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PPC64 / hhvm

A virtual machine designed for executing programs written in Hack and PHP.
http://hhvm.com
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Create an arch-independent SF liveness optimization pass #102

Open gut opened 8 years ago

gut commented 8 years ago

https://reviews.facebook.net/D50415#inline-359421

then new arithmetic vasms that doesn't need to handle the SF (like addi) could be used for sure.

gut commented 7 years ago

A motivation behind this issue: image

As you can see, the "dot variant" takes much longer than the one without it. This SF liveness pass would improve performance for these instructions.

Bonus: compare is very expensive: image

Source: https://www.setphaserstostun.org/power8/POWER8_UM_v1.3_16MAR2016_pub.pdf

gut commented 7 years ago

ps: this idea is already implemented for ARM:

" Strength reduction on flag-setting instructions. On x64, arithmetic and bitwise operations always set the status flags, but this is not the case on ARM. With some better analysis of defs and uses of virtual SF registers, we can avoid unnecessary flag-setting on ARM (and other architectures with multiple strengths of arithmetic instructions)."

http://hhvm.com/blog/2017/03/09/how-the-cyber-elephant-got-his-arm.html