Agner`s CPU blog

Software optimization resources | E-mail subscription to this blog | www.agner.org

SIMD exceptions are fine with masking
Author: Sylvain Collange Date: 2016-08-25 07:43
Agner wrote:
I consider this as implied in (3) because the mask in ForwardCom suppresses all error conditions for disabled elements. Maybe I didn't express this clearly enough. It is still a problem that the behavior depends on the vector length. If a loop generates two exceptions then these exceptions may happen simultaneously if vectors are long, but separately if the same code is run on another processor with shorter vectors. It is difficult to construct the exception handler so that it is guaranteed to generate the same result regardless of the vector length of the microprocessor.
I see. I agree that there are scenarios in which it is important to signal exceptions in the same order as they would occur in the original sequential program. e.g. when speculatively vectorizing a while loop with a data-dependent exit condition.
There are still solutions that are not /that/ difficult:

Option 1. In the exception handler, find out which is the first lane that produced an exception, named i (I assume the ISA gives this information somehow).
- If i=0, report the exception.
- Otherwise, remember the exception, and change the vector length for the current iteration to i-1 (either using VL or a mask register).
At the end of each iteration, check whether there is a pending exception. If there is one, report it. Otherwise, just reset the vector length and continue.

Option 2. Same approach, using hardware instead of software. The "pending exception" becomes a flag register, with 1 flag per lane. The current vector length or mask is automatically reduced as some lanes record exceptions. A new 'commit' instruction is called at the end of each iteration. It raises the recorded exception that would be the first in sequential order.

In practice, I do not expect this to be often necessary, as a speculative vectorizer will need to restore a snapshot to undo all unwanted side effects and restart the loop whenever an exception occurs anyway, regardless on which lane it occurs.

