[llvm-dev] How shall I evaluate the latency of each instruction in LLVM IR?

[Matt Davis via llvm-dev]

Hi Duan,

It sounds like you might be interested in llvm’s  Machine Code Analyzer.  It’s an llvm tool that can calculate reciprocal throughput by simulating an out-of-order instruction pipeline.   It also provides other useful cycle information:
https://llvm.org/docs/CommandGuide/llvm-mca.html

Your question asks about IR.  llvm-mca currently only works on assembly.  However, you can easily generate assembly from IR or make use of MCA as a library.   I suggest just converting your IR to assembly and running that through llvm-mca.

MCA is built on LLVM instruction scheduling information which contains cycle latency information.   If you’re curious of what that data looks like, outside of MCA,  take a peek at the X86 instruction scheduling information located around:  ‘llvm/lib/Target/X86/X86Schedule.td’

-Matt


From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Duan Bing via llvm-dev <llvm-dev at lists.llvm.org>
Reply-To: Duan Bing <hibduan at gmail.com>
Date: Sunday, May 12, 2019 at 6:59 PM
To: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] How shall I evaluate the latency of each instruction in LLVM IR?

Inspired by https://www.agner.org/optimize/instruction_tables.pdf<https://urldefense.proofpoint.com/v2/url?u=https-3A__www.agner.org_optimize_instruction-5Ftables.pdf&d=DwMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=UilupKr8sbkenSLlythLNg&m=DWhb3uSjfkI09NsIvzdD_gqnlaI7VN0NpDYUdEvguDw&s=g7xcpplySDXjLzySPYQQjj3BbmDZdSgQtcKxwgCOh9s&e=>,  which gives us the latency and reciprocal throughput of each instruction in the different architecture of X86, Is there anybody taking the effort to do a similar job for LLVM IR?

Thanks!
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