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authorJason Ekstrand <[email protected]>2015-06-25 08:08:27 -0700
committerJason Ekstrand <[email protected]>2015-06-25 16:42:20 -0700
commit316206ee9ea06419c9a2ea6fe48d66a0b805319d (patch)
tree2c5824b85a819d24b83a0889d050c40a16a1bf38 /docs/utilities.html
parentc1151b18f2dce7c6f238f057e9c4fa8d912ce6b5 (diff)
i965/vec4_live_variables: Do liveness analysis bottom-to-top
From Muchnick's Advanced Compiler Design and Implementation: "To determine which variables are live at each point in a flowgraph, we perform a backward data-flow analysis" Previously, we were walking the blocks forwards and updating the livein and then the liveout. However, the livein calculation depends on the liveout and the liveout depends on the successor blocks. The net result is that it takes one full iteration to go from liveout to livein and then another full iteration to propagate to the predecessors. This works out to an O(n^2) computation where n is the number of blocks. If we run things in the other order, it's O(nl) where l is the maximum loop depth which is practically bounded by 3. In b2c6ba0c4b21391dc35018e1c8c4f7f7d8952bea, we made this same change in the FS backend to great effect. Might as well keep it consistent and make the same change for vec4. Also, this took the time to run the test: ES31-CTS.arrays_of_arrays.InteractionFunctionCalls1 from 6:49.62 to 3:31.40 on Timothy Arceri's machine. Reviewed-by: Matt Turner <[email protected]>
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