What are _mm256_testc_pd, _mm256_testz_pd, _mm256_testnzc_pd for?
I am trying to understand the _mm256_testc_pd, _mm256_testz_pd, and _mm256_testnzc_pd intrinsics, and I have a hard time understanding them.
To analyze _mm256_testc_pd, I have identified the following cases (a is the first, b the second __mm256d argument`):
- If all packed doubles in
bare > 0, thenZF=1,CF=1, except: - If one packed double in each
aandbare mutally < 0, thenZF=0. - If one packed double in each
!aandbare mutually < 0, thenCF=0.
In other words, a value of ZF=1 tells me that either a) b is entirely positive, or b) that for those doubles in b that are not positive, there is a matching double in a that is negative. A value of CF=1 tells me that either c) b is entirely positive, or d) that for those doubles in b that are not positive, there is a matching double in in !a that is negative.
Have I understood this correctly? I am a bit confused by this. What's the point of this check? What would I use these intrinsics for?
x86 sse simd intrinsics avx
edited Nov 21 '18 at 13:10
Paul R
175k24297454
asked Nov 21 '18 at 11:04
mSSM
27438
add a comment |
I am trying to understand the _mm256_testc_pd, _mm256_testz_pd, and _mm256_testnzc_pd intrinsics, and I have a hard time understanding them.
To analyze _mm256_testc_pd, I have identified the following cases (a is the first, b the second __mm256d argument`):
- If all packed doubles in
bare > 0, thenZF=1,CF=1, except: - If one packed double in each
aandbare mutally < 0, thenZF=0. - If one packed double in each
!aandbare mutually < 0, thenCF=0.
In other words, a value of ZF=1 tells me that either a) b is entirely positive, or b) that for those doubles in b that are not positive, there is a matching double in a that is negative. A value of CF=1 tells me that either c) b is entirely positive, or d) that for those doubles in b that are not positive, there is a matching double in in !a that is negative.
Have I understood this correctly? I am a bit confused by this. What's the point of this check? What would I use these intrinsics for?
x86 sse simd intrinsics avx
edited Nov 21 '18 at 13:10
Paul R
175k24297454
asked Nov 21 '18 at 11:04
mSSM
27438
1
The most common use case is testing the result of a compare operation, where the result elements are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using thetestzintrinsic. (I've never found a use for any of the other variants).
– Paul R
Nov 21 '18 at 11:40
@PaulR: Might as well move that to an answer.
– Jason R
Nov 21 '18 at 13:01
@JasonR: yes, you're right - I got carried away with what was originally just going to be a short comment. ;-)
– Paul R
Nov 21 '18 at 13:07
1
Usually you usevptestorvtestpdwith one operand being a constant mask, not two variables. e.g. to check for any element being negative (having its sign bit set). Related: Can PTEST be used to test if two registers are both zero or some other condition?.
– Peter Cordes
Nov 21 '18 at 16:53
add a comment |
I am trying to understand the _mm256_testc_pd, _mm256_testz_pd, and _mm256_testnzc_pd intrinsics, and I have a hard time understanding them.
To analyze _mm256_testc_pd, I have identified the following cases (a is the first, b the second __mm256d argument`):
- If all packed doubles in
bare > 0, thenZF=1,CF=1, except: - If one packed double in each
aandbare mutally < 0, thenZF=0. - If one packed double in each
!aandbare mutually < 0, thenCF=0.
In other words, a value of ZF=1 tells me that either a) b is entirely positive, or b) that for those doubles in b that are not positive, there is a matching double in a that is negative. A value of CF=1 tells me that either c) b is entirely positive, or d) that for those doubles in b that are not positive, there is a matching double in in !a that is negative.
Have I understood this correctly? I am a bit confused by this. What's the point of this check? What would I use these intrinsics for?
x86 sse simd intrinsics avx
edited Nov 21 '18 at 13:10
Paul R
175k24297454
asked Nov 21 '18 at 11:04
mSSM
27438
I am trying to understand the _mm256_testc_pd, _mm256_testz_pd, and _mm256_testnzc_pd intrinsics, and I have a hard time understanding them.
To analyze _mm256_testc_pd, I have identified the following cases (a is the first, b the second __mm256d argument`):
- If all packed doubles in
bare > 0, thenZF=1,CF=1, except: - If one packed double in each
aandbare mutally < 0, thenZF=0. - If one packed double in each
!aandbare mutually < 0, thenCF=0.
In other words, a value of ZF=1 tells me that either a) b is entirely positive, or b) that for those doubles in b that are not positive, there is a matching double in a that is negative. A value of CF=1 tells me that either c) b is entirely positive, or d) that for those doubles in b that are not positive, there is a matching double in in !a that is negative.
Have I understood this correctly? I am a bit confused by this. What's the point of this check? What would I use these intrinsics for?
x86 sse simd intrinsics avx
x86 sse simd intrinsics avx
edited Nov 21 '18 at 13:10
Paul R
175k24297454
asked Nov 21 '18 at 11:04
mSSM
27438
edited Nov 21 '18 at 13:10
Paul R
175k24297454
asked Nov 21 '18 at 11:04
mSSM
27438
edited Nov 21 '18 at 13:10
Paul R
175k24297454
edited Nov 21 '18 at 13:10
Paul R
175k24297454
edited Nov 21 '18 at 13:10
Paul R
175k24297454
175k24297454
asked Nov 21 '18 at 11:04
mSSM
27438
asked Nov 21 '18 at 11:04
mSSM
27438
asked Nov 21 '18 at 11:04
mSSM
27438
27438
1
The most common use case is testing the result of a compare operation, where the result elements are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using thetestzintrinsic. (I've never found a use for any of the other variants).
