Details:
- Added inadvertantly-omitted mention of -r option-equivalent to
--thread-part-jrir to the output for 'configure --help'. Also made
minor edits to the same text.
Details:
- Updated existing macrokernel function names and definitions to
explicitly use slab assignment of micropanels to threads, then created
duplicate versions of macrokernels that explicitly use round-robin
assignment instead of slab. NOTE: As in ac18949, trsm_r macrokernels
were not substantially updated in this commit because they are
currently disabled in bli_trsm_front.c.
- Updated existing packing function (in blk_packm_blk_var1.c) to
explicitly use slab partitioning, and then duplicated for round-robin.
- Updated control tree initialization to use the appropriate macrokernel
and packm function pointers depending on which method (slab or rr) was
enabled at configure-time.
- Updated configure script to accept new --thread-part-jrir=[slab|rr]
option (-m [slab|rr] for short), which allows the user to explicitly
request either slab or round-robin assignment (partitioning) of
micropanels to threads.
- Updated sandbox/ref99 according to above changes.
- Minor updates to build/add-copyright.py.
Details:
- gcc 7 introduced new behavior to the -dumpversion option whereby only
the major version component is output. However, as part of this
change, gcc 7 also introduced a new option, -dumpfullversion, which is
guaranteed to always output the major, minor, and revision numbers. If
we are using gcc 7 or later, we re-query the version string with this
new option and then re-parse the result so as to avoid misleading
output from configure (e.g. using gcc 7.3.0 is reported as 7.7.7).
1) Allow environment variables to be set anywhere in the argument list.
2) Allow any environment variable to be set.
3) Allow LIBPHTREAD to be set to null without getting defaulted to -lpthread.
Details:
- Updated the configure script to allow slashes in version string. This
is needed so that downstream maintainers (such as those for Debian)
can create local tags such as "upstream/0.4.1". Thanks to M. Zhou for
reporting this issue via PR #256 and providing me the information
needed to debug the problem.
Details:
- Corrected feedback echoed to user by configure when libmemkind is
found but not explicitly requested. In these cases, configure would
echo a message that it had received an explicit request to enable
libmemkind, which was not accurate, even if the end result was the
same--that libmemkind is enabled by default when it is found. Thanks
To Devangi Parikh for reporting this issue.
Details:
- Rewrote bli_winsys.c to define bli_setenv() and bli_sleep()
unconditionally, but differently for Windows and non-Windows, but
then disabled the definition of bli_setenv() entirely since BLIS
no longer needs to set environment variables. Updated bli_winsys.h
accordingly, and call bli_sleep() from within testsuite instead of
sleep() directly.
- Use
#if !defined(_POSIX_BARRIERS) || (_POSIX_BARRIERS != 200809L)
instead of
#if !defined(_POSIX_BARRIERS) || (_POSIX_BARRIERS < 0)
when guarding against local definition of pthread barrier in
testsuite. (The description for unistd.h implies that _POSIX_BARRIERS
should always be set to 200809L when barriers are supported, though I
won't be surprised if we encounter a case in the future where it is
set to something else such as 1 while still supported.)
- Removed old _VERS_CONF_INST definitions and installation rules in
top-level Makefile. These are no longer needed because we no longer
output libraries with the version and configuration name as
substrings.
- Comment/whitespace updates in Makefile, config.mk.in, common.mk,
configure, bli_extern_defs.h, and test_libblis.h.
- Added mention of 1m to README.md and other trivial tweaks.
