work from what I have tried since I loose the 30th bit.
Thank you
assembly arm twos-complement absolute-value
edited Nov 15 '18 at 15:52
Peter Cordes
129k18194329
asked Nov 15 '18 at 1:59
mdrjjnmdrjjn
487
add a comment |
2
If I have -2 (11111111111111111111111111111110), is there a neat ARM instruction or a series of such that will make it (00000000000000000000000000000010). OR or XOR would not work from what I have tried since I loose the 30th bit.
Thank you
assembly arm twos-complement absolute-value
edited Nov 15 '18 at 15:52
Peter Cordes
129k18194329
asked Nov 15 '18 at 1:59
mdrjjnmdrjjn
487
1
ARM's negate instruction isrsb dst, src, #0(reverse-subtract). You cantst/ predicated-rsb. I think that's what C++ compilers will do when inlining / optimizingstd::abs, but you should try compiling anabsfunction with optimization enabled.
– Peter Cordes
Nov 15 '18 at 2:21
which arm instruction set? did you look at NEG? you do need the compare as shown in the checked answer...
– old_timer
Nov 15 '18 at 15:48
add a comment |
2
2
2
If I have -2 (11111111111111111111111111111110), is there a neat ARM instruction or a series of such that will make it (00000000000000000000000000000010). OR or XOR would not work from what I have tried since I loose the 30th bit.
Thank you
assembly arm twos-complement absolute-value
edited Nov 15 '18 at 15:52
Peter Cordes
129k18194329
asked Nov 15 '18 at 1:59
mdrjjnmdrjjn
487
If I have -2 (11111111111111111111111111111110), is there a neat ARM instruction or a series of such that will make it (00000000000000000000000000000010). OR or XOR would not work from what I have tried since I loose the 30th bit.
Thank you
assembly arm twos-complement absolute-value
assembly arm twos-complement absolute-value
edited Nov 15 '18 at 15:52
Peter Cordes
129k18194329
asked Nov 15 '18 at 1:59
mdrjjnmdrjjn
487
edited Nov 15 '18 at 15:52
Peter Cordes
129k18194329
asked Nov 15 '18 at 1:59
mdrjjnmdrjjn
487
edited Nov 15 '18 at 15:52
Peter Cordes
129k18194329
edited Nov 15 '18 at 15:52
Peter Cordes
129k18194329
edited Nov 15 '18 at 15:52
Peter Cordes
129k18194329
129k18194329
asked Nov 15 '18 at 1:59
mdrjjnmdrjjn
487
asked Nov 15 '18 at 1:59
mdrjjnmdrjjn
487
asked Nov 15 '18 at 1:59
mdrjjnmdrjjn
487
487
1
ARM's negate instruction isrsb dst, src, #0(reverse-subtract). You cantst/ predicated-rsb. I think that's what C++ compilers will do when inlining / optimizingstd::abs, but you should try compiling anabsfunction with optimization enabled.
– Peter Cordes
Nov 15 '18 at 2:21
which arm instruction set? did you look at NEG? you do need the compare as shown in the checked answer...
– old_timer
Nov 15 '18 at 15:48
add a comment |
1
ARM's negate instruction isrsb dst, src, #0(reverse-subtract). You cantst/ predicated-rsb. I think that's what C++ compilers will do when inlining / optimizingstd::abs, but you should try compiling anabsfunction with optimization enabled.
– Peter Cordes
Nov 15 '18 at 2:21
which arm instruction set? did you look at NEG? you do need the compare as shown in the checked answer...
– old_timer
Nov 15 '18 at 15:48
1
1
ARM's negate instruction is
rsb dst, src, #0 (reverse-subtract). You can tst / predicated-rsb. I think that's what C++ compilers will do when inlining / optimizing std::abs, but you should try compiling an abs function with optimization enabled.– Peter Cordes
Nov 15 '18 at 2:21
ARM's negate instruction is
rsb dst, src, #0 (reverse-subtract). You can tst / predicated-rsb. I think that's what C++ compilers will do when inlining / optimizing std::abs, but you should try compiling an abs function with optimization enabled.– Peter Cordes
Nov 15 '18 at 2:21
which arm instruction set? did you look at NEG? you do need the compare as shown in the checked answer...
– old_timer
Nov 15 '18 at 15:48
which arm instruction set? did you look at NEG? you do need the compare as shown in the checked answer...
– old_timer
Nov 15 '18 at 15:48
add a comment |
1 Answer
1
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4
For finding the absolute value of an integer, use a comparison and a subtraction.