– Paul R
Nov 21 '18 at 11:40
@PaulR: Might as well move that to an answer.
– Jason R
Nov 21 '18 at 13:01
@JasonR: yes, you're right - I got carried away with what was originally just going to be a short comment. ;-)
– Paul R
Nov 21 '18 at 13:07
1
Usually you usevptestorvtestpdwith one operand being a constant mask, not two variables. e.g. to check for any element being negative (having its sign bit set). Related: Can PTEST be used to test if two registers are both zero or some other condition?.
– Peter Cordes
Nov 21 '18 at 16:53
add a comment |
1
The most common use case is testing the result of a compare operation, where the result elements are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using thetestzintrinsic. (I've never found a use for any of the other variants).
– Paul R
Nov 21 '18 at 11:40
@PaulR: Might as well move that to an answer.
– Jason R
Nov 21 '18 at 13:01
@JasonR: yes, you're right - I got carried away with what was originally just going to be a short comment. ;-)
– Paul R
Nov 21 '18 at 13:07
1
Usually you usevptestorvtestpdwith one operand being a constant mask, not two variables. e.g. to check for any element being negative (having its sign bit set). Related: Can PTEST be used to test if two registers are both zero or some other condition?.
– Peter Cordes
Nov 21 '18 at 16:53
1
1
The most common use case is testing the result of a compare operation, where the result elements are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the
testz intrinsic. (I've never found a use for any of the other variants).– Paul R
Nov 21 '18 at 11:40
The most common use case is testing the result of a compare operation, where the result elements are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the
testz intrinsic. (I've never found a use for any of the other variants).– Paul R
Nov 21 '18 at 11:40
@PaulR: Might as well move that to an answer.
– Jason R
Nov 21 '18 at 13:01
@PaulR: Might as well move that to an answer.
– Jason R
Nov 21 '18 at 13:01
@JasonR: yes, you're right - I got carried away with what was originally just going to be a short comment. ;-)
– Paul R
Nov 21 '18 at 13:07
@JasonR: yes, you're right - I got carried away with what was originally just going to be a short comment. ;-)
– Paul R
Nov 21 '18 at 13:07
1
1
Usually you use
vptest or vtestpd with one operand being a constant mask, not two variables. e.g. to check for any element being negative (having its sign bit set). Related: Can PTEST be used to test if two registers are both zero or some other condition?.– Peter Cordes
Nov 21 '18 at 16:53
Usually you use
vptest or vtestpd with one operand being a constant mask, not two variables. e.g. to check for any element being negative (having its sign bit set). Related: Can PTEST be used to test if two registers are both zero or some other condition?.– Peter Cordes
Nov 21 '18 at 16:53
add a comment |
1 Answer
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The most common use case is testing the result of a compare operation, where the result elements from the comparison are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the _mm*_testz_p* intrinsic.
I've never found a use for any of the other variants.
answered Nov 21 '18 at 13:07
Paul R
175k24297454
add a comment |
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The most common use case is testing the result of a compare operation, where the result elements from the comparison are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the _mm*_testz_p* intrinsic.
I've never found a use for any of the other variants.
answered Nov 21 '18 at 13:07
Paul R
175k24297454
add a comment |
The most common use case is testing the result of a compare operation, where the result elements from the comparison are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the _mm*_testz_p* intrinsic.
I've never found a use for any of the other variants.
answered Nov 21 '18 at 13:07
Paul R
175k24297454
add a comment |
The most common use case is testing the result of a compare operation, where the result elements from the comparison are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the _mm*_testz_p* intrinsic.
I've never found a use for any of the other variants.
answered Nov 21 '18 at 13:07
Paul R
175k24297454
The most common use case is testing the result of a compare operation, where the result elements from the comparison are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the _mm*_testz_p* intrinsic.
I've never found a use for any of the other variants.
answered Nov 21 '18 at 13:07
Paul R
175k24297454
answered Nov 21 '18 at 13:07
Paul R
175k24297454
answered Nov 21 '18 at 13:07
Paul R
175k24297454
answered Nov 21 '18 at 13:07
Paul R
175k24297454
175k24297454
add a comment |
add a comment |
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The most common use case is testing the result of a compare operation, where the result elements are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the
testzintrinsic. (I've never found a use for any of the other variants).– Paul R
Nov 21 '18 at 11:40
@PaulR: Might as well move that to an answer.
– Jason R
Nov 21 '18 at 13:01
@JasonR: yes, you're right - I got carried away with what was originally just going to be a short comment. ;-)
– Paul R
Nov 21 '18 at 13:07
1
Usually you use
vptestorvtestpdwith one operand being a constant mask, not two variables. e.g. to check for any element being negative (having its sign bit set). Related: Can PTEST be used to test if two registers are both zero or some other condition?.– Peter Cordes
Nov 21 '18 at 16:53