* Enable shared
* Enable rdp
* Add support for dll
* Use libblis-symbols.def
* Fix building dlls
* Fix libblis-symbols.def
* Fix soname
* Fix Makefile error
* Fix install target
* Fix missing symbols
* Add BLIS_MINUS_TWO
* Add path to dll
* Fix OSX soname
* Add declspec for dll
* Add -DBLIS_BUILD_DLL
* Replace @enable_shared@ in config
* switch to auto for now
* blis_ -> bli_
* Remove BLIS_BUILD_DLL in make check
* change auto->haswell
* enable_shared_01
* Add wno-macro-redefined
* print out.cblat3
* BLIS_BUILD_DLL -> BLIS_IS_BUILDING_LIBRARY
* Use V=1
* Remove fpic for windows
* Remember LIBPTHREAD
* Remove libm for windows
* Remember AR
* Fix remembering libpthread
* Add Wno-maybe-uninitialized in only gcc
* Don't do blastest for shared for now
* Fix install target
And remove unnecessary change
* test auto and x86_64
* Fix install target again
* Use IS_WIN variable
* Remove leading dot from LIBBLIS_SO_MAJ_EXT
* Make is_win yes/no
* Add comments for windows builds
* Change if else blocks location
Details:
- Replaced critical sections that were conditional upon multithreading
being enabled (via pthreads or OpenMP) with unconditional use of
pthreads mutexes. (Why pthreads? Because BLIS already requires it
for its initialization mechanism: pthread_once().) This was done in
bli_error.c, bli_gks.c, bli_l3_ind.c. Also, replaced usage of BLIS's
mtx_t object and bli_mutex_*() API with pthread mutexes in
bli_thread.c. The previous status quo could result in a race condition
if the application called BLIS from more than one thread. The new
pthread-based code should be completely agnostic to the application's
threading configuration. Thanks to AMD for bringing to our attention
the need for a thread-safety review.
- Added an option to the testsuite to simulate application-level
multithreading. Specifically, each thread maintains a counter that is
incremented after each experiment. The thread only executes the
experiment if: counter % n_threads == thread_id. In other words, the
threads simply take turns executing each problem experiment. Also,
POSIX guarantees that fprintf() will not intermingle output, so
output was switched to fprintf() instead of libblis_test_fprintf().
- Changed membrk_t objects to use pthread_mutex_t intead of mtx_t and
replaced use of bli_mutex_init()/_finalize() in bli_membrk.c with
wrappers to pthread_mutex_init()/_destroy().
- Changed the implementation of bli_l3_ind_oper_enable_only() to fix
a race condition; specifically, two threads calling the function with
the same parameters could lead to a non-deterministic outcome.
- Added #include <pthread.h> to bli_cpuid.c and moved the same in
bli_arch.c.
- Added 'const' to declaration of OPT_MARKER in bli_getopt.c.
- Added #include <pthread.h> to bli_system.h.
- Added add-copyright.py script to automate adding new copyright lines
to (and updating existing lines of) source files.
Details:
- Fixed stale filepaths to check-blastest.sh and check-blistest.sh in
travis/do_testsuite.sh and travis/do_sde.sh.
- Create a symbolic link to the 'config' directory so that the top-level
Makefile can find the configs' make_defs.mk files during out-of-tree
builds.
- Added additional case handling to out-of-tree scenario to handle
situations where files 'Makefile', 'common.mk', or 'config' exist but
are not symbolic links. In such cases, configure warns the user and
exits.
- Homogenized various error messages throughout configure.
- Belated thanks to Victor Eijkhout for requesting the feature added
in 0f491e9 whereby lesser Makefiles can compile and link against
an existing installation of BLIS.
Details:
- Updated the build system so that "lesser" Makefiles, such as those in
belonging to example code or the testsuite, may be run even if the
directory is orphaned from the original build tree. This allows a
user to configure, compile, and install BLIS, delete the build tree
(that is, the source distribution, or the build directory for out-
of-tree builds) and then compile example or testsuite code and link
against the installed copy of BLIS (provided the example or testsuite
directory was preserved or obtained from another source). The only
requirement is that make be invoked while setting the
BLIS_INSTALL_PATH variable to the same installation prefix used when
BLIS was configured. The easiest syntax is:
make BLIS_INSTALL_PATH=/install/prefix
though it's also permissible to set BLIS_INSTALL_PATH as an
environment variable prior to running 'make'.
- Updated all lesser Makefiles to implement the new aforementioned build
behavior.
- Relocated check-blastest.sh and check-blistest.sh from build to
blastest and testsuite, respectively, so that if those directories are
copied elsewhere the user can still run 'make check' locally.
- Updated docs/Testsuite.md with language that mentions this new option
of building/linking against an installed copy of BLIS.
Details:
- Changed configure so that the absence of any C++ compiler from the
pre-defined search list does not result in an exit. Instead, in this
situation, the found_cxx variable is assigned 'c++notfound' and the
error message is changed to remind the user that C++ will not be
available in the sandbox. Thanks to Devangi Parikh for reporting this
issue.
