[llvm] 7255ce3 - [SelectionDAG] Fix incorrect condition for shift amount truncation

[Craig Topper via llvm-commits]

Author: Itay Bookstein
Date: 2021-09-28T17:52:30-07:00
New Revision: 7255ce30e48feb07e4e82613f518683fbc247c1c

URL: https://github.com/llvm/llvm-project/commit/7255ce30e48feb07e4e82613f518683fbc247c1c
DIFF: https://github.com/llvm/llvm-project/commit/7255ce30e48feb07e4e82613f518683fbc247c1c.diff

LOG: [SelectionDAG] Fix incorrect condition for shift amount truncation

Comment says:
  // If the operand is larger than the shift count type but the shift
  // count type has enough bits to represent any shift value ...

It clearly talks about the shifted operand, not the shift-amount operand,
but the comparison is performed against Log2_32_Ceil(Op2.getValueSizeInBits())
where Op2 is the shift amount operand. This comparison also doesn't make
sense in the context of the previous one (ShiftsSize > Op2Size) because
Op2Size == Op2.getValueSizeInBits(). Fix to use Op1.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D110509

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 7c16652451b4..83c3c1689b19 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -3146,7 +3146,7 @@ void SelectionDAGBuilder::visitShift(const User &I, unsigned Opcode) {
     // count type has enough bits to represent any shift value, truncate
     // it now. This is a common case and it exposes the truncate to
     // optimization early.
-    else if (ShiftSize >= Log2_32_Ceil(Op2.getValueSizeInBits()))
+    else if (ShiftSize >= Log2_32_Ceil(Op1.getValueSizeInBits()))
       Op2 = DAG.getNode(ISD::TRUNCATE, DL, ShiftTy, Op2);
     // Otherwise we'll need to temporarily settle for some other convenient
     // type.  Type legalization will make adjustments once the shiftee is split.