Previously if-statements were lowered from inner-most to outer-most
(i.e., bottom-up). All assignments within an if-statement would have
the condition of the if-statement appended to its existing condition.
As a result the assignments from a deeply nested if-statement would
have a very long and complex condition.
Several shaders in the OpenGL ES2 conformance test suite contain
non-constant array indexing that has been lowered by the shader
writer. These tests usually look something like:
if (i == 0) {
value = array[0];
} else if (i == 1) {
value = array[1];
} else ...
The IR for the last assignment ends up as:
(assign (expression bool && (expression bool ! (var_ref if_to_cond_assign_condition) ) (expression bool && (expression bool ! (var_ref if_to_cond_assign_condition@20) ) (expression bool && (expression bool ! (var_ref if_to_cond_assign_condition@22) ) (expression bool && (expression bool ! (var_ref if_to_cond_assign_condition@24) ) (var_ref if_to_cond_assign_condition@26) ) ) ) ) (x) (var_ref value) (array_ref (var_ref array) (constant int (5)))
The Mesa IR that is generated from this is just as awesome as you
might expect.
Three changes are made to the way if-statements are lowered.
1. Two condition variables, if_to_cond_assign_then and
if_to_cond_assign_else, are created for each if-then-else structure.
The former contains the "positive" condition, and the later contains
the "negative" condtion. This change was implemented in the previous
patch.
2. Each condition variable is added to a hash-table when it is created.
3. When lowering an if-statement, assignments to existing condtion
variables get the current condition anded. This ensures that nested
condition variables are only set to true when the condition variable
for all outer if-statements is also true.
Changes #1 and #3 combine to ensure the correctness of the resulting
code.
4. When a condition assignment is encountered with a condition that is
a dereference of a previously added condition variable, the condition
is not modified.
Change #4 prevents the continuous accumulation of conditions on
assignments.
If the original if-statements were:
if (x) {
if (a && b && c && d && e) {
...
} else {
...
}
} else {
if (g && h && i && j && k) {
...
} else {
...
}
}
The lowered code will be
if_to_cond_assign_then@1 = x;
if_to_cond_assign_then@2 = a && b && c && d && e
&& if_to_cond_assign_then@1;
...
if_to_cond_assign_else@2 = !if_to_cond_assign_then
&& if_to_cond_assign_then@1;
...
if_to_cond_assign_else@1 = !if_to_cond_assign_then@1;
if_to_cond_assign_then@3 = g && h && i && j;
&& if_to_cond_assign_else@1;
...
if_to_cond_assign_else@3 = !if_to_cond_assign_then
&& if_to_cond_assign_else@1;
...
Depending on how instructions are emitted, there may be an extra
instruction due to the duplication of the '&&
if_to_cond_assign_{then,else}@1' on the nested else conditions. In
addition, this may cause some unnecessary register pressure since in
the simple case (where the nested conditions are not complex) the
nested then-condition variables are live longer than strictly
necessary.
Before this change, one of the shaders in the OpenGL ES2 conformance
test suite's acos_float_frag_xvary generated 348 Mesa IR instructions.
After this change it only generates 124. Many, but not all, of these
instructions would have also been eliminated by CSE.
Reviewed-by: Eric Anholt <eric@anholt.net>
Now the condition (for the then-clause) and the inverse condition (for
the else-clause) get written to separate temporary variables. In the
presence of complex conditions, this shouldn't result in more code
being generated. If the original if-statement was
if (a && b && c && d && e) {
...
} else {
...
}
The lowered code will be
if_to_cond_assign_then = a && b && c && d && e;
...
if_to_cond_assign_else = !if_to_cond_assign_then;
...
Reviewed-by: Eric Anholt <eric@anholt.net>
Previously, if max_depth were 1, the following code would see the
first if-statement (correctly) not get flattened, but the second
if-statement would (incorrectly) get flattened:
void main()
{
if (a)
gl_Position = vec4(0);
if (b)
gl_Position = vec4(1);
}
This is because the visit_leave(ir_if*) method would not decrement the
depth before returning on the first if-statement.
NOTE: This is a candidate for the 7.10 and 7.11 branches.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
This adds a new optional max_depth parameter (defaulting to 0) to
lower_if_to_cond_assign, and makes the pass only flatten if-statements
nested deeper than that.
By default, all if-statements will be flattened, just like before.
This patch also renames do_if_to_cond_assign to lower_if_to_cond_assign,
to match the new naming conventions.
