forked from OSchip/llvm-project
82fb4f4b22
As discussed in https://github.com/llvm/llvm-project/issues/53020 / https://reviews.llvm.org/D116692, SCEV is forbidden from reasoning about 'backedge taken count' if the branch condition is a poison-safe logical operation, which is conservatively correct, but is severely limiting. Instead, we should have a way to express those poison blocking properties in SCEV expressions. The proposed semantics is: ``` Sequential/in-order min/max SCEV expressions are non-commutative variants of commutative min/max SCEV expressions. If none of their operands are poison, then they are functionally equivalent, otherwise, if the operand that represents the saturation point* of given expression, comes before the first poison operand, then the whole expression is not poison, but is said saturation point. ``` * saturation point - the maximal/minimal possible integer value for the given type The lowering is straight-forward: ``` compare each operand to the saturation point, perform sequential in-order logical-or (poison-safe!) ordered reduction over those checks, and if reduction returned true then return saturation point else return the naive min/max reduction over the operands ``` https://alive2.llvm.org/ce/z/Q7jxvH (2 ops) https://alive2.llvm.org/ce/z/QCRrhk (3 ops) Note that we don't need to check the last operand: https://alive2.llvm.org/ce/z/abvHQS Note that this is not commutative: https://alive2.llvm.org/ce/z/FK9e97 That allows us to handle the patterns in question. Reviewed By: nikic, reames Differential Revision: https://reviews.llvm.org/D116766 |
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