[PATCH] D110064: [SelectionDAG] Assume that a GlobalAlias may alias other global values

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope created this revision.
bjope added reviewers: courbet, niravd, efriedma.
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This fixes a bug detected in DAGCombiner when using global alias
variables. Here is an example:

  @foo = global i16 0, align 1
  @aliasFoo = alias i16, i16 * @foo
  define i16 @bar() {
    ...
    store i16 7, i16 * @foo, align 1
    store i16 8, i16 * @aliasFoo, align 1
    ...
  }

BaseIndexOffset::computeAliasing would incorrectly derive NoAlias
for the two accesses in the example above, resulting in DAGCombiner
miscompiles.

This patch fixes the problem by a defensive approach letting
BaseIndexOffset::computeAliasing return false, i.e. that the aliasing
couldn't be determined, when comparing two global values and at least
one is a GlobalAlias. In the future we might improve this with a
deeper analysis to look at the aliasee for the GlobalAlias etc. But
that is a bit more complicated considering that we could have
'local_unnamed_addr' and situations with several 'alias' variables.

Fixes PR51878.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D110064

Files:
  llvm/lib/CodeGen/SelectionDAG/SelectionDAGAddressAnalysis.cpp
  llvm/unittests/CodeGen/SelectionDAGAddressAnalysisTest.cpp

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