[llvm] a6183d0 - [ValueTracking] isKnownNonZero, computeKnownBits for freeze

[Juneyoung Lee via llvm-commits]

Author: Juneyoung Lee
Date: 2020-09-10T08:07:38+09:00
New Revision: a6183d0f028cb73eccc82a7cce9534708a149762

URL: https://github.com/llvm/llvm-project/commit/a6183d0f028cb73eccc82a7cce9534708a149762
DIFF: https://github.com/llvm/llvm-project/commit/a6183d0f028cb73eccc82a7cce9534708a149762.diff

LOG: [ValueTracking] isKnownNonZero, computeKnownBits for freeze

This implements support for isKnownNonZero, computeKnownBits when freeze is involved.

```
  br (x != 0), BB1, BB2
BB1:
  y = freeze x
```

In the above program, we can say that y is non-zero. The reason is as follows:

(1) If x was poison, `br (x != 0)` raised UB
(2) If x was fully undef, the branch again raised UB
(3) If x was non-zero partially undef, say `undef | 1`, `freeze x` will return a nondeterministic value which is also non-zero.
(4) If x was just a concrete value, it is trivial

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D75808

Added: 
    

Modified: 
    llvm/lib/Analysis/ValueTracking.cpp
    llvm/test/Transforms/InstSimplify/known-non-zero.ll
    llvm/unittests/Analysis/ValueTrackingTest.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 469257d91071..1a894959c5bd 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1872,6 +1872,10 @@ static void computeKnownBitsFromOperator(const Operator *I,
       }
     }
     break;
+  case Instruction::Freeze:
+    if (isGuaranteedNotToBePoison(I->getOperand(0), Q.CxtI, Q.DT, Depth + 1))
+      computeKnownBits(I->getOperand(0), Known, Depth + 1, Q);
+    break;
   }
 }
 
@@ -2577,6 +2581,13 @@ bool isKnownNonZero(const Value *V, const APInt &DemandedElts, unsigned Depth,
       return isKnownNonZero(Vec, DemandedVecElts, Depth, Q);
     }
   }
+  // Freeze
+  else if (const FreezeInst *FI = dyn_cast<FreezeInst>(V)) {
+    auto *Op = FI->getOperand(0);
+    if (isKnownNonZero(Op, Depth, Q) &&
+        isGuaranteedNotToBePoison(Op, Q.CxtI, Q.DT, Depth))
+      return true;
+  }
 
   KnownBits Known(BitWidth);
   computeKnownBits(V, DemandedElts, Known, Depth, Q);

diff  --git a/llvm/test/Transforms/InstSimplify/known-non-zero.ll b/llvm/test/Transforms/InstSimplify/known-non-zero.ll
index 524e51be76f5..2af4f2716206 100644
--- a/llvm/test/Transforms/InstSimplify/known-non-zero.ll
+++ b/llvm/test/Transforms/InstSimplify/known-non-zero.ll
@@ -145,3 +145,24 @@ for.body:                                         ; preds = %for.cond
   %inc = add nuw nsw i32 %shift.0, 1
   br label %for.cond
 }
+
+define i1 @freeze_nonzero(i8 %x, i8 %mask) {
+; CHECK-LABEL: @freeze_nonzero(
+; CHECK-NEXT:    [[Y:%.*]] = or i8 [[X:%.*]], [[MASK:%.*]]
+; CHECK-NEXT:    [[C:%.*]] = icmp ne i8 [[Y]], 0
+; CHECK-NEXT:    br i1 [[C]], label [[A:%.*]], label [[B:%.*]]
+; CHECK:       A:
+; CHECK-NEXT:    ret i1 false
+; CHECK:       B:
+; CHECK-NEXT:    ret i1 false
+;
+  %y = or i8 %x, %mask
+  %c = icmp ne i8 %y, 0
+  br i1 %c, label %A, label %B
+A:
+  %fr = freeze i8 %y
+  %c2 = icmp eq i8 %fr, 0
+  ret i1 %c2
+B:
+  ret i1 0
+}

diff  --git a/llvm/unittests/Analysis/ValueTrackingTest.cpp b/llvm/unittests/Analysis/ValueTrackingTest.cpp
index 09faad448459..c45bca1c53bf 100644
--- a/llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ b/llvm/unittests/Analysis/ValueTrackingTest.cpp
@@ -1059,6 +1059,24 @@ TEST_F(ComputeKnownBitsTest, ComputeKnownBitsPtrToIntZext) {
   EXPECT_EQ(Known.One.getZExtValue(), 0u);
 }
 
+TEST_F(ComputeKnownBitsTest, ComputeKnownBitsFreeze) {
+  parseAssembly("define void @test() {\n"
+                "  %m = call i32 @any_num()\n"
+                "  %A = freeze i32 %m\n"
+                "  %n = and i32 %m, 31\n"
+                "  %c = icmp eq i32 %n, 0\n"
+                "  call void @llvm.assume(i1 %c)\n"
+                "  ret void\n"
+                "}\n"
+                "declare void @llvm.assume(i1)\n"
+                "declare i32 @any_num()\n");
+  AssumptionCache AC(*F);
+  KnownBits Known = computeKnownBits(A, M->getDataLayout(), /* Depth */ 0, &AC,
+                                     F->front().getTerminator());
+  EXPECT_EQ(Known.Zero.getZExtValue(), 31u);
+  EXPECT_EQ(Known.One.getZExtValue(), 0u);
+}
+
 class IsBytewiseValueTest : public ValueTrackingTest,
                             public ::testing::WithParamInterface<
                                 std::pair<const char *, const char *>> {