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[MLIR] Add support to use aligned_alloc to lower AllocOp from std to llvm

Authored by bondhugula on Apr 6 2020, 12:40 AM.

Description

[MLIR] Add support to use aligned_alloc to lower AllocOp from std to llvm

Support to recognize and deal with aligned_alloc was recently added to
LLVM's TLI/MemoryBuiltins and its various optimization passes. This
revision adds support for generation of aligned_alloc's when lowering
AllocOp from std to LLVM. Setting 'use-aligned_alloc=1' will lead to
aligned_alloc being used for all heap allocations. An alignment and size
that works with the constraints of aligned_alloc is chosen.

Using aligned_alloc is preferable to "using malloc and adjusting the
allocated pointer to align for indexing" because the pointer access
arithmetic done for the latter only makes it harder for LLVM passes to
deal with for analysis, optimization, attribute deduction, and rewrites.

Differential Revision: https://reviews.llvm.org/D77528