[PATCH] D102267: [SCEV] Apply guards to max with non-unitary steps.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: nikic, reames, mkazantsev.
Herald added a subscriber: hiraditya.
fhahn requested review of this revision.
Herald added a project: LLVM.

We already apply loop-guards when computing the maximum with unitary
steps. This extends the code to also do so when dealing with non-unitary
steps.

This allows us to infer a tighter maximum in some cases.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D102267

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll


Index: llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
===================================================================
--- llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
+++ llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
@@ -200,14 +200,14 @@
 ; CHECK-LABEL: 'test_guard_ule_12_step2'
 ; CHECK-NEXT:  Classifying expressions for: @test_guard_ule_12_step2
 ; CHECK-NEXT:    %iv = phi i64 [ %iv.next, %loop ], [ 0, %entry ]
-; CHECK-NEXT:    --> {0,+,2}<nuw><nsw><%loop> U: [0,-9223372036854775808) S: [0,9223372036854775807) Exits: (2 * (%N /u 2))<nuw> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {0,+,2}<nuw><nsw><%loop> U: [0,13) S: [0,13) Exits: (2 * (%N /u 2))<nuw> LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %idx = getelementptr inbounds i32, i32* %a, i64 %iv
 ; CHECK-NEXT:    --> {%a,+,8}<nuw><%loop> U: full-set S: full-set Exits: ((8 * (%N /u 2)) + %a) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = add nuw nsw i64 %iv, 2
-; CHECK-NEXT:    --> {2,+,2}<nuw><%loop> U: [2,-1) S: [-9223372036854775808,9223372036854775807) Exits: (2 + (2 * (%N /u 2))<nuw>) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {2,+,2}<nuw><nsw><%loop> U: [2,15) S: [2,15) Exits: (2 + (2 * (%N /u 2))<nuw>) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_guard_ule_12_step2
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is (%N /u 2)
-; CHECK-NEXT:  Loop %loop: max backedge-taken count is 9223372036854775807
+; CHECK-NEXT:  Loop %loop: max backedge-taken count is 6
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (%N /u 2)
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -9262,10 +9262,13 @@
       loopHasNoAbnormalExits(AddRec->getLoop())) {
     const SCEV *Exact =
         getUDivExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
-    const SCEV *Max =
-        Exact == getCouldNotCompute()
-            ? Exact
-            : getConstant(getUnsignedRangeMax(Exact));
+    const SCEV *Max = getCouldNotCompute();
+    if (Exact != getCouldNotCompute()) {
+      APInt MaxInt = getUnsignedRangeMax(applyLoopGuards(Exact, L));
+      assert(MaxInt.ule(getUnsignedRangeMax(Exact)) &&
+             "applying loop guards pessimized the found maximum");
+      Max = getConstant(MaxInt);
+    }
     return ExitLimit(Exact, Max, false, Predicates);
   }
 


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