|author||Kenneth Graunke <email@example.com>||2010-11-29 11:00:10 -0800|
|committer||Kenneth Graunke <firstname.lastname@example.org>||2010-12-01 11:52:43 -0800|
glsl: Add comments to lower_jumps (from the commit message).
This is essentially Luca's commit message, but placed at the top of the file.
1 files changed, 31 insertions, 0 deletions
diff --git a/src/glsl/lower_jumps.cpp b/src/glsl/lower_jumps.cpp
index beacfaf8b03..9cd15ef7366 100644
@@ -23,6 +23,37 @@
* \file lower_jumps.cpp
+ * This pass lowers jumps (break, continue, and return) to if/else structures.
+ * It can be asked to:
+ * 1. Pull jumps out of ifs where possible
+ * 2. Remove all "continue"s, replacing them with an "execute flag"
+ * 3. Replace all "break" with a single conditional one at the end of the loop
+ * 4. Replace all "return"s with a single return at the end of the function,
+ * for the main function and/or other functions
+ * Applying this pass gives several benefits:
+ * 1. All functions can be inlined.
+ * 2. nv40 and other pre-DX10 chips without "continue" can be supported
+ * 3. nv30 and other pre-DX10 chips with no control flow at all are better
+ * supported
+ * Continues are lowered by adding a per-loop "execute flag", initialized to
+ * true, that when cleared inhibits all execution until the end of the loop.
+ * Breaks are lowered to continues, plus setting a "break flag" that is checked
+ * at the end of the loop, and trigger the unique "break".
+ * Returns are lowered to breaks/continues, plus adding a "return flag" that
+ * causes loops to break again out of their enclosing loops until all the
+ * loops are exited: then the "execute flag" logic will ignore everything
+ * until the end of the function.
+ * Note that "continue" and "return" can also be implemented by adding
+ * a dummy loop and using break.
+ * However, this is bad for hardware with limited nesting depth, and
+ * prevents further optimization, and thus is not currently performed.