3.1.8.4. clang mode

Here we explain about clang mode.

Attention

Almost all options of Clang / LLVM can be used, but operation is not guaranteed if an option not listed in “C Language User’s Guide”-“Chapter 9 Clang Mode” is specified.

3.1.8.4.1. How to compile

This indicates how to compile C language program.
C compiler uses fccpx. When using MPI library, by using mpifccpx, you don’t need to consider about MPI header file and library.

To translate with clang mode, enable -Nclang option.

  • If using MPI library

[_LNlogin]$ mpifccpx [compile option including -Nclang option] source file name

  • If not using MPI library

[_LNlogin]$ fccpx [compile option including -Nclang option] source file name

See also

There are no restrictions on the order in which options are listed, including -Nclang options, and source file names.

You may specify a mixture of options including -Nclang options and source file names.

3.1.8.4.2. Compilation option

The main compilation options of the C compiler are shown below.

Compile option

Description

-c

Creates an object file.

-o exe_file
Change executable file name / object file name to exe_file.
If omitted executable file name, it will be a.out.

-O [0|1|2|3|fast]
Specify optimization level.
If omitted the word after -O, it will be -O2.
When option is omitted, it is -O2.
Change the default value of C/C++ compiler option that specifies the level of optimization

-fvectorize
Instructs the operation in the loop to generate an object using SIMD extension instructions.
If -O1 option is enabled, -fno-vectorize option is applied when omitted.
Over -O2option is enabled, -fvectorize option is applied when omitted.
It is the same instruction with trad mode’s -Ksimd.

-fopenmp
Enable directives in the OpenMP C specification. Supported specifications are OpenMP 4.5/OpenMP 5.0 (part).
When option omitted, it’s -fnoopenmp.
The -fopenmp option is needed if an object program compiled with it exists in the command line as an input file.

-ffj-ocl
Enables Fujitsu compiler-specific optimization control lines (pragma directives) supported by clang mode.
When option omitted, it’s -ffj-no-ocl.

-I directory

Specify the directory to search for INCLUDE files.

-Rpass=.*
Output optimization information.
It is the same instruction with trad mode’s -Koptmsg=2.

-std=[level]
Specifies the level of language specification that the compiler (including preprocessor) interprets.
For level, specify one of these: c89, c99, c11, gnu89, gnu99 or gnu11. When omitted, -std=gnu11 option is applied.

-ffj-src
Output source list.
It is the same instruction with trad mode’s -Nsrc.

-ffj-lst[=p|t]
Instructs the translation information to be output to a file. The translation information is output to a file with the suffix “.lst”.
If multiple C source files are specified, they are output to each file name.lst file.
If -ffj-lst option is specified with the argument omitted, -ffj-lst=p is applied.
-ffj-lst=p

Instructs the source list and statistical information to be output as translation information.

-ffj-lst=t

In addition to -ffj-lst=p, instructs to output more detailed optimization information.

-ffj-largepage

Indicates whether to create an executable program that uses the large page feature. This option must be specified when linking a program. Default is -ffj-largepage .If not using large page function, use -ffj-nolargepage option. Also -ffj-largepageotpion is unabled when specified -fsanitizeoption at the same time.

-mcpu

Direct outputting the object file for specified processor. For omitted value, -mcpu=a64fxis applied.

-Nlibomp

Uses the LLVM OpenMP library for parallel processing. This option must be specified when linking. When omitted, -Nlibomp option is applied.

--version

Outputs compiler version and copyright information to standard error.

See also

About C compiler clang mode compile option, see C User’s Guide “9.1.2 Translation Option”.

3.1.8.4.4. Environment variable (option specification)

This shows the environment variable which C language compiler (fccpx) uses.

