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authorEric Anholt <eric@anholt.net>2010-09-08 18:04:35 -0700
committerEric Anholt <eric@anholt.net>2010-09-08 18:09:05 -0700
commit5ecd9c70cecc05eaa1fef05f9bd4e8cf50f2c03a (patch)
tree8e2e515b6c52bd1202c36b6eb7aa02644fca7f25 /src/glsl/README
parente04f90712d8e04ce9d2fee46d0cccf818432c7d9 (diff)
glsl: Add info about talloc and optimization passes to the README.
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diff --git a/src/glsl/README b/src/glsl/README
index 7e97b48059..62742bf139 100644
--- a/src/glsl/README
+++ b/src/glsl/README
@@ -189,3 +189,29 @@ You may also need to update the backends if they will see the new expr type:
You can then use the new expression from builtins (if all backends
would rather see it), or scan the IR and convert to use your new
expression type (see ir_mod_to_fract, for example).
+
+Q: How is memory management handled in the compiler?
+
+The hierarchical memory allocator "talloc" developed for the Samba
+project is used, so that things like optimization passes don't have to
+worry about their garbage collection so much. It has a few nice
+features, including low performance overhead and good debugging
+support that's trivially available.
+
+Generally, each stage of the compile creates a talloc context and
+allocates its memory out of that or children of it. At the end of the
+stage, the pieces still live are stolen to a new context and the old
+one freed, or the whole context is kept for use by the next stage.
+
+For IR transformations, a temporary context is used, then at the end
+of all transformations, reparent_ir reparents all live nodes under the
+shader's IR list, and the old context full of dead nodes is freed.
+When developing a single IR transformation pass, this means that you
+want to allocate instruction nodes out of the temporary context, so if
+it becomes dead it doesn't live on as the child of a live node. At
+the moment, optimization passes aren't passed that temporary context,
+so they find it by calling talloc_parent() on a nearby IR node. The
+talloc_parent() call is expensive, so many passes will cache the
+result of the first talloc_parent(). Cleaning up all the optimization
+passes to take a context argument and not call talloc_parent() is left
+as an exercise.