[SCEV] Re-land: Fix isKnownPredicate

[skatkov]

Summary

This is re-land of https://reviews.llvm.org/rL327362 with a fix and regression test.

The crash was due to it is possible that for found MDL loop, LHS or RHS may contain
an invariant unknown SCEV which does not dominate the MDL. Please see regression
test for an example.

[mkazantsev]

LGTM with nits. I've also run this test via our Fuzzer testing and it didn't detect any failures, so I think it's fine now.

lib/Analysis/ScalarEvolution.cpp

8747

It could be one if with && conditions, but it's up to you.

test/Transforms/IndVarSimplify/inner-loop-by-latch-cond.ll

22

How about adding check that @foo is called with %j?

[mkazantsev]

test/Analysis/ScalarEvolution/inner-loop-by-latch-cond-unknown.ll

1

Please add | FileCheck %s and CHECK-LABEL: @foo.

include/llvm/Analysis/ScalarEvolution.h

public:
   bool isKnownNonNegative(const SCEV *S);
 
   /// Test if the given expression is known to be non-positive.
   bool isKnownNonPositive(const SCEV *S);
 
   /// Test if the given expression is known to be non-zero.
   bool isKnownNonZero(const SCEV *S);
 
+  /// Splits SCEV expression \p S into two SCEVs. One of them is obtained from
+  /// \p S by substitution of all AddRec sub-expression related to loop \p L
+  /// with initial value of that SCEV. The second is obtained from \p S by
+  /// substitution of all AddRec sub-expressions related to loop \p L with post
+  /// increment of this AddRec in the loop \p L. In both cases all other AddRec
+  /// sub-expressions (not related to \p L) remain the same.
+  /// If the \p S contains non-invariant unknown SCEV the function returns
+  /// CouldNotCompute SCEV in both values of std::pair.
+  /// For example, for SCEV S={0, +, 1}<L1> + {0, +, 1}<L2> and loop L=L1
+  /// the function returns pair:
+  /// first = {0, +, 1}<L2>
+  /// second = {1, +, 1}<L1> + {0, +, 1}<L2>
+  /// We can see that for the first AddRec sub-expression it was replaced with
+  /// 0 (initial value) for the first element and to {1, +, 1}<L1> (post
+  /// increment value) for the second one. In both cases AddRec expression
+  /// related to L2 remains the same.
+  std::pair<const SCEV *, const SCEV *> SplitIntoInitAndPostInc(const Loop *L,
+                                                                const SCEV *S);
+
   /// Test if the given expression is known to satisfy the condition described
   /// by Pred, LHS, and RHS.
   bool isKnownPredicate(ICmpInst::Predicate Pred, const SCEV *LHS,
                         const SCEV *RHS);
 
   /// Test if the condition described by Pred, LHS, RHS is known to be true on
   /// every iteration of the loop of the recurrency LHS.
   bool isKnownOnEveryIteration(ICmpInst::Predicate Pred,

lib/Analysis/ScalarEvolution.cpp

 bool ScalarEvolution::isKnownNonPositive(const SCEV *S) {
   return !getSignedRangeMax(S).isStrictlyPositive();
 }
 
 bool ScalarEvolution::isKnownNonZero(const SCEV *S) {
   return isKnownNegative(S) || isKnownPositive(S);
 }
 
+std::pair<const SCEV *, const SCEV *>
+ScalarEvolution::SplitIntoInitAndPostInc(const Loop *L, const SCEV *S) {
+  // Compute SCEV on entry of loop L.
+  const SCEV *Start = SCEVInitRewriter::rewrite(S, L, *this);
+  if (Start == getCouldNotCompute())
+    return { Start, Start };
+  // Compute post increment SCEV for loop L.
+  const SCEV *PostInc = SCEVPostIncRewriter::rewrite(S, L, *this);
+  assert(PostInc != getCouldNotCompute() && "Unexpected could not compute");
+  return { Start, PostInc };
+}
+
 bool ScalarEvolution::isKnownPredicate(ICmpInst::Predicate Pred,
                                        const SCEV *LHS, const SCEV *RHS) {
   // Canonicalize the inputs first.
   (void)SimplifyICmpOperands(Pred, LHS, RHS);
 
