[PATCH] D15544: [SelectionDAGBuilder] Set NoUnsignedWrap on ISD::ADD for inbounds gep

[Dan Gohman via llvm-commits]

sunfish created this revision.
sunfish added subscribers: majnemer, llvm-commits.
sunfish set the repository for this revision to rL LLVM.
Herald added subscribers: dschuff, jfb.

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 specifically useful to the WebAssembly backend, since it permits stronger constant offset folding.

Repository:
  rL LLVM

http://reviews.llvm.org/D15544

Files:
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  test/CodeGen/WebAssembly/offset.ll

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