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Analysis: Don't look through aliases when simplifying GEPs.

Description

Analysis: Don't look through aliases when simplifying GEPs.

It is not safe in general to replace an alias in a GEP with its aliasee
if the alias can be replaced with another definition (i.e. via strong/weak
resolution (linkonce_odr) or via symbol interposition (default visibility
in ELF)) while the aliasee cannot. An example of how this can go wrong is
in the included test case.

I was concerned that this might be a load-bearing misoptimization (it's
possible for us to use aliases to share vtables between base and derived
classes, and on Windows, vtable symbols will always be aliases in RTTI
mode, so this change could theoretically inhibit trivial devirtualization
in some cases), so I built Chromium for Linux and Windows with and without
this change. The file sizes of the resulting binaries were identical, so it
doesn't look like this is going to be a problem.

Differential Revision: https://reviews.llvm.org/D65118

Details

Committed
pccJul 22 2019, 3:13 PM
Differential Revision
D65118: Analysis: Don't look through aliases when simplifying GEPs.
Parents
rL366753: Fixing build error from commit 9285295.
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