-  // If LHS or RHS is an addrec, check to see if the condition is true in
-  // every iteration of the loop.
-  // If LHS and RHS are both addrec, both conditions must be true in
-  // every iteration of the loop.
-  const SCEVAddRecExpr *LAR = dyn_cast<SCEVAddRecExpr>(LHS);
-  const SCEVAddRecExpr *RAR = dyn_cast<SCEVAddRecExpr>(RHS);
-  bool LeftGuarded = false;
-  bool RightGuarded = false;
-  if (LAR) {
-    const Loop *L = LAR->getLoop();
-    if (isAvailableAtLoopEntry(RHS, L) &&
-        isKnownOnEveryIteration(Pred, LAR, RHS)) {
-      if (!RAR) return true;
-      LeftGuarded = true;
-    }
-  }
-  if (RAR) {
-    const Loop *L = RAR->getLoop();
-    auto SwappedPred = ICmpInst::getSwappedPredicate(Pred);
-    if (isAvailableAtLoopEntry(LHS, L) &&
-        isKnownOnEveryIteration(SwappedPred, RAR, LHS)) {
-      if (!LAR) return true;
-      RightGuarded = true;
+  // We'd like to check the predicate on every iteration of the most dominated
+  // loop between loops used in LHS and RHS.
+  // To do this we use the following list of steps:
+  // 1. Collect set S all loops on which either LHS or RHS depend.
+  // 2. If S is non-empty
+  // a. Let PD be the element of S which is dominated by all other elements of S
+  // b. Let E(LHS) be value of LHS on entry of PD.
+  //    To get E(LHS), we should just take LHS and replace all AddRecs that are
+  //    attached to PD on with their entry values.
+  //    Define E(RHS) in the same way.
+  // c. Let B(LHS) be value of L on backedge of PD.
+  //    To get B(LHS), we should just take LHS and replace all AddRecs that are
+  //    attached to PD on with their backedge values.
+  //    Define B(RHS) in the same way.
+  // d. Note that E(LHS) and E(RHS) are automatically available on entry of PD,
+  //    so we can assert on that.
+  // e. Return true if isLoopEntryGuardedByCond(Pred, E(LHS), E(RHS)) &&
+  //                   isLoopBackedgeGuardedByCond(Pred, B(LHS), B(RHS))
+
+  // First collect all loops.
+  SmallPtrSet<const Loop *, 8> LoopsUsed;
+  getUsedLoops(LHS, LoopsUsed);
+  getUsedLoops(RHS, LoopsUsed);
+
+  // Domination relationship must be a linear order on collected loops.
+#ifndef NDEBUG
+  for (auto *L1 : LoopsUsed)
+    for (auto *L2 : LoopsUsed)
+      assert((DT.dominates(L1->getHeader(), L2->getHeader()) ||
+              DT.dominates(L2->getHeader(), L1->getHeader())) &&
+             "Domination relationship is not a linear order");
+#endif
+  if (!LoopsUsed.empty()) {
+    const Loop *MDL = *std::max_element(LoopsUsed.begin(), LoopsUsed.end(),
+                                        [&](const Loop *L1, const Loop *L2) {
+                         return DT.dominates(L1->getHeader(), L2->getHeader());
+                       });
+
+    // Get init and post increment value for LHS.
+    auto SplitLHS = SplitIntoInitAndPostInc(MDL, LHS);
+    if (SplitLHS.first != getCouldNotCompute()) {
+      // if LHS does not contain unknown non-invariant SCEV then
+      // get init and post increment value for RHS.
+      auto SplitRHS = SplitIntoInitAndPostInc(MDL, RHS);
+      if (SplitRHS.first != getCouldNotCompute()) {
+        // if RHS does not contain unknown non-invariant SCEV then
+        // check whether implication is possible.
+        // It is possible that init SCEV contains an invariant load but it does
+        // not dominate MDL and is not available at MDL loop entry, so we should
+        // check it here.
+        if (isAvailableAtLoopEntry(SplitLHS.first, MDL) &&
+            isAvailableAtLoopEntry(SplitRHS.first, MDL)) {
+          if (isLoopEntryGuardedByCond(MDL, Pred, SplitLHS.first,
+                                       SplitRHS.first) &&
+              isLoopBackedgeGuardedByCond(MDL, Pred, SplitLHS.second,
+                                          SplitRHS.second))
+            return true;
+        }
+      }
     }
   }
-  if (LeftGuarded && RightGuarded)
-    return true;
 
   if (isKnownPredicateViaSplitting(Pred, LHS, RHS))
     return true;
 
   // Otherwise see what can be done with some simple reasoning.
   return isKnownViaNonRecursiveReasoning(Pred, LHS, RHS);
 }
 

test/Analysis/ScalarEvolution/inner-loop-by-latch-cond-unknown.ll

+; RUN: opt < %s -analyze -iv-users -S
+
+; This is a regression test for the commit rL327362.
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @foo(i64)
+
+define void @test(i64 %a, i64* %p) {
+entry:
+  br label %first_loop
+
+first_loop:
+  %i = phi i64 [20, %entry], [%i.next, %first_loop]
+  %i.next = add nuw nsw i64 %i, 1
+  %cond1 = icmp ult i64 %i.next, %a
+  br i1 %cond1, label %first_loop, label %middle_block
+
+middle_block:
+  %b = load i64, i64* %p
+  %cmp = icmp ult i64 %i, %b
+; When SCEV will try to compute the initial value for %j
+; it will observe umax generated by this select.
+; When it will try to simplify this umax it will invoke
+; isKnownPredicate with AddRec for %i and unknonw SCEV for %b.
+; As a result we find MDL == first_loop where %b is not available
+; at loop entry.
+  %s = select i1 %cmp, i64 %i, i64 %b
+  br label %second_loop
+
+second_loop:
+  %j = phi i64 [%s, %middle_block], [%j.next, %second_loop]
+  %j.next = add nuw nsw i64 %j, 1
+  %cond2 = icmp ult i64 %j.next, 100
+  br i1 %cond2, label %second_loop, label %return
+
+return:
+  ret void
+}

test/Transforms/IndVarSimplify/inner-loop-by-latch-cond.ll

+; RUN: opt < %s -indvars -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @foo(i64)
+
+define void @test(i64 %a) {
+entry:
+  br label %outer_header
+
+outer_header:
+  %i = phi i64 [20, %entry], [%i.next, %outer_latch]
+  %i.next = add nuw nsw i64 %i, 1
+  br label %inner_header
+
+inner_header:
+  %j = phi i64 [1, %outer_header], [%j.next, %inner_header]
+  %cmp = icmp ult i64 %j, %i.next
+; CHECK-NOT: select
+  %s = select i1 %cmp, i64 %j, i64 %i
+  call void @foo(i64 %s)
+  %j.next = add nuw nsw i64 %j, 1
+  %cond = icmp ult i64 %j, %i
+  br i1 %cond, label %inner_header, label %outer_latch
+
+outer_latch:
+  %cond2 = icmp ne i64 %i.next, 40
+  br i1 %cond2, label %outer_header, label %return
+
+return:
+  ret void
+}