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PAGE 1 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Open Source | Open Possibilities Building the.

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Presentation on theme: "PAGE 1 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Open Source | Open Possibilities Building the."— Presentation transcript:

1 PAGE 1 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Open Source | Open Possibilities Building the Linux Kernel and User Space for the Hexagon™ DSP with LLVM Anshu Dasgupta and Pavel Potoplyak Linux Plumbers Conference September 19, 2013 Credits: Thomas Brezinski and Anand Kodnani

2 PAGE 2 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Qualcomm Innovation Center, Inc. 5775 Morehouse Drive San Diego, CA 92121 U.S.A. © 2013 Qualcomm Innovation Center, Inc. Not to be used, copied, reproduced, or modified in whole or in part, nor its contents revealed in any manner to others without the express written permission of Qualcomm Innovation Center, Inc. Qualcomm is a trademark of QUALCOMM Incorporated, registered in the United States and other countries. All QUALCOMM Incorporated trademarks are used with permission. Other product and brand names may be trademarks or registered trademarks of their respective owners. This technical data may be subject to U.S. and international export, re-export or transfer (“export”) laws. Diversion contrary to U.S. and international law is strictly prohibited. Presentation Title 80-BAxxx-x Rev. x

3 PAGE 3 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Introduction Who are we?  Anshu Dasgupta – manage Qualcomm’s Hexagon™ DSP compiler team  Pavel Potoplyak – engineer working with the Hexagon tools team

4 PAGE 4 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Introduction What are we doing? Bringing up a LLVM compiler for the Hexagon™ DSP  GCC is a mature toolset with thousands of hours of testing behind it  How do we achieve that level of robustness and maturity? One solution: Build Linux kernel and user space with LLVM for the Hexagon™ DSP

5 PAGE 5 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Introduction Why are we building the Linux kernel and user space with LLVM? Compiler testing Need extensive testing to deploy commercially VLIW code paths and optimizations a challenge for correctness, performance Compiler bugs are very difficult to track down in embedded applications Hexagon codebases transitioning from GCC to LLVM Programmers code to compiler behavior not to C standard

6 PAGE 6 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Hexagon™ DSP Hexagon™ DSP: Qualcomm’s multithreaded VLIW DSP Part of the Snapdragon platform C, C++ compiler Processor designed to be programmed in C and C++ LLVM-based compiler (transitioned from GCC compiler) VLIW architecture provides several opportunities and challenges for the compiler

7 PAGE 7 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Compiler Correctness Primary goal of compiler: Generate correct code Relax correctness and any performance goal can be met :-) We have an elaborate set of internal tests for correctness that runs every night Compiling Hexagon DSP Linux kernel and user space with LLVM  Stress test for compiler correctness Several bugs uncovered during Hexagon Linux bringup Example packet: { r7 += mpyi(r21, r20) r8 = add(r8, #16) r12 = add(r12, #8) if (cmp.gt(r2, r12.new)) jump:t.LBB0_4 }

8 PAGE 8 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Example of Unsupported Feature: Global Register Variables #ifndef __llvm__ register struct thread_info *__current_thread_info asm(QUOTED_THREADINFO_REG); #define current_thread_info() __current_thread_info #else inline struct thread_info *current_thread_info() { struct thread_info *x; asm ("%0 = " QUOTED_THREADINFO_REG : "=r"(x)); return x; } #endif Several other GCC extensions not implemented in LLVM  http://clang.llvm.org/docs/UsersManual.html#gcc-extensions-not- implemented-yethttp://clang.llvm.org/docs/UsersManual.html#gcc-extensions-not- implemented-yet

9 PAGE 9 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Example of Compiler Exploiting C99 Undefined Behavior char *killer = NULL; #ifndef __llvm__ *killer = 1; #else __asm__ __volatile__( "r0 = #0\n\t" "r1 = #1\n\t" "memb(r0+#0) = r1\n\t" ::: "r0", "r1", "memory"); #endif

10 PAGE 10 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Esoteric Difference #1: File Scope Inline ASM void sleep1() { sleep(1); } /* notice nop placed between two functions */ asm("nop"); void sleep2() { sleep(2); }

11 PAGE 11 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Esoteric Difference #1: GCC code generation with –fno-toplevel-reorder.file "example1.c".text.type sleep1, @function sleep1: allocframe(#0) r0 = #1 call sleep deallocframe jumpr r31.size sleep1,.-sleep1 //APP nop # NOTICE NOP PLACEMENT //NO_APP.p2align 2.globl sleep2.type sleep2, @function sleep2: allocframe(#0)...

12 PAGE 12 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Esoteric Difference #1: LLVM code generation.file "example1.c" nop # NOTICE NOP PLACEMENT.text.type sleep1,@function sleep1: allocframe(#8) r0 = #1 call sleep memw(r29+#4) = r0 dealloc_return.Ltmp0:.type sleep1,@function allocframe(#8)...

13 PAGE 13 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Esoteric Difference #2: Controlling Names in Assembler Code void f1() { f2(); } extern int f2() asm ("f3");

14 PAGE 14 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Esoteric Difference #2: GCC code generation.file "example2.c".text.p2align 2.globl f1.type f1, @function f1: // saved LR + FP regs size (bytes) = 8 allocframe(#0) call f3 # NOTICE CALL TO f3 deallocframe jumpr r31.size f1,.-f1

15 PAGE 15 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Esoteric Difference #2: LLVM code generation.file "example2.c".text.globl f1.falign.type f1,@function f1: allocframe(#8) call f2 # NOTICE CALL TO f2 memw(r29+#4) = r0 dealloc_return.Ltmp0:.size f1,.Ltmp0-f1

16 PAGE 16 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Esoteric Difference #3: Preprocessing $ echo '#include "foo.h"' | \ hexagon-clang -E -dM -MD -MP -MF foo.out -xc - -o out.out -MT 'a b' $ cat foo.out a b: foo.h foo.h: $ echo '#include "foo.h"' | \ hexagon-gcc -E -dM -MD -MP -MF foo.out -xc - -o out.out -MT a b' $ cat foo.out a b: foo.h foo.h:

17 PAGE 17 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Conclusion Our conclusion  Built more than 55 user space packages with LLVM for Hexagon DSP  Uncovered several latent compiler bugs  Buiding and running large codebases through toolset is extremely beneficial  Significantly improves the quality of the compiler shipped to our customers  Would like to automate the build without any source code changes

18 PAGE 18 Open Source | Open Possibilities MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION Open Questions How do you quantify the quality of the toolset, kernel and user space? Components change. For instance: new compiler, new optimizations What are the different dimensions of measurable quality Correctness Build time Code size Execution time How do we capture, archive, and analyze these metrics on a macro scale? Which GCC-isms should LLVM support? Which GCC-isms should be eliminated from Linux user space?


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