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[NFC] Change SelectionDAG::getMemBasePlusOffset() to use int64_t

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Summary:
This change is preparatory work to use this helper functions in more places.
Currently the function only allows positive offsets, but there are cases
where we want to subtract an offset from an existing pointer.

The motivation for this change is our out-of-tree CHERI backend
(https://github.com/CTSRD-CHERI/llvm-project). We use a separate register
type to store pointers (128-bit capabilities, which are effectively
unforgeable and monotonic fat pointers). These capabilities permit a
reduced set of operations and therefore use a separate ValueType (iFATPTR).
to represent pointers implemented as capabilities.
Therefore, we need to avoid using ISD::ADD for our patterns that operate
on pointers and need to use a function that chooses ISD::ADD or a new
ISD::PTRADD opcode depending on the value type.

We originally added a new DAG.getPointerAdd() function, but after this
patch series we can modify the implementation of getMemBasePlusOffset()
instead. Avoiding direct uses of ISD::ADD for pointer types will
significantly reduce the amount of assertion/instruction selection
failures for us in future upstream merges.

Reviewers: spatel

Reviewed By: spatel

Subscribers: merge_guards_bot, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71204
This commit is contained in:
Alex Richardson 2019-12-13 19:42:58 +00:00
parent dd8a7fcdd7
commit d9bb70acd7
2 changed files with 3 additions and 2 deletions
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3
llvm/include/llvm/CodeGen/SelectionDAG.h
View File

@ -1138,7 +1138,8 @@ public:
SDValue Offset, ISD::MemIndexedMode AM);
/// Returns sum of the base pointer and offset.
SDValue getMemBasePlusOffset(SDValue Base, unsigned Offset, const SDLoc &DL);
/// Unlike getObjectPtrOffset this does not set NoUnsignedWrap by default.
SDValue getMemBasePlusOffset(SDValue Base, int64_t Offset, const SDLoc &DL);
SDValue getMaskedLoad(EVT VT, const SDLoc &dl, SDValue Chain, SDValue Base,
SDValue Offset, SDValue Mask, SDValue Src0, EVT MemVT,

2
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
View File

@ -5732,7 +5732,7 @@ static SDValue getMemsetStringVal(EVT VT, const SDLoc &dl, SelectionDAG &DAG,
return SDValue(nullptr, 0);
}
SDValue SelectionDAG::getMemBasePlusOffset(SDValue Base, unsigned Offset,
SDValue SelectionDAG::getMemBasePlusOffset(SDValue Base, int64_t Offset,
const SDLoc &DL) {
EVT VT = Base.getValueType();
return getNode(ISD::ADD, DL, VT, Base, getConstant(Offset, DL, VT));