Agner`s CPU blog

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

32B store-forwarding is slower than 16B
Author: Agner Date: 2017-06-28 23:57
Fabian Giesen wrote:
I believe this is, ultimately, an artifact of how 256-bit vectors are implemented internally. Namely, I believe that the lower and upper 128-bit data paths are 1 cycle offset from each other
I have found no evidence of the two 128-bit lanes being offset by one clock. Why would they do this if all execution units are 256 bits? It's a matter of physical distances on the chip. I think that all units belonging to the same 128-bit lane are clustered together to minimize the length of data paths within the same lane. Any instruction that transports data between different 128-bit lanes has an extra clock cycle delay for moving data from one lane-cluster to another. I guess the 256-bit store somehow uses the permute machinery or some lane-crossing paths even though the write port is 256 bits.
 
thread Test results for Broadwell and Skylake new - Agner - 2015-12-26
replythread Sustained 64B loads per cycle on Haswell & Sky new - Nathan Kurz - 2015-12-26
last replythread Sustained 64B loads per cycle on Haswell & Sky new - Agner - 2015-12-27
last replythread Sustained 64B loads per cycle on Haswell & Sky new - Nathan Kurz - 2015-12-27
reply Sustained 64B loads per cycle on Haswell & Sky new - John D. McCalpin - 2016-01-04
reply Sustained 64B loads per cycle on Haswell & Sky new - T - 2016-06-18
last reply Sustained 64B loads per cycle on Haswell & Sky new - Jens Nurmann - 2017-01-12
replythread Test results for Broadwell and Skylake new - Peter Cordes - 2015-12-28
last reply Test results for Broadwell and Skylake new - Agner - 2015-12-29
replythread Test results for Broadwell and Skylake new - Tacit Murky - 2016-01-04
last replythread Test results for Broadwell and Skylake new - Agner - 2016-01-05
last replythread Test results for Broadwell and Skylake new - Tacit Murky - 2016-03-09
last reply Test results for Broadwell and Skylake new - Tacit Murky - 2016-06-05
replythread Minor bug in the microarchitecture manual new - SHK - 2016-01-10
last reply Minor bug in the microarchitecture manual new - Agner - 2016-01-16
replythread Test results for Broadwell and Skylake new - John D. McCalpin - 2016-01-12
last replythread Test results for Broadwell and Skylake new - Jess - 2016-02-11
last reply Description of discrepancy new - Nathan Kurz - 2016-03-13
reply Test results for Broadwell and Skylake new - Russell Van Zandt - 2016-02-22
replythread Instruction Throughput on Skylake new - Nathan Kurz - 2016-04-23
last replythread Instruction Throughput on Skylake new - Agner - 2016-04-24
last replythread Instruction Throughput on Skylake new - Nathan Kurz - 2016-04-26
last replythread Instruction Throughput on Skylake new - Agner - 2016-04-27
last replythread Instruction Throughput on Skylake new - T - 2016-06-18
reply Instruction Throughput on Skylake new - Agner - 2016-06-19
last replythread Instruction Throughput on Skylake new - Nathan Kurz - 2016-07-08
last replythread Instruction Throughput on Skylake new - Nathan Kurz - 2016-07-11
replythread Instruction Throughput on Skylake new - Tacit Murky - 2016-07-17
last replythread Haswell register renaming / unfused limits new - Peter Cordes - 2017-05-11
reply Haswell register renaming / unfused limits new - Tacit Murky - 2017-05-11
last reply Haswell register renaming / unfused limits new - Peter Cordes - 2017-05-12
last reply Instruction Throughput on Skylake new - T - 2016-08-08
reply Unlamination of micro-fused ops in SKL and earlier new - Travis - 2016-09-09
replythread 32B store-forwarding is slower than 16B new - Peter Cordes - 2017-05-11
last replythread 32B store-forwarding is slower than 16B new - Fabian Giesen - 2017-06-28
last reply 32B store-forwarding is slower than 16B - Agner - 2017-06-28
reply SHL/SHR r,cl latency is lower than throughput new - Peter Cordes - 2017-05-27
replythread Test results for Broadwell and Skylake new - Bulat Ziganshin - 2017-05-30
last replythread Test results for Broadwell and Skylake new - Agner - 2017-05-30
last replythread Test results for Broadwell and Skylake new - Bulat Ziganshin - 2017-05-30
last replythread Test results for Broadwell and Skylake new - - - 2017-06-19
replythread Test results for Broadwell and Skylake new - Jorcy Neto - 2017-06-20
last reply Test results for Broadwell and Skylake new - Jorcy Neto - 2017-06-20
replythread Test results for Broadwell and Skylake new - Bulat Ziganshin - 2017-06-21
reply Test results for Broadwell and Skylake new - Jorcy Neto - 2017-06-26
last replythread Test results for Broadwell and Skylake new - - - 2017-07-05
last replythread Test results for Broadwell and Skylake new - - - 2017-07-12
last reply Test results for Broadwell and Skylake new - Jorcy Neto - 2017-07-19
last replythread Test results for Broadwell and Skylake new - Xing Liu - 2017-06-28
last replythread Test results for Broadwell and Skylake new - Travis - 2017-06-29
last replythread Test results for Broadwell and Skylake new - Xing Liu - 2017-06-30
last reply Test results for Broadwell and Skylake new - Travis - 2017-07-13
reply Official information about uOps and latency SNB+ new - SEt - 2017-07-17
last replythread Test results for Broadwell and Skylake new - Armand Behroozi - 2020-10-07
last reply Test results for Broadwell and Skylake new - Agner - 2020-10-11