[llvm] [InstCombine] Remove shl if we only demand known signbits of shift source (PR #79014)

[Nikita Popov via llvm-commits]

================
@@ -640,25 +640,31 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
                                                     DemandedMask, Known))
             return R;
 
-      // TODO: If we only want bits that already match the signbit then we don't
+      uint64_t ShiftAmt = SA->getLimitedValue(BitWidth - 1);
+      // If we only want bits that already match the signbit then we don't
       // need to shift.
+      if (DemandedMask.countr_zero() >= ShiftAmt) {
+        unsigned NumHiDemandedBits = BitWidth - DemandedMask.countr_zero();
+        unsigned SignBits =
+            ComputeNumSignBits(I->getOperand(0), Depth + 1, CxtI);
+        if (SignBits > ShiftAmt && SignBits - ShiftAmt >= NumHiDemandedBits)
+          return I->getOperand(0);
 
-      // If we can pre-shift a right-shifted constant to the left without
-      // losing any high bits amd we don't demand the low bits, then eliminate
-      // the left-shift:
-      // (C >> X) << LeftShiftAmtC --> (C << RightShiftAmtC) >> X
-      uint64_t ShiftAmt = SA->getLimitedValue(BitWidth-1);
-      Value *X;
-      Constant *C;
-      if (DemandedMask.countr_zero() >= ShiftAmt &&
-          match(I->getOperand(0), m_LShr(m_ImmConstant(C), m_Value(X)))) {
-        Constant *LeftShiftAmtC = ConstantInt::get(VTy, ShiftAmt);
-        Constant *NewC = ConstantFoldBinaryOpOperands(Instruction::Shl, C,
-                                                      LeftShiftAmtC, DL);
-        if (ConstantFoldBinaryOpOperands(Instruction::LShr, NewC, LeftShiftAmtC,
-                                         DL) == C) {
-          Instruction *Lshr = BinaryOperator::CreateLShr(NewC, X);
-          return InsertNewInstWith(Lshr, I->getIterator());
+        // If we can pre-shift a right-shifted constant to the left without
+        // losing any high bits amd we don't demand the low bits, then eliminate
+        // the left-shift:
+        // (C >> X) << LeftShiftAmtC --> (C << RightShiftAmtC) >> X
----------------
nikic wrote:

```suggestion
        // losing any high bits and we don't demand the low bits, then eliminate
        // the left-shift:
        // (C >> X) << LeftShiftAmtC --> (C << LeftShiftAmtC) >> X
```
Pre-existing, but if you're touching the code anyway...

https://github.com/llvm/llvm-project/pull/79014