@ input in r0
cmp r0, #0 @ is r0 < 0?
rsbmi r0, r0 #0 @ if yes, r0 = 0 - r0
answered Nov 15 '18 at 7:13
fuzfuz
58.5k14127258
mvna bitwise NOT, ones-complement inversion rather than two's complement. infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204j/…. Like I commented, you can usersbmi r0, #0
– Peter Cordes
Nov 15 '18 at 11:36
@PeterCordes Clearly you are right. I though for a second thatmvnstood for “move negated.” That said, did you get my email?
– fuz
Nov 15 '18 at 11:39
I did, thanks for the reminder. I should have some time to look at that today.
– Peter Cordes
Nov 15 '18 at 11:43
@PeterCordes I'm looking forwards to your comments!
– fuz
Nov 15 '18 at 11:44
2
Worth noting that if you're using an ARMv7 or ARMv8 device, you will be using the Thumb-2 instruction set which does not include conditional execution of non-branch instructions. You will need anit mibefore thersbmi. Many assemblers will insert this for you, though some will warn because you're getting an instruction you didn't explicitly request. See community.arm.com/processors/b/blog/posts/…
– cooperised
Nov 15 '18 at 13:44
add a comment |
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1 Answer
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1 Answer
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oldest
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oldest
votes
4
For finding the absolute value of an integer, use a comparison and a subtraction.
@ input in r0
cmp r0, #0 @ is r0 < 0?
rsbmi r0, r0 #0 @ if yes, r0 = 0 - r0
answered Nov 15 '18 at 7:13
fuzfuz
58.5k14127258
mvna bitwise NOT, ones-complement inversion rather than two's complement. infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204j/…. Like I commented, you can usersbmi r0, #0
– Peter Cordes
Nov 15 '18 at 11:36
@PeterCordes Clearly you are right. I though for a second thatmvnstood for “move negated.” That said, did you get my email?
– fuz
Nov 15 '18 at 11:39
I did, thanks for the reminder. I should have some time to look at that today.
– Peter Cordes
Nov 15 '18 at 11:43
@PeterCordes I'm looking forwards to your comments!
– fuz
Nov 15 '18 at 11:44
2
Worth noting that if you're using an ARMv7 or ARMv8 device, you will be using the Thumb-2 instruction set which does not include conditional execution of non-branch instructions. You will need anit mibefore thersbmi. Many assemblers will insert this for you, though some will warn because you're getting an instruction you didn't explicitly request. See community.arm.com/processors/b/blog/posts/…
– cooperised
Nov 15 '18 at 13:44
add a comment |
4
For finding the absolute value of an integer, use a comparison and a subtraction.
@ input in r0
cmp r0, #0 @ is r0 < 0?
rsbmi r0, r0 #0 @ if yes, r0 = 0 - r0
answered Nov 15 '18 at 7:13
fuzfuz
58.5k14127258
mvna bitwise NOT, ones-complement inversion rather than two's complement. infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204j/…. Like I commented, you can usersbmi r0, #0
– Peter Cordes
Nov 15 '18 at 11:36
@PeterCordes Clearly you are right. I though for a second thatmvnstood for “move negated.” That said, did you get my email?
– fuz
Nov 15 '18 at 11:39
I did, thanks for the reminder. I should have some time to look at that today.
– Peter Cordes
Nov 15 '18 at 11:43
@PeterCordes I'm looking forwards to your comments!
– fuz
Nov 15 '18 at 11:44
2
Worth noting that if you're using an ARMv7 or ARMv8 device, you will be using the Thumb-2 instruction set which does not include conditional execution of non-branch instructions. You will need anit mibefore thersbmi. Many assemblers will insert this for you, though some will warn because you're getting an instruction you didn't explicitly request. See community.arm.com/processors/b/blog/posts/…
– cooperised
Nov 15 '18 at 13:44
add a comment |
4
4
4
For finding the absolute value of an integer, use a comparison and a subtraction.
@ input in r0
cmp r0, #0 @ is r0 < 0?
rsbmi r0, r0 #0 @ if yes, r0 = 0 - r0
answered Nov 15 '18 at 7:13
fuzfuz
58.5k14127258
For finding the absolute value of an integer, use a comparison and a subtraction.
