From 5ecd9c70cecc05eaa1fef05f9bd4e8cf50f2c03a Mon Sep 17 00:00:00 2001 From: Eric Anholt Date: Wed, 8 Sep 2010 18:04:35 -0700 Subject: [PATCH] glsl: Add info about talloc and optimization passes to the README. --- src/glsl/README | 26 ++++++++++++++++++++++++++ 1 file changed, 26 insertions(+) diff --git a/src/glsl/README b/src/glsl/README index 7e97b48059c..62742bf1396 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. -- 2.30.2