[RISCV] Custom-legalise 32-bit variable shifts on RV64
The previous DAG combiner-based approach had an issue with infinite loops
between the target-dependent and target-independent combiner logic (see
PR40333). Although this was worked around in rL351806, the combiner-based
approach is still potentially brittle and can fail to select the 32-bit shift
variant when profitable to do so, as demonstrated in the pr40333.ll test case.
This patch instead introduces target-specific SelectionDAG nodes for
SHLW/SRLW/SRAW and custom-lowers variable i32 shifts to them. pr40333.ll is a
good example of how this approach can improve codegen.
This adds DAG combine that does SimplifyDemandedBits on the operands (only
lower 32-bits of first operand and lower 5 bits of second operand are read).
This seems better than implementing SimplifyDemandedBitsForTargetNode as there
is no guarantee that would be called (and it's not for e.g. the anyext return
test cases). Also implements ComputeNumSignBitsForTargetNode.
There are codegen changes in atomic-rmw.ll and atomic-cmpxchg.ll but the new
instruction sequences are semantically equivalent.
Differential Revision: https://reviews.llvm.org/D57085
llvm-svn: 352169
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.h b/llvm/lib/Target/RISCV/RISCVISelLowering.h
index 319c270..340993c 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.h
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.h
@@ -31,7 +31,12 @@
SELECT_CC,
BuildPairF64,
SplitF64,
- TAIL
+ TAIL,
+ // RV64I shifts, directly matching the semantics of the named RISC-V
+ // instructions.
+ SLLW,
+ SRAW,
+ SRLW
};
}
@@ -57,9 +62,16 @@
// Provide custom lowering hooks for some operations.
SDValue LowerOperation(SDValue Op, SelectionDAG &DAG) const override;
+ void ReplaceNodeResults(SDNode *N, SmallVectorImpl<SDValue> &Results,
+ SelectionDAG &DAG) const override;
SDValue PerformDAGCombine(SDNode *N, DAGCombinerInfo &DCI) const override;
+ unsigned ComputeNumSignBitsForTargetNode(SDValue Op,
+ const APInt &DemandedElts,
+ const SelectionDAG &DAG,
+ unsigned Depth) const override;
+
// This method returns the name of a target specific DAG node.
const char *getTargetNodeName(unsigned Opcode) const override;