[Lexicon] Add GVN

[modocache]

Summary

Add a lexicon entry for global value numbering.

[davide]

Rather than practice, I'd say analysis. Also, maybe just mention expressions (you can look up several papers for the exact definition https://arxiv.org/pdf/1303.1880.pdf, or Kildall's one, which I think has the first definition).
Technically speaking, GVN is just an analysis (although the GVN pass in LLVM currently does also elimination and PRE) [and also NewGVN, but that will change soon, hopefully]

[inouehrs]

docs/Lexicon.rst

113

nit: Global value numbering -> Global Value Numbering (capitalize each word)

[modocache]

Capitalize & only mention expressions. Thanks for the link to the paper!

[davide]

If you don't mind, I like the description at the top of NewGVN better, i.e.

/// GVN partitions values computed by a function into congruence classes.
/// Values ending up in the same congruence class are guaranteed to be the same
/// for every execution of the program. In that respect, congruency is a
/// compile-time approximation of equivalence of values at runtime.

Can you use this one? I also would remove the bit about transformations (many thinks as GVN as PRE but it's actually not).
With that, LGTM, you don't need to resubmit for another look.

[modocache]

Sure! I updated this to use the description from NewGVN. (I'm updating Phabticator since that's my normal workflow.)

docs/Lexicon.rst

 
 G
 -
 
 **GC**
     Garbage Collection. The practice of using reachability analysis instead of
     explicit memory management to reclaim unused memory.
 
+**GVN**
+    Global value numbering. The practice of assigning a value number to
+    variables and expressions. Provably equivalent variables and expressions are
+    assigned the same value number, and may then be transformed or eliminated.
+
 H
 -
 
 .. _heap:
 
 **Heap**
     In garbage collection, the region of memory which is managed using
     reachability analysis.