- Also tweaked the message when a C++ compiler *is* found to remind any
would-be confused user that BLIS will only use C++ if it is needed by
code in the sandbox.
Details:
- Disable sandbox-related obj directory creation, directory mirroring,
and makefile fragment generation when a sandbox is not enabled.
- Prevent various duplicate actions by configure (such as those
mentioned above for sandboxes above).
Details:
- Fixed some stale code that was preventing the -p option to configure
from working as expected (though the --prefix option was unaffected).
This bug was was most likely introduced in 7e5648c (May 7 2018).
Thanks to Dave Love for reporting this issue.
Details:
- Added support for --enable-arg-max-hack to configure, which will
change the behavior of make when building BLIS so that rather than
invoke the archiver/linker with all of the object files as command
line arguments, those object files are echoed to a temporary file
and then the archiver/linker is fed that temporary file via the @
notation. An example of this can be found in the GNU make docs at
https://www.gnu.org/software/make/manual/make.html#File-Function
- Thanks to Isuru Fernando for prompting this feature.
* Add appveyor file
* Build script
* Remove fPIC for now
* copy as
* set CC and CXX
* Change the order of immintrin.h
* Fix testsuite header
* Move testsuite defs to .c
* Fix appveyor file
* Remove fPIC again and fix strerror_r missing bug
* Remove appveyor script
* cd to blis directory
* Fix sleep implementation
* Add f2c_types_win.h
* Fix f2c compilation
* Remove rdp and rename appveyor.yml
* Remove setenv declaration in test header
* set CPICFLAGS to empty
* Fix another immintrin.h issue
* Escape CFLAGS and LDFLAGS
* Fix more ?mmintrin.h issues
* Build x86_64 in appveyor
* override LIBM LIBPTHREAD AR AS
* override pthreads in configure
* Move windows definitions to bli_winsys.h
* Fix LIBPTHREAD default value
* Build intel64 in appveyor for now
Details:
- This commit fixes several bugs in configure relating to selecting a C
compiler. By dumb luck, two of the two bugs sort of cancelled each
other out in most use cases, which manifested as the expected behavior.
Thanks to Mathieu Poumeyrol for bringing this issue to our attention,
and to Devin Matthews for suggesting the more portable way of
capturing both stdout and stderr and suggesting a return code check
instead of testing stdout/stderr.
- The first bug: As the values of the compiler search list are iterated
over, only stderr is captured when querying a compiler with --version
rather than both stdout and stderr.
- The second bug: After each query, a conditional attempted to test
whether the query resulted in anything being output. That conditional
erroneously was using "-z" instead of "-n" for non-emptiness. Thus,
most of the time, stderr was empty (because the --version info was
being output on stdout), and since it was empty, the -z conditional
(intended to execute only when a compiler was found to be responsive)
executed.
- A third bug was also fixed in the way that the merged stdout/stderr
output was tested for non-emptiness (moving the 'cat' invocation to
another line and testing the contents of a variable instead).
- The three bugs above have been fixed as part of a partial rewrite of
the select_cc() function in terms of a return code check, which
obviated the need to save the output of stdout and stderr.
- The fourth bug involved a misnamed variable in the right-hand side
of a statement intended to prepend CC to search_list when CC was
non-empty. This typically did not manifest as a bug since usually CC
(if it was set) was set to a value that was known to work.
Details:
- Added mention of the new support for preset CFLAGS, LDFLAGS to the
bottom of the text output by './configure --help'.
- Updated usage example to use 'haswell' instead of 'sandybridge'.
Details:
- Any preexisting values set to the CFLAGS environment variable (or the
CFLAGS variable if given on the command line) are saved by configure
for later inclusion (prepending, to be precise) along with the
compiler flags automatically determined by the BLIS build system.
LDFLAGS is treated in a similar manner.) Thanks to Dave Love for
requesting this feature in issue #223 and Mathieu Poumeyrol for his
support on this and a previous related issue.
- Comment updates to build/config.mk.in.
- Strip whitespace from return value of various cflags functions in
common.mk.