 
thread Proposal for instruction set - now on Github new - Agner - 2016-06-26
replythread Proposal for instruction set - now on Github new - Joe Duarte - 2016-07-04
last replythread Proposal for instruction set - now on Github new - Agner - 2016-07-04
replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-07-06
last replythread Proposal for instruction set - now on Github new - Agner - 2016-07-06
last replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-07-07
last reply Proposal for instruction set - now on Github new - Agner - 2016-07-07
replythread Whole-function vectorization and conditionals new - Sylvain Collange - 2016-08-15
last replythread Whole-function vectorization and conditionals new - Agner - 2016-08-15
last replythread Whole-function vectorization and conditionals new - Sylvain Collange - 2016-08-15
last replythread Whole-function vectorization and conditionals new - Agner - 2016-08-15
last replythread Whole-function vectorization and conditionals new - Sylvain Collange - 2016-08-15
last replythread Whole-function vectorization and conditionals new - Agner - 2016-08-15
reply Number of input dependencies new - Agner - 2016-08-16
last replythread Whole-function vectorization and conditionals new - Sylvain Collange - 2016-08-16
last replythread Whole-function vectorization and conditionals new - Agner - 2016-08-17
last replythread Merging with first operand new - Sylvain Collange - 2016-08-18
last replythread Merging with first operand new - Agner - 2016-08-19
replythread SIMD exceptions are fine with masking new - Sylvain Collange - 2016-08-19
last replythread SIMD exceptions are fine with masking new - Agner - 2016-08-20
reply SIMD exceptions are fine with masking new - Hubert Lamontagne - 2016-08-20
last reply SIMD exceptions are fine with masking - Sylvain Collange - 2016-08-25
last reply Merging with first operand new - Hubert Lamontagne - 2016-08-19
last replythread Proposal for instruction set - now on Github new - Joe Duarte - 2016-08-17
last replythread Proposal for instruction set - now on Github new - Agner - 2016-08-18
last replythread Proposal for instruction set - now on Github new - Joe Duarte - 2016-08-31
reply Proposal for instruction set - now on Github new - Agner - 2016-08-31
last reply Proposal for instruction set - now on Github new - Jorcy Neto - 2016-09-01
replythread Proposal for instruction set - now on Github new - Yuhong Bao - 2016-07-12
last reply Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-07-12
replythread Things from MIPS (and novel things) new - Anonymous - 2016-07-28
replythread Things from MIPS (and novel things) new - Agner - 2016-07-28
last reply Things from MIPS (and novel things) new - Hubert Lamontagne - 2016-07-28
last replythread Matrix multiplication new - Agner - 2016-07-29
reply Matrix multiplication new - Hubert Lamontagne - 2016-07-29
last replythread Matrix multiplication new - John D. McCalpin - 2016-07-29
last reply Matrix multiplication new - Agner - 2016-07-29
replythread Introduction website new - Agner - 2016-08-01
last replythread Introduction website new - EricTL - 2017-07-17
last replythread Introduction website new - Agner - 2017-07-18
last replythread Introduction website new - EricTL - 2017-07-20
last reply Introduction website new - Agner - 2017-07-20
replythread Proposal for instruction set - now on Github new - Joe Duarte - 2016-08-04
last replythread Proposal for instruction set - now on Github new - Agner - 2016-08-04
last replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-08-05
replythread Proposal for instruction set - now on Github new - Agner - 2016-08-06
last replythread Proposal for instruction set - now on Github new - fanoI - 2016-08-08
last replythread Proposal for instruction set - now on Github new - Agner - 2016-08-08
last reply Proposal for instruction set - now on Github new - fanoI - 2016-08-09
last replythread Proposal for instruction set - now on Github new - Joe Duarte - 2016-08-08
last replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-08-09
last replythread Proposal for instruction set - now on Github new - Joe Duarte - 2016-08-11
last replythread Proposal for instruction set - now on Github new - Agner - 2016-08-12
last reply Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-08-12
replythread Proposal for instruction set - now on Github new - grant galitz - 2016-08-22
reply Proposal for instruction set - now on Github new - Agner - 2016-08-22
last reply Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-08-24
replythread ARM with scalable vector extensions new - Agner - 2016-08-23
replythread ARM with scalable vector extensions new - Jorcy Neto - 2016-08-23
last reply ARM with scalable vector extensions new - Hubert Lamontagne - 2016-08-26
last reply ARM with scalable vector extensions new - Jorcy Neto - 2016-12-20
replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-09-05
replythread Proposal for instruction set - now on Github new - Agner - 2016-09-05
replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-09-05
last replythread Proposal for instruction set - now on Github new - Agner - 2016-09-06
reply Proposal for instruction set - now on Github new - Bigos - 2016-09-06
last replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-09-06
last replythread Proposal for instruction set - now on Github new - Agner - 2016-09-07
last replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-09-07
last replythread Proposal for instruction set - now on Github new - Agner - 2016-09-08
last reply Proposal for instruction set - now on Github new - Hubert Lamontagne - 2016-09-08
last replythread Proposal for instruction set - now on Github new - Commenter - 2016-09-07
last reply Proposal for instruction set - now on Github new - Bigos - 2016-09-08
last replythread Paging new - Kurt Baumgardner - 2016-09-09
replythread Paging new - Agner - 2016-09-10
reply Paging new - Hubert Lamontagne - 2016-09-11
last replythread Paging new - Kurt Baumgardner - 2016-09-13
replythread Paging new - Agner - 2016-09-13
last reply Paging new - Kurt Baumgardner - 2016-09-13
