[llvm] b8ac19c - [SCEV] Limited support for unsigned preds in isImpliedViaOperations

[Max Kazantsev via llvm-commits]

Author: Max Kazantsev
Date: 2020-10-02T10:20:57+07:00
New Revision: b8ac19cf1cca5faec8b4404bb0f666cb63c9e1de

URL: https://github.com/llvm/llvm-project/commit/b8ac19cf1cca5faec8b4404bb0f666cb63c9e1de
DIFF: https://github.com/llvm/llvm-project/commit/b8ac19cf1cca5faec8b4404bb0f666cb63c9e1de.diff

LOG: [SCEV] Limited support for unsigned preds in isImpliedViaOperations

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.

Differential Revision: https://reviews.llvm.org/D88087
Reviewed By: reames

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index a3e454fefcf0..70d37cb73fd1 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10236,13 +10236,31 @@ bool ScalarEvolution::isImpliedViaOperations(ICmpInst::Predicate Pred,
   // We want to avoid hurting the compile time with analysis of too big trees.
   if (Depth > MaxSCEVOperationsImplicationDepth)
     return false;
-  // 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;
+
+  // We only want to work with GT comparison so far.
+  if (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_SLT) {
+    Pred = CmpInst::getSwappedPredicate(Pred);
     std::swap(LHS, RHS);
     std::swap(FoundLHS, FoundRHS);
   }
+
+  // For unsigned, try to reduce it to corresponding signed comparison.
+  if (Pred == ICmpInst::ICMP_UGT)
+    // 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)) {
+      // Knowing that both FoundLHS and FoundRHS are non-negative, and knowing
+      // FoundLHS >u FoundRHS, we also know that FoundLHS >s FoundRHS. Let us
+      // use this fact to prove that LHS and RHS are non-negative.
+      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::ICMP_SGT;
+    }
+
   if (Pred != ICmpInst::ICMP_SGT)
     return false;
 

diff  --git a/llvm/unittests/Analysis/ScalarEvolutionTest.cpp b/llvm/unittests/Analysis/ScalarEvolutionTest.cpp
index e5ffc21fb664..be8941838f71 100644
--- a/llvm/unittests/Analysis/ScalarEvolutionTest.cpp
+++ b/llvm/unittests/Analysis/ScalarEvolutionTest.cpp
@@ -1283,4 +1283,37 @@ TEST_F(ScalarEvolutionsTest, ImpliedViaAddRecStart) {
   });
 }
 
+TEST_F(ScalarEvolutionsTest, UnsignedIsImpliedViaOperations) {
+  LLVMContext C;
+  SMDiagnostic Err;
+  std::unique_ptr<Module> M =
+      parseAssemblyString("define void @foo(i32* %p1, i32* %p2) { "
+                          "entry: "
+                          "  %x = load i32, i32* %p1, !range !0 "
+                          "  %cond = icmp ne i32 %x, 0 "
+                          "  br i1 %cond, label %guarded, label %exit "
+                          "guarded: "
+                          "  %y = add i32 %x, -1 "
+                          "  ret void "
+                          "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 *Guarded = getInstructionByName(F, "y")->getParent();
+    ASSERT_TRUE(Guarded);
+    EXPECT_TRUE(
+        SE.isBasicBlockEntryGuardedByCond(Guarded, ICmpInst::ICMP_ULT, Y, X));
+    EXPECT_TRUE(
+        SE.isBasicBlockEntryGuardedByCond(Guarded, ICmpInst::ICMP_UGT, X, Y));
+  });
+}
+
 }  // end namespace llvm