[PATCH] D43175: [SCEV] Favor isKnownViaSimpleReasoning over constant ranges check

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: sanjoy, skatkov, reames.

There is a more powerful but still simple function `isKnownViaSimpleReasoning ` that
does constant range check and few more additional checks. We use it some places (e.g.
when proving implications) and in some other places we only check constant ranges.

Currently, indvar simplifier fails to remove the check in following loop:

  int inc = ...;
  for (int i = inc, j = inc - 1; i < 200; ++i, ++j)
    if (i > j) { ... }

This patch replaces all usages of `isKnownPredicateViaConstantRanges` with
`isKnownViaSimpleReasoning` to have smarter proofs. In particular, it fixes the
case above.


https://reviews.llvm.org/D43175

Files:
  lib/Analysis/ScalarEvolution.cpp
  test/Transforms/IndVarSimplify/loop-invariant-conditions.ll


Index: test/Transforms/IndVarSimplify/loop-invariant-conditions.ll
===================================================================
--- test/Transforms/IndVarSimplify/loop-invariant-conditions.ll
+++ test/Transforms/IndVarSimplify/loop-invariant-conditions.ll
@@ -355,6 +355,38 @@
   ret void
 }
 
+; check that we can prove that a recurrency is greater than another recurrency
+; in the same loop, with the same step, and with smaller starting value.
+define void @test12(i64* %inc_ptr) {
+; CHECK-LABEL: @test12
+entry:
+  %inc = load i64, i64* %inc_ptr, !range !0
+  %inc.minus.1 = sub i64 %inc, 1
+  br label %loop
+
+loop:
+  %iv = phi i64 [ %inc, %entry ], [ %iv.next, %backedge ]
+  %iv.minus.1 = phi i64 [ %inc.minus.1, %entry ], [ %iv.minus.1.next, %backedge ]
+  %iv.next = add i64 %iv, 1
+  %iv.minus.1.next = add i64 %iv.minus.1, 1
+  %brcond = icmp sgt i64 %iv.next, %iv.minus.1.next
+  ; CHECK: br i1 true, label %if.true, label %if.false
+  br i1 %brcond, label %if.true, label %if.false
+
+if.true:
+  br label %backedge
+
+if.false:
+  br label %backedge
+
+backedge:
+  %loopcond = icmp slt i64 %iv, 200
+  br i1 %loopcond, label %loop, label %exit
+
+exit:
+  ret void
+}
+
 !1 = !{i64 -1, i64 100}
 
 
Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -8691,8 +8691,8 @@
   if (isKnownPredicateViaSplitting(Pred, LHS, RHS))
     return true;
 
-  // Otherwise see what can be done with known constant ranges.
-  return isKnownPredicateViaConstantRanges(Pred, LHS, RHS);
+  // Otherwise see what can be done with some simple reasoning.
+  return isKnownViaSimpleReasoning(Pred, LHS, RHS);
 }
 
 bool ScalarEvolution::isMonotonicPredicate(const SCEVAddRecExpr *LHS,
@@ -8959,7 +8959,7 @@
   // (interprocedural conditions notwithstanding).
   if (!L) return true;
 
-  if (isKnownPredicateViaConstantRanges(Pred, LHS, RHS))
+  if (isKnownViaSimpleReasoning(Pred, LHS, RHS))
     return true;
 
   BasicBlock *Latch = L->getLoopLatch();
@@ -9070,7 +9070,7 @@
   assert(isAvailableAtLoopEntry(RHS, L) &&
          "RHS is not available at Loop Entry");
 
-  if (isKnownPredicateViaConstantRanges(Pred, LHS, RHS))
+  if (isKnownViaSimpleReasoning(Pred, LHS, RHS))
     return true;
 
   // If we cannot prove strict comparison (e.g. a > b), maybe we can prove
@@ -9085,9 +9085,9 @@
 
   if (ProvingStrictComparison) {
     ProvedNonStrictComparison =
-        isKnownPredicateViaConstantRanges(NonStrictPredicate, LHS, RHS);
+        isKnownViaSimpleReasoning(NonStrictPredicate, LHS, RHS);
     ProvedNonEquality =
-        isKnownPredicateViaConstantRanges(ICmpInst::ICMP_NE, LHS, RHS);
+        isKnownViaSimpleReasoning(ICmpInst::ICMP_NE, LHS, RHS);
     if (ProvedNonStrictComparison && ProvedNonEquality)
       return true;
   }


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