Details:
- Fixed configure so that MK_ENABLE_MEMKIND is assigned "no" when the
option is disabled due to libmemkind not being present. This wasn't
affecting anything since the one use of the variable (in common.mk)
was formulated as "ifeq ($(MK_ENABLE_MEMKIND),yes)". That is, the
variable being empty was effectively equivalent to it being set to
"no".
- Comment updates to build/config.mk.in, common.mk.
Details:
- Return early from function, pass_config_kernel_registries(), that
implements indirect blacklisting of subconfigurations (during pass 0).
In short, I realized that indirect blacklisting is not needed in the
situations I envisioned, and can actually cause problems under certain
circumstances. Thanks to Tony Skjellum for reporting the issue (#214)
that led to this commit, and to Devin Matthews for prompting me to
realize that indirect blacklisting was unnecessary, at least as
originally envisioned.
Detail:
- configure:
- add support for --enable-sandbox=NAME to configure script, where NAME
is a subdirectory of a new 'sandbox' directory that contains an
alternative implementation of gemm. (For now, only implementations of
gemm may be provided via a sandbox.);
- add support for C++ compiler. C++ compilers are handled in a manner
similar to that of C compilers, in that a default search order is
used, and that CXX is searched for first, if the variable is set. In
practice, the C++ compiler that is selected should correspond to the
selected C compiler. (Example: If gcc is selected for C, g++ should
be selected for C++.) The result of the search is output to config.mk
via build/config.mk.in. NOTE: The use of C++ in BLIS is still
hypothetical, but may eventually move to being experimental. This
support was intended only for use of C++ within a gemm sandbox.
- build/config.mk.in:
- define SANDBOX variable containing sandbox subdirectory name.
- build/bli_config.in:
- define either of the BLIS_ENABLE_SANDBOX or BLIS_DISABLE_SANDBOX
macros in bli_config.h.
- common.mk:
- include makefile fragments that were propagated into the specified
sandbox subdirectory;
- generate different CFLAGS for sandboxes, as well as a separate
CXXFLAGS variable for sandboxes when C++ source files are compiled;
- isolate into a single location lists of file suffixes for various
purposes.
- reorganized/clean up code related to identifying header files and
paths.
- Makefile:
- generate object filepaths for and compile source code files found in
sandbox sub-directory;
- remove makefile fragments placed in sandbox sub-directory (cleanmk);
- various other cleanups.
- Added .cc, .cpp, and .cxx to list of suffixes of files to recognize in
makefile fragments (via build/gen-make-frags/suffix_list).
- Updated blis.h to conditionally #include bli_sandbox.h (via a new file,
bli_sbox.h), which each sandbox is assumed to use for any type
definitions and function prototypes it wishes to export out to blis.h.
- Conditionally disable bli_gemmnat() implementation in frame/3 when
BLIS_ENABLE_SANDBOX is defined.
Details:
- Added logic to bli_arch.c that will call what was previously the body
of bli_arch_query_id() only once and then cache the value in a static
variable local to the file. (Previously, the arch_t associated with
the hardware/configuration was queried every time bli_arch_query_id()
was called, which was at least once per level-3 function call. Thanks
to Devin Matthews for suggesting this feature via issue #175.
- Added -lpthread to the compile/link command line of the compiler
invocation that compiles build/detect/config/config_detect.c, which
prints the string identifying the detected configuration, since it
is now needed due to new pthread_once() logic in bli_arch.c.
- Implementation note: I chose to implement this arch_t caching feature
via pthread_once(), using a separate pthread_once_t variable local to
the file, rather than calling bli_init_once(). The reason is that I
did not want to require bli_init() as a prerequisite to this function.
bli_init() already calls several sub-components, some of which make use
of bli_arch_query_id(), and therefore it would be easy to fall into a
circular self-init situation (which usually causes pthreads to hang
indefinitely).
Details:
- Echo the ${cc_vendor} when informing the user of the compiler's version.
Previously, the actual ${cc} (which could be a path to the executable)
was being printed, which has already been printed by that point in the
configure script.
