[GlobalISel] Make use of KnownBits::computeForAddSub

jayfoad · jayfoad · commit cbbbd5b5f617 · 2020-01-27T22:22:56.000Z
Summary: This is mostly NFC. computeForAddSub may give more precise results in some cases, but that doesn't seem to affect any existing GlobalISel tests. Subscribers: rovka, hiraditya, volkan, Petar.Avramovic, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D73431
diff --git a/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp b/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
@@ -169,18 +169,12 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
     break;
   }
   case TargetOpcode::G_SUB: {
-    // If low bits are known to be zero in both operands, then we know they are
-    // going to be 0 in the result. Both addition and complement operations
-    // preserve the low zero bits.
-    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known, DemandedElts,
                          Depth + 1);
-    unsigned KnownZeroLow = Known2.countMinTrailingZeros();
-    if (KnownZeroLow == 0)
-      break;
     computeKnownBitsImpl(MI.getOperand(2).getReg(), Known2, DemandedElts,
                          Depth + 1);
-    KnownZeroLow = std::min(KnownZeroLow, Known2.countMinTrailingZeros());
-    Known.Zero.setLowBits(KnownZeroLow);
+    Known = KnownBits::computeForAddSub(/*Add*/ false, /*NSW*/ false, Known,
+                                        Known2);
     break;
   }
   case TargetOpcode::G_XOR: {
@@ -204,24 +198,12 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
     LLVM_FALLTHROUGH;
   }
   case TargetOpcode::G_ADD: {
-    // Output known-0 bits are known if clear or set in both the low clear bits
-    // common to both LHS & RHS.  For example, 8+(X<<3) is known to have the
-    // low 3 bits clear.
-    // Output known-0 bits are also known if the top bits of each input are
-    // known to be clear. For example, if one input has the top 10 bits clear
-    // and the other has the top 8 bits clear, we know the top 7 bits of the
-    // output must be clear.
-    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known, DemandedElts,
                          Depth + 1);
-    unsigned KnownZeroHigh = Known2.countMinLeadingZeros();
-    unsigned KnownZeroLow = Known2.countMinTrailingZeros();
     computeKnownBitsImpl(MI.getOperand(2).getReg(), Known2, DemandedElts,
                          Depth + 1);
-    KnownZeroHigh = std::min(KnownZeroHigh, Known2.countMinLeadingZeros());
-    KnownZeroLow = std::min(KnownZeroLow, Known2.countMinTrailingZeros());
-    Known.Zero.setLowBits(KnownZeroLow);
-    if (KnownZeroHigh > 1)
-      Known.Zero.setHighBits(KnownZeroHigh - 1);
+    Known =
+        KnownBits::computeForAddSub(/*Add*/ true, /*NSW*/ false, Known, Known2);
     break;
   }
   case TargetOpcode::G_AND: {
