Revert "[VectorUtils] Query number of sign bits to allow more truncations"

This was a fairly simple patch but on closer inspection was seriously flawed and caused PR27690.

This reverts commit r268921.

llvm-svn: 269051

1 files changed, 4 insertions, 14 deletions

diff --git a/llvm/lib/Analysis/VectorUtils.cpp b/llvm/lib/Analysis/VectorUtils.cpp
index 2c03f1a05ce..23a0de856bc 100644
--- a/llvm/lib/Analysis/VectorUtils.cpp
+++ b/llvm/lib/Analysis/VectorUtils.cpp
@@ -320,9 +320,6 @@ llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
   SmallPtrSet<Instruction *, 4> InstructionSet;
   MapVector<Instruction *, uint64_t> MinBWs;
 
-  assert(Blocks.size() > 0 && "Must have at least one block!");
-  const DataLayout &DL = Blocks[0]->getModule()->getDataLayout();
-  
   // Determine the roots. We work bottom-up, from truncs or icmps.
   bool SeenExtFromIllegalType = false;
   for (auto *BB : Blocks)
@@ -366,19 +363,12 @@ llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
 
     // If we encounter a type that is larger than 64 bits, we can't represent
     // it so bail out.
-    APInt NeededBits = DB.getDemandedBits(I);
-    unsigned BW = NeededBits.getBitWidth();
-    if (BW > 64)
+    if (DB.getDemandedBits(I).getBitWidth() > 64)
       return MapVector<Instruction *, uint64_t>();
 
-    auto NSB = ComputeNumSignBits(I, DL);
-
-    // Query demanded bits for the bits required by the instruction. Remove
-    // any bits that are equal to the sign bit, because we can truncate the
-    // instruction without changing their value.
-    NeededBits &= APInt::getLowBitsSet(BW, BW - NSB);
-    DBits[Leader] |= NeededBits.getZExtValue();
-    DBits[I] |= NeededBits.getZExtValue();
+    uint64_t V = DB.getDemandedBits(I).getZExtValue();
+    DBits[Leader] |= V;
+    DBits[I] = V;
 
     // Casts, loads and instructions outside of our range terminate a chain
     // successfully.