Charu Chandrasekaran, Apple Marcello Maggioni, Apple

71 Pages·2017·0.66 MB·English

Apple LLVM GPU Compiler:
Embedded Dragons
Charu Chandrasekaran, Apple
Marcello Maggioni, Apple
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Agenda
• How Apple uses LLVM to build a GPU Compiler
• Factors that affect GPU performance
• The Apple GPU compiler
• Pipeline passes
• Challenges
2
How Apple uses LLVM
• Live on Trunk and merge continuously
• Benefit from latest improvements on trunk
• Identify any regressions immediately and report back
• Minimize changes to open source llvm code
• Reuse as much as possible
3
Continuous Integration
LLVM Trunk
GPU Compiler
Year 1
production
compiler
4
Continuous Integration
LLVM Trunk
GPU Compiler
Year 1
production
compiler
5
Continuous Integration
LLVM Trunk
GPU Compiler
Year 1 Year 2 Year 3
production production production
compiler compiler compiler
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3
Testing
• Regression testing involves:
• register count
• instruction count
FileCheck : correctness
•
compile time
•
• compiler size
runtime performance
•
7
The GPU SW Stack
IR
XPC Service
iOS / watchOS / tvOS Process
Metal-FE
App
Interacts
Metal Framework,
User
GPU Driver XPC Service
Shader
Result
Backend
.metal .exec IR
.obj
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About GPUs
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About GPUs
Shader Core
PC
LANE 0 LANE 1 LANE 2 LANE 3 LANE 4 LANE 5 LANE 6 LANE 7
GPUs are massively parallel vector processors
Threads are grouped together and execute in lockstep (they share the same PC)
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