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[InstCombine] Let combineLoadToNewType preserve ABI alignment of the load…

Authored by aqjune on Jan 14 2020, 8:11 AM.

Description

[InstCombine] Let combineLoadToNewType preserve ABI alignment of the load (PR44543)

Summary:
If aligment on LoadInst isn't specified, load is assumed to be ABI-aligned.
And said aligment may be different for different types.
So if we change load type, but don't pay extra attention to the aligment
(i.e. keep it unspecified), we may either overpromise (if the default aligment
of the new type is higher), or underpromise (if the default aligment
of the new type is smaller).

Thus, if no alignment is specified, we need to manually preserve the implied ABI alignment.

This addresses https://bugs.llvm.org/show_bug.cgi?id=44543 by making combineLoadToNewType preserve ABI alignment of the load.

Reviewers: spatel, lebedev.ri

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72710