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authorIan Romanick <[email protected]>2011-08-01 14:13:10 -0700
committerIan Romanick <[email protected]>2011-08-15 11:44:27 -0700
commita352e2d08e0a141298275e77f25541218a97afb7 (patch)
tree7b5e74350cc9169fb88253d9c3f88dcfc949c2eb /scons
parent4a026d6ba50034c326eb6cfd0b555f57d83ab609 (diff)
glsl: Modify strategy for accumulating conditions when lowering if-statements
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 <[email protected]>
Diffstat (limited to 'scons')
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