Agner`s CPU blog

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

Proposal for instruction set - now on Github
Author:  Date: 2017-09-21 07:54
>Many x86 processors now have a micro-op cache because the decoding of instructions is a bottleneck. The ForwardCom ISA is designed so that decoding is very simple, never requiring more than one pipeline stage or one clock cycle.

Yes, but decoding is still sequential and not parallel. My proposal makes decoding ridiculously parallel.

New number proposal: 8 words, a half cache-line. The beginning of every 8 word block must begin a new instruction. Only the beginning of 8-word blocks are valid jump targets. Hence, I can never jump into the middle of an instruction. (Alternative proposal: Decoding of instruction lengths is always relative to the most recent 8-word boundary. Entry into the middle of an instruction is a fault. Worst case price: The decoder must look back 7 words on a jump and decode the instruction lengths in order to determine whether the jump was faulty. This does not require a lot of extra instruction fetching because these 7 previous words are in the same cache-line anyway).

Worst case overhead for instruction alignment restriction: 25% is NOP. Worst case for jump target restriction: Harder to compute, but compilers should be able to manage without too many NOPs.

Gain: Decoding is ridiculously parallel, because I can decode every 8-word block independently. If decoding becomes the bottleneck, I just plug in more decoding units (i.e. we might still be limited by decoding latency, but never by decoding throughput). The longest possible dependency chain in decoding is 7. Your proposal has infinite dependency chains in decoding (you must know the reading frame).


I can statically analyze code coming in 8 word-blocks and be sure to never have mis-decoded an instruction.

Another bag of problems we can avoid: Suppose some program uses code in two valid decodings, dependent on entrypoint. This means that your branch predictor, uop-cache, etc must work on the tuple (memory location, reading frame), where reading frame is one of 0,1,2. With my proposal you cut this down to just memory location, which simplifies the logic a lot-- because the logic for caching uops or other processor generated metadata is the same logic as for caching data and just maps address -> stuff.

The only real-life code that does such stuff (e.g. uses a 2-word instruction as a different instruction by jumping into the middle) is shellcode. While shellcode is fun and all, this is maybe not the most important customer group. You target "real" compilers, and not return oriented programming compilers.

 
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 new - 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 - 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