Building Clang/LLVM efficiently Tilmann Scheller LLVM Compiler - - PowerPoint PPT Presentation

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Building Clang/LLVM efficiently Tilmann Scheller LLVM Compiler Engineer t.scheller@samsung.com Samsung Open Source Group Samsung Research UK EuroLLVM 2015 London, United Kingdom, April 13 14, 2015 Samsung Open Source Group 1 Overview

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Building Clang/LLVM efficiently

Tilmann Scheller LLVM Compiler Engineer t.scheller@samsung.com Samsung Open Source Group Samsung Research UK EuroLLVM 2015 London, United Kingdom, April 13 – 14, 2015

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Overview

Introduction
Performance
Summary

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Introduction

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Introduction

Clang/LLVM are large pieces of C++ software
Long turnaround time harms developer productivity
Squeeze maximum performance out of the toolchain
Speed up the build without buying new hardware
Focus on x86-64 Linux
Test machine: Fedora 21 on i7-4770K CPU @ 3.50GHz,

16GB RAM, 1TB 7200RPM HDD

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Ideas

Build with Clang instead of GCC
Only build what you really need (e.g. only build relevant

backends)

Use a faster linker: GNU gold instead of GNU ld
Use a heavily optimized compiler binary for the build

(LTO, PGO, LTO+PGO)

For incremental debug builds: shared library build instead
f static build

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Performance

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Measurement

GCC 4.9.2 shipping with Fedora 21
Clang trunk snapshot (r234392 from April 8, 2015)
Standard debug/release builds of Clang (CMake build

with default generator) unless otherwise noted

Using GNU gold 2.24 for linking
Make invoked with -j8
Best of five runs

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Clang vs. GCC compile time

Both Clang and GCC are compiled with GCC 4.9.2

Release build Debug build 100 200 300 400 500 600 700 800 538 509 726 755 Clang GCC 4.9.2

seconds

1.48x faster 1.34x faster

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Only build a subset of Clang/LLVM

Only build relevant backends
Host Clang built with GCC 4.9.2

Release build Debug build 100 200 300 400 500 600 700 800 583 591 726 755 X86 backend only All backends

seconds

1.27x faster 1.24x faster

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Use a faster linker

Using GNU gold instead of GNU ld
Building Clang with GCC 4.9.2

Release build Debug build 100 200 300 400 500 600 700 800 900 1000 726 755 731 873 746 GNU gold + split DWARF GNU gold GNU ld

seconds

1.17x faster

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Optimize host compiler binary aggressively

Building Clang with a heavily optimized host Clang build
Host Clang was compiled with Clang or GCC at the various different
ptimization levels

Clang -O3 GCC -O3 Clang LT O GCC LT O GCC PGO GCC LT O+PGO 400 420 440 460 480 500 520 540 560 537 538 522 547 462 482

Release

seconds

Clang -O3 GCC -O3 Clang LT O GCC LT O GCC PGO GCC LT O+PGO 400 420 440 460 480 500 520 510 509 498 512 450 467

Debug

seconds

PGO 1.16x faster PGO 1.13x faster

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Speeding up incremental builds

Shared library build rather than static build
Debug build of Clang with GCC 4.9.2
Incremental build after changing a single file in the ARM backend

(ARMISelLowering.cpp)

Build time after edit 5 10 15 20 25 30 5 27 Shared build Static build

seconds

5.4x faster

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Summary

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Summary

Always build with Clang if you care about compile time
Use GNU gold rather than GNU ld
Building Clang with GCC and PGO produces fastest

binary

Shared library builds speed up incremental debug builds

tremendously

Overall speedup: 1.58x for release builds and 1.94x for

debug builds (Switching from GCC + GNU ld to a PGO build of Clang + GNU gold)

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Thank you.

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