Computer Architecture / Assembly
See also note on:
- Concurrency
Architecture Course
arch vs microarch
data types and size addresses packed vector data fp - cray, intel extended
isa fixed - mips arm variable length - x86 1-18bytes compressed - thumb vliw
nmber of operands
reg-reg reg-mem
pipelining
Like car assembly line Make processing instructions deep. Different stages. You need to stall the pipeline sometimes Better throughput, worse latency Appendix C classic 5 stage RISC pipline
- Instruction fetch
- instruction decode
- execute
- Memory
- Write
Hazards
structural - if only have so many units that do a certain thing Data interrupts are an interesting one you can forget about, because they are so implicit
Control hazards Branch prediction
superscalar
Make processor wide, multiple pipelines. Parallelism is decided at runtime. Check for data dependencies
Out of order execution
Scoreboard Is this where the idea of register file really comes into play? To remember dependencies
VLIW
pushes scheduling to compile time. Removes lots of complicated scheduling circuitt Software pipelining vs Loop unrolling. Sortware pipelining removes the loop head? No, software pipelining reorders the statements perhaps after a loop unroll to give dependencies space. We can also sort of reorganize the association of loop variable to statements
Trace scheduling
equal scheduling model - when does operation finish? Exactly at its latency less-equal scheduling - can’t pack something in that less than windoww anymore. The looawe the spec, the less ahead of time reeasonig we can do
compressed instructions
predication
partial predication - Conditional move instructions for example
full predication
avoid small chunks of branched code
When is computing both sides of a branch worth it? balanced or longest is frequently executed
The it instructions in arm
ALAT code hammock
appendix H. Dependency analysis. The indices are Loop dependence analysis I wonder if this is what llvm wants presburger for True dependence - read after write antidependance - write after read output dependence- write after write
Use z3 for analysis
loop carried dependence For going across loops we need to know the order. i <= i’ dependence
antidependeance
branch prediction
static prediction 0 Coin toss or just assuming fallthrough may work 50% of time Alternative to delay slots. 2 delay slots can be hard to fill Backwards jumps occur commonly in loops. More commonly taken than not taken Hint flags in instructions - can be profiled to make better. Maybe %80 accuracy
1-bit predictor. did you most recently jump or not n-bit history predictor - keep table table based on jump address Predictor is trained. A table is built Time adn spatial correlation Combinding methods - boosting. gets you to ~99% which you need for deep pipelines
predicting address of indirect jumps
EPIC
Cache
associative cache cache coherence victim cache write buffer multi level cache prefetching
SMT - simultaneous multi threading
Intel hyperthreading TLBleed programs hyperhtreading can side channel info to each other
Misc
[dan luu what cpus have since the 80s(https://danluu.com/new-cpu-features/)]
dhrystone -benchmark suite. Patterson kind of rails on it.
delay slots - something that makes more sense when you’re aware of microarchecture
https://arxiv.org/pdf/1911.03282.pdf nanobench https://developer.amd.com/amd-uprof/ amd uprof https://people.freebsd.org/~lstewart/articles/cpumemory.pdf what every programmer should know baout memory
https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-823-computer-system-architecture-fall-2005/lecture-notes/ https://www.youtube.com/c/OnurMutluLectures/playlists Onur Mutlu lectures, courses Should I do Gem5, verilog, vhdl, other?
Communication Locality in Computation: Software, Chip Multiprocessors and Brains - Greenfield thesis