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[IR] Do not assume that function pointers are aligned

Description

[IR] Do not assume that function pointers are aligned

Summary:
The value tracking analysis uses function alignment to infer that the
least significant bits of function pointers are known to be zero.
Unfortunately, this is not correct for ARM targets: the least
significant bit of a function pointer stores the ARM/Thumb state
information (i.e., the LSB is set for Thumb functions and cleared for
ARM functions).

The original approach (https://reviews.llvm.org/D44781) introduced a
new field for function pointer alignment in the DataLayout structure
to address this. But it seems unlikely that optimizations based on
function pointer alignment would bring much benefit in practice to
justify the additional maintenance burden, so this patch simply
assumes that function pointer alignment is always unknown.

Reviewers: javed.absar, efriedma

Reviewed By: efriedma

Subscribers: kristof.beyls, llvm-commits, hfinkel, rogfer01

Differential Revision: https://reviews.llvm.org/D46110

Details

Committed
miyukiApr 27 2018, 2:12 AM
Reviewer
efriedma
Differential Revision
D46110: [IR] Do not assume that function pointers are aligned
Parents
rL331024: [mips] Add support for Virtualization ASE
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