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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| .. | ||
| AliasSet | ||
| AssumptionCache | ||
| BasicAA | ||
| BlockFrequencyInfo | ||
| BranchProbabilityInfo | ||
| CFLAliasAnalysis | ||
| CallGraph | ||
| CostModel | ||
| CycleInfo | ||
| DDG | ||
| Delinearization | ||
| DemandedBits | ||
| DependenceAnalysis | ||
| DivergenceAnalysis | ||
| DominanceFrontier | ||
| Dominators | ||
| FunctionPropertiesAnalysis | ||
| GlobalsModRef | ||
| IRSimilarityIdentifier | ||
| IVUsers | ||
| LazyCallGraph | ||
| LazyValueAnalysis | ||
| LegacyDivergenceAnalysis | ||
| Lint | ||
| LoopAccessAnalysis | ||
| LoopCacheAnalysis/PowerPC | ||
| LoopInfo | ||
| LoopNestAnalysis | ||
| MemoryDependenceAnalysis | ||
| MemorySSA | ||
| MustExecute | ||
| PhiValues | ||
| PostDominators | ||
| ProfileSummary | ||
| RegionInfo | ||
| ScalarEvolution | ||
| ScopedNoAliasAA | ||
| StackSafetyAnalysis | ||
| TypeBasedAliasAnalysis | ||
| ValueTracking | ||
| alias-analysis-uses.ll | ||