1 files changed, 47 insertions, 0 deletions
diff --git a/content/zettel/3a3.md b/content/zettel/3a3.md
new file mode 100644
index 0000000..ec8d9a5
--- /dev/null
+++ b/content/zettel/3a3.md
@@ -0,0 +1,47 @@
++++
+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>