[PATCH] D58853: [SCEV] Handle case where MaxBECount is less precise than ExactBECount for OR.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: sanjoy, efriedma, mkazantsev.
Herald added subscribers: jdoerfert, javed.absar, hiraditya.
Herald added a project: LLVM.

In some cases, MaxBECount can be less precise than ExactBECount for AND
and OR (the AND case was PR26207). In the OR test case, both ExactBECounts are
undef, but MaxBECount are different, so we hit the assertion below. This
patch uses the same solution the AND case already uses.

Assertion failed:

  ((isa<SCEVCouldNotCompute>(ExactNotTaken) || !isa<SCEVCouldNotCompute>(MaxNotTaken))
    && "Exact is not allowed to be less precise than Max"), function ExitLimit

This patch also consolidates test cases for both AND and OR in a single
test case.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13245


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D58853

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/ScalarEvolution/exact-exit-count-more-precise.ll
  llvm/test/Transforms/IndVarSimplify/pr26207.ll


Index: llvm/test/Transforms/IndVarSimplify/pr26207.ll
===================================================================
--- llvm/test/Transforms/IndVarSimplify/pr26207.ll
+++ /dev/null
@@ -1,20 +0,0 @@
-; RUN: opt -S -indvars < %s | FileCheck %s
-
-target triple = "x86_64-unknown-linux-gnu"
-
-define void @main(i16 %in) {
-; CHECK-LABEL: @main(
-  br label %bb2
-
-bb2:                                              ; preds = %bb1.i, %bb2, %0
-  %_tmp44.i = icmp slt i16 %in, 2
-  br i1 %_tmp44.i, label %bb1.i, label %bb2
-
-bb1.i:                                            ; preds = %bb1.i, %bb2
-  %_tmp25.i = phi i16 [ %in, %bb2 ], [ %_tmp6.i, %bb1.i ]
-  %_tmp6.i = add nsw i16 %_tmp25.i, 1
-  %_tmp10.i = icmp sge i16 %_tmp6.i, 2
-  %exitcond.i = icmp eq i16 %_tmp6.i, 2
-  %or.cond = and i1 %_tmp10.i, %exitcond.i
-  br i1 %or.cond, label %bb2, label %bb1.i
-}
Index: llvm/test/Analysis/ScalarEvolution/exact-exit-count-more-precise.ll
===================================================================
--- /dev/null
+++ llvm/test/Analysis/ScalarEvolution/exact-exit-count-more-precise.ll
@@ -0,0 +1,49 @@
+; RUN: opt -analyze -scalar-evolution %s | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @test_and(i16 %in) {
+  br label %bb2
+
+bb2:                                              ; preds = %bb1.i, %bb2, %0
+  %_tmp44.i = icmp slt i16 %in, 2
+  br i1 %_tmp44.i, label %bb1.i, label %bb2
+
+bb1.i:                                            ; preds = %bb1.i, %bb2
+  %_tmp25.i = phi i16 [ %in, %bb2 ], [ %_tmp6.i, %bb1.i ]
+  %_tmp6.i = add nsw i16 %_tmp25.i, 1
+  %_tmp10.i = icmp sge i16 %_tmp6.i, 2
+  %exitcond.i = icmp eq i16 %_tmp6.i, 2
+  %or.cond = and i1 %_tmp10.i, %exitcond.i
+  br i1 %or.cond, label %bb2, label %bb1.i
+}
+
+; CHECK-LABEL: Determining loop execution counts for: @test_and
+; CHECK-NEXT: Loop %bb1.i: backedge-taken count is (1 + (-1 * %in))
+; CHECK-NEXT: Loop %bb1.i: max backedge-taken count is -1
+; CHECK-NEXT: Loop %bb1.i: Predicated backedge-taken count is (1 + (-1 * %in))
+
+
+define void @test_or() {
+  %C10 = icmp slt i1 undef, undef
+  br i1 %C10, label %BB, label %exit
+
+BB:                                               ; preds = %BB, %0
+  %indvars.iv = phi i64 [ -1, %BB ], [ -1, %0 ]
+  %sum.01 = phi i32 [ %2, %BB ], [ undef, %0 ]
+  %1 = trunc i64 %indvars.iv to i32
+  %2 = add nsw i32 %1, %sum.01
+  %B3 = add i32 %1, %2
+  %C11 = icmp ult i32 %2, %1
+  %C5 = icmp sle i32 %1, %B3
+  %B = or i1 %C5, %C11
+  br i1 %B, label %BB, label %exit
+
+exit:                                      ; preds = %BB, %0
+  ret void
+}
+
+; CHECK-LABEL: Determining loop execution counts for: @test_or
+; CHECK-NEXT: Loop %BB: backedge-taken count is undef
+; CHECK-NEXT: Loop %BB: max backedge-taken count is -1
+; CHECK-NEXT: Loop %BB: Predicated backedge-taken count is undef
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -7272,6 +7272,14 @@
         if (EL0.ExactNotTaken == EL1.ExactNotTaken)
           BECount = EL0.ExactNotTaken;
       }
+      // There are cases (e.g. PR26207) where computeExitLimitFromCond is able
+      // to be more aggressive when computing BECount than when computing
+      // MaxBECount.  In these cases it is possible for EL0.ExactNotTaken and
+      // EL1.ExactNotTaken to match, but for EL0.MaxNotTaken and EL1.MaxNotTaken
+      // to not.
+      if (isa<SCEVCouldNotCompute>(MaxBECount) &&
+          !isa<SCEVCouldNotCompute>(BECount))
+        MaxBECount = getConstant(getUnsignedRangeMax(BECount));
 
       return ExitLimit(BECount, MaxBECount, false,
                        {&EL0.Predicates, &EL1.Predicates});


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