  1. fccpx_ENV

    You can set compile options in the environment variable fccpx_ENV. The compile options defined in fccpx_ENV are automatically passed to the compiler. Compiler options defined in environment variables and systems have the following precedence:

    [Priority]

    1. Compile command operand

    2. Environment variable for setting translation option (Mode unique:fccpx_clang_ENV,fcc_clang_ENV)

    3. Environment variable for setting translation option (Mode common:fccpx_ENV,fcc_ENV)

    4. Translation profile file (Mode unique:/etc$FJSVXTCLANGA/fccpx_clang_PROF)

    5. Translation profile file (Mode common:/etc$FJSVXTCLANGA/fccpx_PROF)

    6. Omitted value

    In the following example, the recommended option is set in the environment variable fccpx_ENV.

    [_LNlogin]$ export fccpx_ENV=-Ofast -mllvm -polly -mllvm -polly-parallel

Enabled option is confirmable with standard outputting by specifying -v or -### option.

See also

If specified -v option, command (translation) is executed.

If specified -### option, command (translation) is not executed.

[Output example when specifying -v]

[_LNlogin]$ fccpx -Nclang -v clang_sample.c
clang: Fujitsu C/C++ Compiler 4.0.0 (Jun  6 2019 15:04:58) (based on LLVM 6.0.0)
Target: aarch64-unknown-linux-gnu
Thread model: posix

InstalledDir: (omitted)
Found candidate GCC installation: (omitted)
Found candidate GCC installation: (omitted)
Selected GCC installation: (omitted)
Candidate multilib: .;@m64
Selected multilib: .;@m64
 "/opt/FJSVxtclanga/fujitsu_compilers_sve_cross_20191226/sve_cross/clang-comp/bin/clang-7"
 -cc1 -triple aarch64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free
 -disable-llvm-verifier -discard-value-names -main-file-name clang_sample.c
 -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno
 -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array
 -target-cpu a64fx -mllvm -enable-prefetch-hpc-tag -target-feature +v8.2a
 -target-feature +fp-armv8 -target-feature +neon -target-feature +crc
 -target-feature +crypto -target-feature +ras -target-feature +lse
 -target-feature +rdm -target-feature +sve -target-abi aapcs
 -mllvm -use-sve-vectorizer -fallow-half-arguments-and-returns -ffj-line
 -dwarf-column-info -debug-info-kind=line-tables-only -dwarf-version=4 -debugger-tuning=gdb -v
 -resource-dir /opt/FJSVxtclanga/fujitsu_compilers_sve_cross_20191226/sve_cross/clang-comp/lib64/clang/7.1.0
 -I /opt/FJSVxtclanga/.common/SELN003/include -I /opt/FJSVxtclanga/.common/TELI006/include
 -isysroot /opt/FJSVxos/devkit/aarch64/rfs -internal-isystem /opt/FJSVxos/devkit/aarch64/rfs/usr/local/include
 -internal-isystem /opt/FJSVxtclanga/fujitsu_compilers_sve_cross_20191226/sve_cross/clang-comp/bin/../include/external
 -internal-isystem /opt/FJSVxtclanga/fujitsu_compilers_sve_cross_20191226/sve_cross/clang-comp/lib64/clang/7.1.0/include
 -internal-externc-isystem /opt/FJSVxos/devkit/aarch64/rfs/include
 -internal-externc-isystem /opt/FJSVxos/devkit/aarch64/rfs/usr/include
 -fdebug-compilation-dir /home/users/fj0025/05_C/clang -ferror-limit 19 -fmessage-length 157
 -fno-signed-char -fobjc-runtime=gcc -fdiagnostics-show-option -fcolor-diagnostics
 -o /home/users/xxxxxx/clang_sample-903f6a.o -x c clang_sample.c -faddrsig
clang -cc1 version 7.1.0 based upon LLVM 7.1.0 default target aarch64-unknown-linux-gnu
(Omitted)

  1. TMPDIR

    The temporary directory used by the compiler can be changed by using the environment variable TMPDIR. /etc/profile to set the home directory in TMPDIR.

    When changing the temporary directory, please avoid using /tmp. For TMPDIR, specify a writable directory of /home/ or /vol0n0m/data/.

