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Let -basic-block-sections=labels emit basicblock metadata in a new .bb_addr_map…
Audit Required7841e21c9849

Authored by rahmanl on Sep 14 2020, 10:16 AM.

Description

Let -basic-block-sections=labels emit basicblock metadata in a new .bb_addr_map section, instead of emitting special unary-encoded symbols.

This patch introduces the new .bb_addr_map section feature which allows us to emit the bits needed for mapping binary profiles to basic blocks into a separate section.
The format of the emitted data is represented as follows. It includes a header for every function:

Address of the function-> 8 bytes (pointer size)
Number of basic blocks in this function (>0)-> ULEB128

The header is followed by a BB record for every basic block. These records are ordered in the same order as MachineBasicBlocks are placed in the function. Each BB Info is structured as follows:

Offset of the basic block relative to function begin-> ULEB128
Binary size of the basic block-> ULEB128
BB metadata-> ULEB128 [ MBB.isReturn() OR MBB.hasTailCall() << 1 OR MBB.isEHPad() << 2 ]

The new feature will replace the existing "BB labels" functionality with -basic-block-sections=labels.
The .bb_addr_map section scrubs the specially-encoded BB symbols from the binary and makes it friendly to profilers and debuggers.
Furthermore, the new feature reduces the binary size overhead from 70% bloat to only 12%.

For more information and results please refer to the RFC: https://lists.llvm.org/pipermail/llvm-dev/2020-July/143512.html

Reviewed By: MaskRay, snehasish

Differential Revision: https://reviews.llvm.org/D85408