ref: 939f632c16cadc94f03173c274c2a55535088bb6
parent: fbabc93aa114749eb5e995c0ce54cfc10681e6e6
author: Ori Bernstein <[email protected]>
date: Sat Aug 18 17:56:48 EDT 2012
Write a bit about flattening.
--- a/6/simp.c
+++ b/6/simp.c
@@ -1282,6 +1282,7 @@
printf("END ----------------\n");}
}
+
cfg = mkcfg(s->stmts, s->nstmts);
if (debug)
dumpcfg(cfg, stdout);
--- a/doc/compiler.txt
+++ b/doc/compiler.txt
@@ -193,12 +193,43 @@
3. FLATTENING:
This phase is invoked repeatedly on each top level declaration that we
- want to generate code for.
+ want to generate code for. There is a good chance that this flattening
+ phase should be made machine independent, and passed as a parameter
+ a machine description describing known integer and pointer sizes, among
+ other machine attributes. However, for now, it is machine dependent,
+ and lives in 6/simp.c.
+ The goal of flattening a tree is to take semantically involved constructs
+ such as looping, and simplify things into something that is easy to
+ generate code for, as well as something that is easier to analyze for
+ optimization.
+
3.1. Control Flow:
- All control flow is simplified to
+ All if statements, loops, and other complex constructs are simplified
+ to jumps and conditional jumps. Loops are generally simplified from
+ something that would look like this:
+ loop
+ init
+ cond
+ inc
+ body
+
+ To something that would look like this:
+
+ init
+ jmp cond
+ .loop:
+ body
+ inc
+ .cond:
+ cjmp cond .loop .end
+ .end:
+
+ Boolean expressions are simplified as described in section 8.4 of the
+ Dragon book[1]
+
3.2. Complex Expressions:
4. OPTIMIZATION:
@@ -222,4 +253,5 @@
6.5. Instruction Selection:
-
+[1] Aho, Sethi, Ullman: Compilers: Principles, Techniques, and Tools, 1988.
+ ISBN 0-201-10088-6