Front End Performance Consideration (x86)

In the?last article, we discussed the architecture of the x86 processor, focusing on the #performance implications on software. In this article, I will focus on the Front End (FE) part of the #x86 processor. ?

To recap, the FE is responsible for fetching the instructions, decoding them into the opcodes for the processor, and feeding them into the execution ports. There is a scheduler responsible for scheduling the instructions, and the high-level goal is to ensure that the ports are optimally utilized, including keeping decoded instructions ready for the execution engine to pick up.?

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All software, no matter if it is written in a high-level language such as #python or a more system level language such as #c or C++, is eventually converted to machine code. In a CISC architecture like x86, this means translation into variable length instructions that are executed by the processor. Those who are familiar with the internals of the x86 architecture will recall that the %rip register holds the address of the currently executing instruction. Although the %rip register is incremented sequentially, the logic of the program will dictate what the next instruction to execute will be. For example, within a function there may be near or far jumps, corresponding to the if and else clauses in the code. Functions call other functions, and in some cases such as in Python, the Python code may invoke C/C++ libraries for efficiency reasons. NumPy (Numerical Python) for example uses math libraries implemented in C/C++. From a program execution perspective, all this means that it is not by any means simple to predict the next set of instructions that will be executed, and if the wrong set of instructions have been decoded, and in some cases even executed (out of order, speculative/eager execution), they have to be discarded and the FE pipeline stalls for the new instructions to be fetched from memory (worst case faulted in from storage to disk first) before the pipeline can kick off again. 英特尔 provides a tool called vTune that provides insights into execution bottlenecks and can tell if the system is Front End Bound.?

Considering the above, there are a few things to keep an eye for removing Front End issues:?

1. Instructions are brought into the L1I (L1 Instruction) cache before they are decoded. A processor with a bigger L1I cache will in general have a better FE profile.?
2. The #hardware has a branch prediction engine, and CPU vendors often improve the performance of these engines.?
3. In compiled languages such as C/C++, profile guided optimization (PGO) uses profiling data collected from a performance run and uses information on mis predicted branches to automatically reorder the code.?
4. Finally, in C/C++, programmers can use built-in functions such as __builtin_expect() in GCC to guide the compiler to layout code for a better execution profile. This is especially important for functions that are uncommonly executed, or not covered by PGO optimization mentioned above.?

#performanceengineering #performanceoptimization ?