[PATCH] D88087: [SCEV] Limited support for unsigned preds in isImpliedViaOperations

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: fhahn, lebedev.ri, reames.
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mkazantsev requested review of this revision.

The logic there only considers `SLT/SGT` predicates. We can use the same logic
for proving `ULT/UGT` predicates if all involved values are non-negative.

Adding full-scale support for unsigned might be challenging because of code amount,
so we can consider this in the future.


https://reviews.llvm.org/D88087

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/unittests/Analysis/ScalarEvolutionTest.cpp


Index: llvm/unittests/Analysis/ScalarEvolutionTest.cpp
===================================================================
--- llvm/unittests/Analysis/ScalarEvolutionTest.cpp
+++ llvm/unittests/Analysis/ScalarEvolutionTest.cpp
@@ -1186,4 +1186,34 @@
   });
 }
 
+TEST_F(ScalarEvolutionsTest, UnsignedIsImpliedViaOperations) {
+  LLVMContext C;
+  SMDiagnostic Err;
+  std::unique_ptr<Module> M =
+      parseAssemblyString("define void @foo(i32* %p) { "
+                          "entry: "
+                          "  %x = load i32, i32* %p, !range !0 "
+                          "  %cond = icmp ne i32 %x, 0 "
+                          "  br i1 %cond, label %loop, label %exit "
+                          "loop: "
+                          "  %y = add i32 %x, -1 "
+                          "  br label %loop "
+                          "exit:"
+                          "  ret void "
+                          "} "
+                          "!0 = !{i32 0, i32 2147483647}",
+                          Err, C);
+
+  ASSERT_TRUE(M && "Could not parse module?");
+  ASSERT_TRUE(!verifyModule(*M) && "Must have been well formed!");
+
+  runWithSE(*M, "foo", [](Function &F, LoopInfo &LI, ScalarEvolution &SE) {
+    auto *X = SE.getSCEV(getInstructionByName(F, "x"));
+    auto *Y = SE.getSCEV(getInstructionByName(F, "y"));
+    auto *Loop = LI.getLoopFor(getInstructionByName(F, "y")->getParent());
+    ASSERT_TRUE(Loop);
+    EXPECT_TRUE(SE.isLoopEntryGuardedByCond(Loop, ICmpInst::ICMP_UGT, X, Y));
+  });
+}
+
 }  // end namespace llvm
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10148,8 +10148,21 @@
   // We want to avoid hurting the compile time with analysis of too big trees.
   if (Depth > MaxSCEVOperationsImplicationDepth)
     return false;
+
+  if (ICmpInst::isUnsigned(Pred))
+    // We can replace unsigned predicate with its signed counterpart if all
+    // involved values are non-negative.
+    // TODO: We could have better support for unsigned.
+    if (isKnownNonNegative(FoundLHS) && isKnownNonNegative(FoundRHS)) {
+      const SCEV *MinusOne = getNegativeSCEV(getOne(LHS->getType()));
+      if (isImpliedCondOperands(ICmpInst::ICMP_SGT, LHS, MinusOne, FoundLHS,
+                                FoundRHS) &&
+          isImpliedCondOperands(ICmpInst::ICMP_SGT, RHS, MinusOne, FoundLHS,
+                                FoundRHS))
+        Pred = ICmpInst::getSignedPredicate(Pred);
+    }
+
   // We only want to work with ICMP_SGT comparison so far.
-  // TODO: Extend to ICMP_UGT?
   if (Pred == ICmpInst::ICMP_SLT) {
     Pred = ICmpInst::ICMP_SGT;
     std::swap(LHS, RHS);


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