3.1.8.4.5. C library for parallel processing

With Supercomputer Fugaku, two libraries are provided for parallel processing.

Library name

Description

LLVM OpenMP library

  • A library for parallel functions based on LLVM OpenMP Runtime Library, which is open source software.

  • Supported specifications are OpenMP 4.5/OpenMP 5.0 (part).

  • Available in clang mode and trad mode.

  • The trad mode is available in OpenMP 3.1/OpenMP 4.5 (part).

  • For the specifications of the LLVM OpenMP library, please read “Chapter 4 Multiprocessing” in the C User’s Guide.

Fujitsu OpenMP library

  • Available on only trad mode.

  • For the specifications of the Fujitsu OpenMP library, please read “Appendix J Fujitsu OpenMP Library” in the C User’s Guide.
Specify the library to be used for parallel processing.
This option must be specified when linking.

Option

Description

-Nlibomp

Indicates that the LLVM OpenMP library is to be used as the OpenMP library.

When omitted, -Nlibomp option is applied.

-Nfjomplib

Not available in clang mode.

Attention

When -Nclang option is available, -Nfjomplib option is disabled and -Nlibomp option is enabled.

Note

The following environment variables added in OpenMP 4.0 and later are available in the LLVM OpenMP library. On the other hand, the Fujitsu OpenMP library does not support these environment variables. These environment variables are ignored when using the Fujitsu OpenMP library.

  • OMP_CANCELLATION

  • OMP_DISPLAY_ENV

  • OMP_DEFAULT_DEVICE

  • OMP_MAX_TASK_PRIORITY

3.1.8.4.6. Compilation example

Shows how to compile a C program.
Here is a general C language compilation procedure.

Multi node job (hybrid parallel)

[_LNlogin]$ mpifccpx -Nclang -Ofast -mllvm -polly -mllvm -polly-parallel -fopenmp  sample.c

Single node job (sequential)

[_LNlogin]$ fccpx -Nclang -Ofast sample.c

Single node job (OpenMP)

[_LNlogin]$ fccpx -Nclang -Ofast -fopenmp  sample.c

3.1.8.4.7. C sample program

Here is an example from compilation to job execution using a sample program.

3.1.8.4.8. Built-in debug function

The built-in debugging function performs various inspections by compiling a program with debugging options at compile time and executing an execution module.

  • About built-in debug function

Changing the execution environment may cause abnormal termination. In such cases, the following are possible causes:

  1. The variable is quoted without setting an initial value.

  2. Array subscript exceeds array size

We introduce the built-in debugging function to check these.

  • When using C / C ++, use the built-in debugging function with the following inspection options.

  • -fsanitize=undefined option

When using an array, check the size of the declared array and the validity of the index range when used

  • -fsanitize=address option

Inspect heap memory for illegal memory release, out-of-area write, and memory leak

For more detail, see “9.1.2.2.1 General Options for Compiler” of “Chapter 9 Clang Mode” in the manual “C User’s Guide”.

This is an example of message which output with sample program that is created to match fsanitize=undefined option . (Sample program is easy to understand to output message.)

  1. Sample program

1 #include <stdio.h>
2 int main() {
3   int data[3];
4   int p = 5;
5   data[p] = 20;
6 }

  1. compiling

[_LNlogin]$ fccpx -Nclang -Ofast -fsanitize=undefined -o sample_debug sample_debug.c

Note

When compiling, warining which means the below -ffj-largepage option is disabled will be output.

clang-6.0: warning: '-fsanitize' specified, '-ffj-largepage' ignored. [-Woption-ignored]

  1. Execution example

[_LNlogin]$ cat sample_debug.sh.5568.err
sample_debug.c:5:3: runtime error: index 5 out of bounds for type 'int [3]'
sample_debug.c:5:3: runtime error: store to address 0xffffffffe608 with insufficient space for an object of type 'int'
0xffffffffe608: note: pointer points here
00 40 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  8c 32 40 00 00 00 00 00  00 00 00 00