[Backend] Fix device assert inside reduction/scan region#4811
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Currently the reduction codegen unconditionally executes the combine region which can create problems because we conditionally load from shared memory, so this uses uninitialized registers. Generally combine regions should be pure, so this shouldn't be observable but with the overflow sanitizer the frontend injects assertions into the combine region. This changes the `accumulate` function to take a predicate and if the combine region isn't speculateble we only run it on threads where the predicate is true. In the common case, the codegen is unchanged.
pawelszczerbuk
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Thanks! Looks great!
peterbell10
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Nov 5, 2024
…on (#5033) Reductions have special handling for side effectful "combine ops" (e.g. "add" for a sum reduction). In the presence of side effects, a predicate is computed to determine whether a thread should participate in the reduction, to ensure that invalid/uninitialized data is not operated on. See #4811 for more details. ~Previously, the predicate logic was incorrect for 2D reductions. This PR fixes the logic and adds a python test.~ Edit: after additional discussion with @peterbell10, we removed the lanePred logic. Here's our thinking on why this is valid: * lanePred info is computed based entirely on the blocked layout info and properties of the reduction * the blocked layout won't tell you which threads do or don't have uninitialized data Instead, it sounds like the motivation for #4811 is based on uninitialized values that can be indicated by the `pred` variable passed into `warpReduce()`.
bertmaher
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Nov 5, 2024
…on (#5033) Reductions have special handling for side effectful "combine ops" (e.g. "add" for a sum reduction). In the presence of side effects, a predicate is computed to determine whether a thread should participate in the reduction, to ensure that invalid/uninitialized data is not operated on. See #4811 for more details. ~Previously, the predicate logic was incorrect for 2D reductions. This PR fixes the logic and adds a python test.~ Edit: after additional discussion with @peterbell10, we removed the lanePred logic. Here's our thinking on why this is valid: * lanePred info is computed based entirely on the blocked layout info and properties of the reduction * the blocked layout won't tell you which threads do or don't have uninitialized data Instead, it sounds like the motivation for #4811 is based on uninitialized values that can be indicated by the `pred` variable passed into `warpReduce()`.
Luosuu
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Nov 13, 2024
…#4811) Currently the reduction codegen unconditionally executes the combine region which can create problems because we conditionally load from shared memory, so this uses uninitialized registers. Generally combine regions should be pure, so this shouldn't be observable but with the overflow sanitizer the frontend injects assertions into the combine region. This changes the `accumulate` function to take a predicate and if the combine region isn't speculateble we only run it on threads where the predicate is true. In the common case, the codegen is unchanged.
Luosuu
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Nov 13, 2024
…on (triton-lang#5033) Reductions have special handling for side effectful "combine ops" (e.g. "add" for a sum reduction). In the presence of side effects, a predicate is computed to determine whether a thread should participate in the reduction, to ensure that invalid/uninitialized data is not operated on. See triton-lang#4811 for more details. ~Previously, the predicate logic was incorrect for 2D reductions. This PR fixes the logic and adds a python test.~ Edit: after additional discussion with @peterbell10, we removed the lanePred logic. Here's our thinking on why this is valid: * lanePred info is computed based entirely on the blocked layout info and properties of the reduction * the blocked layout won't tell you which threads do or don't have uninitialized data Instead, it sounds like the motivation for triton-lang#4811 is based on uninitialized values that can be indicated by the `pred` variable passed into `warpReduce()`.
guacamoleo
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Nov 14, 2024
…on (triton-lang#5033) Reductions have special handling for side effectful "combine ops" (e.g. "add" for a sum reduction). In the presence of side effects, a predicate is computed to determine whether a thread should participate in the reduction, to ensure that invalid/uninitialized data is not operated on. See triton-lang#4811 for more details. ~Previously, the predicate logic was incorrect for 2D reductions. This PR fixes the logic and adds a python test.~ Edit: after additional discussion with @peterbell10, we removed the lanePred logic. Here's our thinking on why this is valid: * lanePred info is computed based entirely on the blocked layout info and properties of the reduction * the blocked layout won't tell you which threads do or don't have uninitialized data Instead, it sounds like the motivation for triton-lang#4811 is based on uninitialized values that can be indicated by the `pred` variable passed into `warpReduce()`.
bertmaher
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Dec 10, 2024
…#4811) Currently the reduction codegen unconditionally executes the combine region which can create problems because we conditionally load from shared memory, so this uses uninitialized registers. Generally combine regions should be pure, so this shouldn't be observable but with the overflow sanitizer the frontend injects assertions into the combine region. This changes the `accumulate` function to take a predicate and if the combine region isn't speculateble we only run it on threads where the predicate is true. In the common case, the codegen is unchanged.
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Currently the reduction codegen unconditionally executes the combine region which can create problems because we conditionally load from shared memory, so this uses uninitialized registers.
Generally combine regions should be pure, so this shouldn't be observable but with the overflow sanitizer the frontend injects assertions into the combine region.
This changes the
accumulatefunction to take a predicate and if the combine region isn't speculateble we only run it on threads where the predicate is true. In the common case, the codegen is unchanged.