[SelectionDAG] Let ComputeKnownSignBits handle (shl (ext X), C) #97695
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@llvm/pr-subscribers-backend-x86 @llvm/pr-subscribers-llvm-selectiondag Author: Björn Pettersson (bjope) ChangesAdd simple support for looking through ZEXT/ANYEXT/SEXT when doing ComputeKnownSignBits for SHL. This is valid for the case when all extended bits are shifted out, because then the number of sign bits can be found by analysing the EXT operand. A future improvement could be to pass along the "shifted left by" information in the recursive calls to ComputeKnownSignBits. Allowing us to handle this more generically. Full diff: https://github.com/llvm/llvm-project/pull/97695.diff 1 Files Affected:
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 96242305e9eab..991df60a5e650 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -4617,6 +4617,21 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
case ISD::SHL:
if (std::optional<uint64_t> ShAmt =
getValidMaximumShiftAmount(Op, DemandedElts, Depth + 1)) {
+ if (Op.getOperand(0).getOpcode() == ISD::ANY_EXTEND ||
+ Op.getOperand(0).getOpcode() == ISD::ZERO_EXTEND ||
+ Op.getOperand(0).getOpcode() == ISD::SIGN_EXTEND) {
+ SDValue Src = Op.getOperand(0);
+ EVT SrcVT = Src.getValueType();
+ SDValue ExtendedOp = Op.getOperand(0).getOperand(0);
+ EVT ExtendedOpVT = ExtendedOp.getValueType();
+ uint64_t ExtendedWidth =
+ SrcVT.getScalarSizeInBits() - ExtendedOpVT.getScalarSizeInBits();
+ if (ExtendedWidth <= *ShAmt) {
+ Tmp = ComputeNumSignBits(ExtendedOp, DemandedElts, Depth + 1);
+ if (*ShAmt - ExtendedWidth < Tmp)
+ return Tmp - (*ShAmt - ExtendedWidth);
+ }
+ }
// shl destroys sign bits, ensure it doesn't shift out all sign bits.
Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1);
if (*ShAmt < Tmp)
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No regression tests yet. If anyone has a good idea how and where to put them, let me know. The need for this was found downstream as DAG Combiner prefer |
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Added some test cases based on the |
| @@ -0,0 +1,158 @@ | |||
| ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5 | |||
| ; RUN: llc < %s -mtriple=x86_64-linux | FileCheck %s --check-prefix=X64 | |||
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(pedantic) rename to known-signtbits-shl.ll to
| getValidMaximumShiftAmount(Op, DemandedElts, Depth + 1)) { | ||
| if (Op.getOperand(0).getOpcode() == ISD::ANY_EXTEND || | ||
| Op.getOperand(0).getOpcode() == ISD::ZERO_EXTEND || | ||
| Op.getOperand(0).getOpcode() == ISD::SIGN_EXTEND) |
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Use ISD::isExtOpcode(Op.getOperand(0).getOpcode())
| SDValue Src = Op.getOperand(0); | ||
| EVT SrcVT = Src.getValueType(); | ||
| SDValue ExtendedOp = Op.getOperand(0).getOperand(0); | ||
| EVT ExtendedOpVT = ExtendedOp.getValueType(); |
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(pedantic) ExtendedOp isn't extended - it's the pre-extended source value
| EVT ExtendeeVT = Extendee.getValueType(); | ||
| uint64_t SizeDifference = | ||
| ExtVT.getScalarSizeInBits() - ExtendeeVT.getScalarSizeInBits(); | ||
| if (SizeDifference <= MinShAmt) { |
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Add a explanation comment (similar to the patch summary)
…lvm#97695) Adding test cases aiming at showing possibility to look through ZERO_EXTEND/ANY_EXTEND when computing number of sign bits for an SHL node. If all extended bits are shifted out we can analyze the operand that is extended.
…#97695) Add simple support for looking through ZEXT/ANYEXT/SEXT when doing ComputeKnownSignBits for SHL. This is valid for the case when all extended bits are shifted out, because then the number of sign bits can be found by analysing the EXT operand. A future improvement could be to pass along the "shifted left by" information in the recursive calls to ComputeKnownSignBits. Allowing us to handle this more generically.
Thanks! |
…lvm#97695) Adding test cases aiming at showing possibility to look through ZERO_EXTEND/ANY_EXTEND when computing number of sign bits for an SHL node. If all extended bits are shifted out we can analyze the operand that is extended.
…#97695) Add simple support for looking through ZEXT/ANYEXT/SEXT when doing ComputeKnownSignBits for SHL. This is valid for the case when all extended bits are shifted out, because then the number of sign bits can be found by analysing the EXT operand. A future improvement could be to pass along the "shifted left by" information in the recursive calls to ComputeKnownSignBits. Allowing us to handle this more generically.
Add simple support for looking through ZEXT/ANYEXT/SEXT when doing ComputeKnownSignBits for SHL. This is valid for the case when all extended bits are shifted out, because then the number of sign bits can be found by analysing the EXT operand.
A future improvement could be to pass along the "shifted left by" information in the recursive calls to ComputeKnownSignBits. Allowing us to handle this more generically.