[InstCombine] dropRedundantMaskingOfLeftShiftInput(): pat. c/d/e with mask (PR42563)

Summary:
If we have a pattern `(x & (-1 >> maskNbits)) << shiftNbits`,
we already know (have a fold) that will drop the `& (-1 >> maskNbits)`
mask iff `(shiftNbits-maskNbits) s>= 0` (i.e. `shiftNbits u>= maskNbits`).

So even if `(shiftNbits-maskNbits) s< 0`, we can still
fold, we will just need to apply a **constant** mask afterwards:
```
Name: c, normal+mask
  %t0 = lshr i32 -1, C1
  %t1 = and i32 %t0, %x
  %r = shl i32 %t1, C2
=>
  %n0 = shl i32 %x, C2
  %n1 = i32 ((-(C2-C1))+32)
  %n2 = zext i32 %n1 to i64
  %n3 = lshr i64 -1, %n2
  %n4 = trunc i64 %n3 to i32
  %r = and i32 %n0, %n4
```
https://rise4fun.com/Alive/gslRa

Naturally, old `%masked` will have to be one-use.
This is not valid for pattern f - where "masking" is done via `ashr`.

https://bugs.llvm.org/show_bug.cgi?id=42563

Reviewers: spatel, nikic, xbolva00

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67725

llvm-svn: 372630

1 files changed, 28 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 3f466495c5e..2dea4c45cbd 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -202,10 +202,35 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
     // shall be unset in the root value (OuterShift). If ShAmtsDiff is negative,
     // we'll need to also produce a mask to unset ShAmtsDiff high bits.
     // So, does *any* channel need a mask? (is ShiftShAmt u>= MaskShAmt ?)
-    if (!match(ShAmtsDiff, m_NonNegative()))
-      return nullptr; // FIXME.
+    if (!match(ShAmtsDiff, m_NonNegative())) {
+      // This sub-fold (with mask) is invalid for 'ashr' "masking" instruction.
+      if (match(Masked, m_AShr(m_Value(), m_Value())))
+        return nullptr;
+      // For a mask we need to get rid of old masking instruction.
+      if (!Masked->hasOneUse())
+        return nullptr; // Else we can't perform the fold.
+      Type *Ty = X->getType();
+      unsigned BitWidth = Ty->getScalarSizeInBits();
+      // We should produce compute the mask in wider type, and truncate later!
+      // Get type twice as wide element-wise (same number of elements!).
+      Type *ExtendedScalarTy = Type::getIntNTy(Ty->getContext(), 2 * BitWidth);
+      Type *ExtendedTy =
+          Ty->isVectorTy()
+              ? VectorType::get(ExtendedScalarTy, Ty->getVectorNumElements())
+              : ExtendedScalarTy;
+      auto *ExtendedNumHighBitsToClear = ConstantExpr::getZExt(
+          ConstantExpr::getAdd(
+              ConstantExpr::getNeg(ShAmtsDiff),
+              ConstantInt::get(Ty, BitWidth, /*isSigned=*/false)),
+          ExtendedTy);
+      // And compute the mask as usual: (-1 l>> (ShAmtsDiff))
+      auto *ExtendedAllOnes = ConstantExpr::getAllOnesValue(ExtendedTy);
+      auto *ExtendedMask =
+          ConstantExpr::getLShr(ExtendedAllOnes, ExtendedNumHighBitsToClear);
+      NewMask = ConstantExpr::getTrunc(ExtendedMask, Ty);
+    } else
+      NewMask = nullptr; // No mask needed.
     // All good, we can do this fold.
-    NewMask = nullptr; // No mask needed.
   } else
     return nullptr; // Don't know anything about this pattern.