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

Finkel, Hal J. via llvm-dev llvm-dev at lists.llvm.org
Mon May 13 09:57:28 PDT 2019

We have an interface for estimating this, although it depends on the targets providing appropriate information through the TargetTransformInfo interface.

Call TTI->getInstructionCost with the kind parameter set to TCK_Latency.


Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Matt Davis via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Monday, May 13, 2019 10:04 AM
To: Duan Bing; LLVM Developers Mailing List
Subject: Re: [llvm-dev] How shall I evaluate the latency of each instruction in LLVM IR?

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:


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’


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?

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