Details:
- Historically, the compiler selection has happened statically in the
various make_defs.mk and would only be overriden by setting CC (either
prior to running configure or as a configure argument). However, in
the last couple months, configure has evolved to contain rather
sophisticated compiler detection logic for the purposes of blacklisting
sub-configurations. It only makes sense that configure now fully take
over the responsibility of selecting a compiler from the GNU make side
of the build system. Thanks to Alex Arslan for his help exposing this
issue.
- Substitute found_cc into CC in config.mk via configure.
- Set a new variable, CC_VENDOR, in config.mk via substitution from
configure, and disable the corresponding CC_VENDOR code in common.mk.
- Disabled default compiler selection (usually gcc) in the sub-configs'
various make_def.mk files.
Details:
- Set a compiler search list (and order) as a function of the OS detected
via 'uname -s'. By default, this list and order is 'gcc clang cc' for
Linux and Darwin (OS X), and any other OS except OpenBSD). On OpenBSD,
we use 'cc gcc clang' because OpenBSD's default installation of gcc
(4.2.1) is too old for BLIS. Thanks to Alex Arslan for reporting this
issue and suggesting a fix.
Details:
- Use the .dylib shared library suffix on OS X (instead of .so in Linux).
- Link with the -dynamiclib and -install_name options on OS X (instead of
-shared and -soname in Linux).
- Determine operating system (e.g. Linux, Darwin) during configure and
substitute into config.mk.in rather than run 'uname -s' during make.
- Echo operating system during configure.
Details:
- Tweaked configure so that the shared library is generated by default.
- Updated --help text and configure's feedback messages reporting the
status of the static/shared builds.
- Changed the order of build product installation so that headers are
installed last, after libraries and symlinks.
Details:
- Changed the naming conventions used for installed libraries and
symlinks to more closely mirror patterns used by typical GNU/Linux
libraries. Whereas previously static and shared libraries were
installed and symlinked as follows:
(library) libblis-0.3.2-15-haswell.a
(library) libblis-0.3.2-15-haswell.so
(symlink) libblis.a -> libblis-0.3.2-15-haswell.a
(symlink) libblis.so -> libblis-0.3.2-15-haswell.so
we now use the following naming conventions:
(library) libblis.a
(symlink) libblis.so -> libblis.so.0.1.2
(symlink) libblis.so.0 -> libblis.so.0.1.2
(library) libblis.so.0.1.2
where 0.1.2 indicates shared library major, minor, and build versions
of 0, 1, and 2, respectively. The conventional version string can
still be queried by linking to the library in question and then calling
bli_info_get_version_str(). (The testsuite binary does this
automatically at startup.)
- Added logic to common.mk to set the soname field in the shared library
via the -soname linker flag.
- Added a 'so_version' file to the top-level directory containing two
lines. The first line specifies the .so major version number, and the
second line specifies the minor and build version numbers joined with
a '.'. This file is read by configure and those values substituted
into build/config.mk.in to define SO_MAJOR, SO_MINORB, and SO_MMB
variables.
Details:
- Renamed the following variables in config.mk (via build/config.mk.in):
BLIS_ENABLE_VERBOSE_MAKE_OUTPUT -> ENABLE_VERBOSE
BLIS_ENABLE_STATIC_BUILD -> MK_ENABLE_STATIC
BLIS_ENABLE_SHARED_BUILD -> MK_ENABLE_SHARED
BLIS_ENABLE_BLAS2BLIS -> MK_ENABLE_BLAS
BLIS_ENABLE_CBLAS -> MK_ENABLE_CBLAS
BLIS_ENABLE_MEMKIND -> MK_ENABLE_MEMKIND
and also renamed all uses of these variables in makefiles and makefile
fragments. Notice that we use the "MK_" prefix so that those variables
can be easily differentiated (such as via grep) from their "BLIS_" C
preprocessor macro counterparts.
- Other whitespace changes to build/config.mk.in.
- Renamed the following C preprocessor macros in bli_config.h (via
build/bli_config.h.in):
BLIS_ENABLE_BLAS2BLIS -> BLIS_ENABLE_BLAS
BLIS_DISABLE_BLAS2BLIS -> BLIS_DISABLE_BLAS
BLIS_BLAS2BLIS_INT_TYPE_SIZE -> BLIS_BLAS_INT_TYPE_SIZE
and also renamed all relevant uses of these macros in BLIS source
files.