last replythread Paging new - Hubert Lamontagne - 2016-09-13
last reply Paging new - Kurt Baumgardner - 2016-09-14
replythread Paging new - Hubert Lamontagne - 2016-09-11
last reply Paging new - Kurt Baumgardner - 2016-09-13
last replythread Paging new - Agner - 2016-09-14
last reply Paging new - Jorcy Neto - 2016-09-18
replythread A null register? new - csdt - 2016-09-23
last replythread A null register? new - Agner - 2016-09-24
last replythread A null register? new - Hubert Lamontagne - 2016-09-24
replythread A null register? new - csdt - 2016-09-26
last reply A null register? new - Agner - 2016-09-27
last replythread Indexed registers new - Kurt Baumgardner - 2016-09-26
last replythread Indexed registers new - Agner - 2016-09-27
replythread Indexed registers new - Kurt Baumgardner - 2016-09-27
last reply Indexed registers new - Agner - 2016-09-28
last replythread Indexed registers new - Hubert Lamontagne - 2016-09-28
last replythread Indexed registers new - Kurt Baumgardner - 2016-10-03
reply Indexed registers new - Agner - 2016-10-03
last replythread Indexed registers new - Hubert Lamontagne - 2016-10-04
last replythread Bilinear Interpolation new - Hubert Lamontagne - 2016-10-28
last replythread Bilinear Interpolation new - Agner - 2016-10-29
last replythread Bilinear Interpolation new - Hubert Lamontagne - 2016-10-29
last replythread Bilinear Interpolation new - Agner - 2016-10-30
last reply Bilinear Interpolation new - Hubert Lamontagne - 2016-10-30
replythread ForwardCom version 1.04 new - Agner - 2016-12-08
replythread ForwardCom version 1.04 new - Matthias Bentrup - 2016-12-12
last replythread ForwardCom version 1.04 new - Agner - 2016-12-12
last reply ForwardCom version 1.04 new - Matthias Bentrup - 2016-12-14
last replythread Async system calls; horizontal packing instruction new - Joe Duarte - 2016-12-14
reply Async system calls; horizontal packing instruction new - Agner - 2016-12-15
last replythread Comparison of instruction sets new - Agner - 2016-12-17
replythread Comparison of instruction sets new - Joe Duarte - 2016-12-28
reply Comparison of instruction sets new - Agner - 2016-12-29
last reply Comparison of instruction sets new - Hubert Lamontagne - 2016-12-30
last reply Comparison of instruction sets new - Hubert Lamontagne - 2017-01-05
replythread ForwardCom version 1.05 new - Agner - 2017-01-22
replythread Syscall/ISR acceleration new - Jonathan Brandmeyer - 2017-01-22
last replythread Syscall/ISR acceleration new - Agner - 2017-01-23
last replythread Syscall/ISR acceleration new - Jonathan Brandmeyer - 2017-01-25
last reply Syscall/ISR acceleration new - Agner - 2017-01-25
replythread ForwardCom version 1.05 new - Jiří Moravec - 2017-01-23
last reply ForwardCom version 1.05 new - Agner - 2017-01-24
last replythread Jump prefetch? new - csdt - 2017-01-27
last replythread Jump prefetch? new - Agner - 2017-01-27
last replythread Jump prefetch? new - csdt - 2017-01-30
last replythread Jump prefetch? new - Agner - 2017-01-30
last replythread Jump prefetch? new - csdt - 2017-01-30
replythread Jump prefetch? new - Agner - 2017-01-31
reply Jump prefetch? new - csdt - 2017-01-31
last replythread Jump prefetch? new - Hubert Lamontagne - 2017-02-01
last replythread Jump prefetch? new - Agner - 2017-02-01
last replythread Jump prefetch? new - Hubert Lamontagne - 2017-02-01
last replythread Jump prefetch? new - Agner - 2017-02-02
last reply Jump prefetch? new - Agner - 2017-02-14
last replythread Jump prefetch? new - Hubert Lamontagne - 2017-01-31
last replythread High precision arithmetic new - fanoI - 2017-03-21
last reply High precision arithmetic new - Agner - 2017-03-21
replythread Intel's Control-flow Enforcement Technology new - Joe Duarte - 2017-04-13
last reply Intel's Control-flow Enforcement Technology new - Agner - 2017-04-14
reply Proposal for instruction set - now on Github new - Agner - 2017-04-27
replythread Assembler with metaprogramming features new - Agner - 2017-07-27
last replythread Assembler with metaprogramming features new - Kai Rese - 2017-08-11
last replythread Assembler with metaprogramming features new - Agner - 2017-08-11
last replythread Assembler with metaprogramming features new - Kai Rese - 2017-08-14
last replythread Assembler with metaprogramming features new - Agner - 2017-08-14
last reply Assembler with metaprogramming features new - Kai Rese - 2017-08-15
replythread Number of register file ports in implementations new - Hubert Lamontagne - 2017-08-22
last replythread Number of register file ports in implementations new - Agner - 2017-08-23
last replythread Number of register file ports in implementations new - Hubert Lamontagne - 2017-08-27
last replythread Number of register file ports in implementations new - Agner - 2017-08-28
reply Number of register file ports in implementations new - Bigos - 2017-08-28
last reply Number of register file ports in implementations new - Hubert Lamontagne - 2017-08-28
last replythread Proposal for instruction set - now on Github new - yeengief - 2017-09-20
replythread Proposal for instruction set - now on Github new - Agner - 2017-09-20
last replythread Proposal for instruction set - now on Github new - yeengief - 2017-09-20
last replythread Proposal for instruction set - now on Github new - Agner - 2017-09-20
last replythread Proposal for instruction set - now on Github new - yeengief - 2017-09-21
last replythread Proposal for instruction set - now on Github new - Agner - 2017-09-21
last replythread Proposal for instruction set - now on Github new - yeengief - 2017-09-21
last reply Proposal for instruction set - now on Github new - Agner - 2017-09-23
replythread Proposal for instruction set - now on Github new - - - 2017-09-22
last reply Proposal for instruction set - now on Github new - Agner - 2017-09-23
last replythread Proposal for instruction set - now on Github new - Hubert Lamontagne - 2017-09-25
last replythread Proposal for instruction set - now on Github new - Agner - 2017-09-26
last reply Proposal for instruction set - now on Github new - Hubert Lamontagne - 2017-09-26