@ input in r0
cmp r0, #0 @ is r0 < 0?
rsbmi r0, r0 #0 @ if yes, r0 = 0 - r0
answered Nov 15 '18 at 7:13
fuzfuz
58.5k14127258
edited Nov 15 '18 at 14:09
answered Nov 15 '18 at 7:13
fuzfuz
58.5k14127258
answered Nov 15 '18 at 7:13
fuzfuz
58.5k14127258
answered Nov 15 '18 at 7:13
fuzfuz
58.5k14127258
58.5k14127258
mvna bitwise NOT, ones-complement inversion rather than two's complement. infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204j/…. Like I commented, you can usersbmi r0, #0
– Peter Cordes
Nov 15 '18 at 11:36
@PeterCordes Clearly you are right. I though for a second thatmvnstood for “move negated.” That said, did you get my email?
– fuz
Nov 15 '18 at 11:39
I did, thanks for the reminder. I should have some time to look at that today.
– Peter Cordes
Nov 15 '18 at 11:43
@PeterCordes I'm looking forwards to your comments!
– fuz
Nov 15 '18 at 11:44
2
Worth noting that if you're using an ARMv7 or ARMv8 device, you will be using the Thumb-2 instruction set which does not include conditional execution of non-branch instructions. You will need anit mibefore thersbmi. Many assemblers will insert this for you, though some will warn because you're getting an instruction you didn't explicitly request. See community.arm.com/processors/b/blog/posts/…
– cooperised
Nov 15 '18 at 13:44
add a comment |
mvna bitwise NOT, ones-complement inversion rather than two's complement. infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204j/…. Like I commented, you can usersbmi r0, #0
– Peter Cordes
Nov 15 '18 at 11:36
@PeterCordes Clearly you are right. I though for a second thatmvnstood for “move negated.” That said, did you get my email?
– fuz
Nov 15 '18 at 11:39
I did, thanks for the reminder. I should have some time to look at that today.
– Peter Cordes
Nov 15 '18 at 11:43
@PeterCordes I'm looking forwards to your comments!
– fuz
Nov 15 '18 at 11:44
2
Worth noting that if you're using an ARMv7 or ARMv8 device, you will be using the Thumb-2 instruction set which does not include conditional execution of non-branch instructions. You will need anit mibefore thersbmi. Many assemblers will insert this for you, though some will warn because you're getting an instruction you didn't explicitly request. See community.arm.com/processors/b/blog/posts/…
– cooperised
Nov 15 '18 at 13:44
mvn a bitwise NOT, ones-complement inversion rather than two's complement. infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204j/…. Like I commented, you can use rsbmi r0, #0– Peter Cordes
Nov 15 '18 at 11:36
mvn a bitwise NOT, ones-complement inversion rather than two's complement. infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204j/…. Like I commented, you can use rsbmi r0, #0– Peter Cordes
Nov 15 '18 at 11:36
@PeterCordes Clearly you are right. I though for a second that
mvn stood for “move negated.” That said, did you get my email?– fuz
Nov 15 '18 at 11:39
@PeterCordes Clearly you are right. I though for a second that
mvn stood for “move negated.” That said, did you get my email?– fuz
Nov 15 '18 at 11:39
I did, thanks for the reminder. I should have some time to look at that today.
– Peter Cordes
Nov 15 '18 at 11:43
I did, thanks for the reminder. I should have some time to look at that today.
– Peter Cordes
Nov 15 '18 at 11:43
@PeterCordes I'm looking forwards to your comments!
– fuz
Nov 15 '18 at 11:44
@PeterCordes I'm looking forwards to your comments!
– fuz
Nov 15 '18 at 11:44
2
2
Worth noting that if you're using an ARMv7 or ARMv8 device, you will be using the Thumb-2 instruction set which does not include conditional execution of non-branch instructions. You will need an
it mi before the rsbmi. Many assemblers will insert this for you, though some will warn because you're getting an instruction you didn't explicitly request. See community.arm.com/processors/b/blog/posts/…– cooperised
Nov 15 '18 at 13:44
Worth noting that if you're using an ARMv7 or ARMv8 device, you will be using the Thumb-2 instruction set which does not include conditional execution of non-branch instructions. You will need an
it mi before the rsbmi. Many assemblers will insert this for you, though some will warn because you're getting an instruction you didn't explicitly request. See community.arm.com/processors/b/blog/posts/…– cooperised
Nov 15 '18 at 13:44
add a comment |
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1
ARM's negate instruction is
rsb dst, src, #0(reverse-subtract). You cantst/ predicated-rsb. I think that's what C++ compilers will do when inlining / optimizingstd::abs, but you should try compiling anabsfunction with optimization enabled.– Peter Cordes
Nov 15 '18 at 2:21
which arm instruction set? did you look at NEG? you do need the compare as shown in the checked answer...
– old_timer
Nov 15 '18 at 15:48