From 797f639e79a90b59c3eea67e1577012b4cde77e0 Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Wed, 6 Jan 2016 00:43:06 +0000 Subject: [SelectionDAGBuilder] Set NoUnsignedWrap for inbounds gep and load/store offsets. In an inbounds getelementptr, when an index produces a constant non-negative offset to add to the base, the add can be assumed to not have unsigned overflow. This relies on the assumption that addresses can't occupy more than half the address space, which isn't possible in C because it wouldn't be possible to represent the difference between the start of the object and one-past-the-end in a ptrdiff_t. Setting the NoUnsignedWrap flag is theoretically useful in general, and is specifically useful to the WebAssembly backend, since it permits stronger constant offset folding. Differential Revision: http://reviews.llvm.org/D15544 llvm-svn: 256890 --- llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) (limited to 'llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp') diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp index 0872d7a9a22..bc2405b952a 100644 --- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp +++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp @@ -6843,9 +6843,13 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) { uint64_t PtrOff = ShAmt / 8; unsigned NewAlign = MinAlign(LN0->getAlignment(), PtrOff); SDLoc DL(LN0); + // The original load itself didn't wrap, so an offset within it doesn't. + SDNodeFlags Flags; + Flags.setNoUnsignedWrap(true); SDValue NewPtr = DAG.getNode(ISD::ADD, DL, PtrType, LN0->getBasePtr(), - DAG.getConstant(PtrOff, DL, PtrType)); + DAG.getConstant(PtrOff, DL, PtrType), + &Flags); AddToWorklist(NewPtr.getNode()); SDValue Load; -- cgit v1.2.3