https://github.com/erikd/gcc-aarch64-optimization-bug

Code to demonstrate a GCC Aarch64 optimization bug
https://github.com/erikd/gcc-aarch64-optimization-bug

Last synced: 26 days ago
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Code to demonstrate a GCC Aarch64 optimization bug

• Host: GitHub
• URL: https://github.com/erikd/gcc-aarch64-optimization-bug
• Owner: erikd
• Created: 2014-12-06T04:54:51.000Z (almost 10 years ago)
• Default Branch: master
• Last Pushed: 2014-12-06T22:18:06.000Z (almost 10 years ago)
• Last Synced: 2024-05-08T20:35:57.373Z (6 months ago)
• Language: C
• Size: 156 KB
• Stars: 0
• Watchers: 3
• Forks: 1
• Open Issues: 0
• Metadata Files:
  • Readme: Readme.md

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README

        
# gcc-aarch-optimization-bug

This repo contains code (from [libsndfile][libsndfile]) to show a GCC on Aarch64

compiler bug that is present in gcc version 4.9.2 (Debian/Linaro 4.9.2-5). The

bug seems to be some sort of incorrect interaction between the inliner and the

`-fschedule-insns` compiler flag.

The repo contains:

* ag_enc.c : The file displaying the problem.

* aglib.h : A header file.

* dyn_comp_test.c : The driver program.

* Makefile

# The Problem

The probelm is that the file `ag_enc.c` compiled with GCC 4.9.2 from Debian,

with optimisation flags of `-O1` returns the correct result, while if it is

compiled with `-O1 -fschedule-insns` it returns an incorrect result.

On a Debian Aarch64 system you can just run:

    CC=gcc-4.9 make clean check

which will compile the test program with the two different sets of compiler

flags, one which passes one which fails.

On my system (a QEMU user space chroot) the result (with some lines trimmed)

looks like:

    rm -f ag_enc.o

    OPT_FLAGS="-O" make dyn_comp_test && ./dyn_comp_test

    make[1]: Entering directory '/home/erikd/Git/gcc-aarch64-optimization-bug'

    gcc-4.9 -O -Isrc -std=c99 -Wcast-align -Wcast-qual -Wshadow .....

    gcc-4.9 -O -Isrc -std=c99 -Wcast-align -Wcast-qual -Wshadow .....

    gcc-4.9 dyn_comp_test.o ag_enc.o -lm -o dyn_comp_test

    make[1]: Leaving directory '/home/erikd/Git/gcc-aarch64-optimization-bug'

    Checksum value 0x1c1c : Correct!

    rm -f ag_enc.o

    OPT_FLAGS="-O -fschedule-insns" make dyn_comp_test && ./dyn_comp_test

    make[1]: Entering directory '/home/erikd/Git/gcc-aarch64-optimization-bug'

    gcc-4.9 -O -fschedule-insns -Isrc -std=c99 -Wcast-align -Wcast-qual .....

    gcc-4.9 dyn_comp_test.o ag_enc.o -lm -o dyn_comp_test

    make[1]: Leaving directory '/home/erikd/Git/gcc-aarch64-optimization-bug'

    Checksum value 0x0fb9 : Error, should be 0x1c1c!

With gcc-4.8 on Aarch64 or gcc of any version of any other architecture, the

program will pass with both sets of compiler flags. Eg on Aarch64:

    CC=gcc-4.8 make clean check

I've also test this with Debian's gcc-snapshot (`gcc version 5.0.0 20141118

(experimental) [trunk revision 217719] (Debian 20141118-1)`) and it does not

display the problem either.

[libsndfile]: https://github.com/erikd/libsndfile/