[LoopVectorize] Preserve SCEV

[nikic]

Summary

As far as I can tell, LoopVectorize preserves SCEV, mainly by dint of forgetting the loop being vectorized. We should mark it as preserved in the pass manager.

This is a very small compile-time improvement: http://llvm-compile-time-tracker.com/compare.php?from=a1677bda7975a0f690292587a04b9e053aacd1dc&to=bb3b36e5587a636f51e59a76d2e26db07ceea9ff&stat=instructions:u

[fhahn]

LGTM, thanks! As mentioned in the description, LV should properly invalidate SCEV for the IR it touches.

[fhahn]

(it might surface additional general missed SCEV invalidation patterns, but it's better to bite that bullet and fix those going forward IMO)

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

PreservedAnalyses LoopVectorizePass::run(Function &F,
 
     // We currently do not preserve loopinfo/dominator analyses with outer loop
     // vectorization. Until this is addressed, mark these analyses as preserved
     // only for non-VPlan-native path.
     // TODO: Preserve Loop and Dominator analyses for VPlan-native path.
     if (!EnableVPlanNativePath) {
       PA.preserve<LoopAnalysis>();
       PA.preserve<DominatorTreeAnalysis>();
+      PA.preserve<ScalarEvolutionAnalysis>();
+
+#ifdef EXPENSIVE_CHECKS
+      SE.verify();
+#endif
     }
 
     if (Result.MadeCFGChange) {
       // Making CFG changes likely means a loop got vectorized. Indicate that
       // extra simplification passes should be run.
       // TODO: MadeCFGChanges is not a prefect proxy. Extra passes should only
       // be run if runtime checks have been added.
       AM.getResult<ShouldRunExtraVectorPasses>(F);

llvm/test/Transforms/LoopVectorize/novect-lcssa-cfg-invalidation.ll

 ; RUN: opt -S -passes="loop-vectorize,jump-threading" -debug-pass-manager < %s 2>&1 | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 
 ; Checks what analyses are invalidated after Loop Vectorization when no actual
 ; vectorization happens, and the only change LV makes is LCSSA formation.
 
 define i32 @novect(ptr %p) {
 
 ; CHECK:           Running pass: LoopVectorizePass on novect
-; CHECK:           Invalidating analysis: ScalarEvolutionAnalysis on novect
+; CHECK-NOT:       Invalidating analysis: ScalarEvolutionAnalysis on novect
 ; CHECK-NOT:       Invalidating analysis: BranchProbabilityAnalysis on novect
 ; CHECK-NOT:       Invalidating analysis: BlockFrequencyAnalysis on novect
 ; CHECK:           Invalidating analysis: DemandedBitsAnalysis on novect
 ; CHECK:           Invalidating analysis: LoopAccessAnalysis on novect
 ; CHECK:           Running pass: JumpThreadingPass on novect
 
 ; CHECK:           entry:
 ; CHECK:             br label %middle