+++
title = "Back End"
date = "2020-12-10"
author = "Yann Herklotz"
tags = []
categories = []
backlinks = ["3a2"]
forwardlinks = ["3a4"]
zettelid = "3a3"
+++

This is where most of the optimisations happen, and where the
intermediate CFG language is finally converted to proper assembly. The
main optimisations are done in RTL, which is a language that models a
CFG and has infinite registers it can choose from, therefore not using a
stack. Optimisations such as inlining, constant propagation and dead
code elimination are all performed at this level, using Kildall's
algorithm to get the necessary analysis to perform these optimisations
\[1\].

RTL is then translated to LTL, which is where physical registers are
assigned, and where the rest of the variables are put into the stack. At
this level, the CFG is also translated into a CFG that uses basic blocks
instead of instructions. This is an interesting addition, and I am not
sure why that is not a property of RTL, as basic blocks can make some
optimisations simpler. Optimisations that would affect basic blocks,
such as lazy code motion (LCM) optimisations would be harder if the
language contained basic blocks, as these would have to be changed. As
these optimisations are performed at the RTL level, it does make sense
to have a simpler representation of code at that level and make these
transformations simpler.

<div id="refs" class="references csl-bib-body" markdown="1">

<div id="ref-bertot06_struc_approac_provin_compil_optim"
class="csl-entry" markdown="1">

<span class="csl-left-margin">\[1\]
</span><span class="csl-right-inline">Y. Bertot, B. Grégoire, and X.
Leroy, “A structured approach to proving compiler optimizations based on
dataflow analysis,” in *Types for proofs and programs*, J.-C. Filliâtre,
C. Paulin-Mohring, and B. Werner, Eds., Berlin, Heidelberg: Springer
Berlin Heidelberg, 2006, pp. 66–81.</span>

</div>

</div>