- Renamed "blas2blis" variable occurrences in configure to "blas", as
was done in build/config.mk.in and build/bli_config.h.in.
- Renamed the following functions in frame/base/bli_info.c:
bli_info_get_enable_blas2blis() -> bli_info_get_enable_blas()
bli_info_get_blas2blis_int_type_size()
-> bli_info_get_blas_int_type_size()
- Remove bli_config.h during 'make cleanh' target of top-level Makefile.
Details:
- Added support for two new configure options: --libdir and --includedir.
They specify the precise install directories for libraries and header
files, respectively, and override any location implied by the --prefix
option (including the default install prefix, if --prefix was not
given). Thanks to Nico Schlömer for suggesting this via issue #195.
- Removed the INSTALL_PREFIX definition/anchor from build/config.mk.in
and replaced it with corresponding definitions/anchors for libdir and
includedir.
- Updated top-level Makefile to use the new variables, INSTALL_LIBDIR
and INSTALL_INCDIR, instead of INSTALL_PREFIX (which is now no longer
needed by make).
- Set default sane values for INSTALL_LIBDIR and INSTALL_INCDIR in
common.mk when configure has not been run, as is already done for
DIST_PATH. This is to safeguard against statements in the top-level
Makefile that use 'find' to locate old libraries and headers for the
uninstall targets, which run regardless of make target. Without setting
INSTALL_LIBDIR and INSTALL_INCDIR, those variables are empty and the
'find' ends up looking at '/', which is obviously not what we want.
(Also enclosed those definitions in an IS_CONFIGURED guard so that they
won't get evaluated unless configure has been run.)
- Rearranged "ifeq ($(IS_CONFIGURED),yes)" conditionals in Makefile to
reduce occurrences and separated "local" and top-level components of
cleanblastest and cleanblistest targets to improve readability.
- Adjusted out-of-tree builds so that they are no longer oblivious to
the .git directories, if present, and thus now properly augment version
strings with the appropriate patch number.
- Include missing version string in 'configure --help' output.
Details:
- Recycled the core functionality of build/update-version-file.sh into a
function in configure, disabling the updating of the 'version' file in
the process. Instead of writing the patched version string back to the
version file and then reading it again from within configure, the
patched version string is now saved directly to a variable in the main()
function in configure. This will prevent developers from accidentally
committing configure-induced changes to the version file in between
releases.
Details:
- Previously forgot to add explicit enforcement of a minimum gcc version
in configure script when 'knl' sub-configuration is requested.
- Comment updates to configure.
Details:
- Imported the 24x16 knl sgemm microkernel (and its corresonding spackm
kernel) from TBLIS and enabled its use in the knl sub-config. Also
Added sgemm microkernel prototype to bli_kernels_knl.h.
- Updated dgemm and dpackm microkernels from TBLIS, which included an
important change regarding the offsets array (changed from extern
declaration to static declaration/definition).
- Activated use of level-1v and -1f zen kernels in skx and knl
sub-configs.
- Removed some old macros no longer needed in bli_family_skx.h now that
libmemkind support exists in configure.
- Moved bli_avx512_macros.h to frame/include and adjusted #includes in
skx and knl kernels accordingly.
- Moved unused kernels in kernels/knl/3 to kernels/knl/3/other
directory.
- Fixed a minor bug in the 'make' output per compile when verboseness
is not turned on. The rule-generating function 'make-kernel-rule' was
previously passing in the name of the config, rather than the name of
the kernel set returned by get-config-for-kset, which could give
misleading information to the user when the kconfig_map mapped a
kernel set to a sub-configuration that did not share the same name.
(This didn't affect the CFLAGS that were actually used.)
- Updated test/3m4m/Makefile, removing acml targets and renaming the
remaining targets.
Details:
- Swiched from querying version of 'objdump' to 'as' (e.g. the
assembler).
- Fixed the outputting of the version of 'as' on OS X, which required
this beauty:
...=$(as -v /dev/null -o /dev/null 2>&1)
- Only add sub-configs to blacklist if the sub-config hasn't already
been added.
Details:
- Added logic to configure that attempts to assemble various small files
containing select instructions designed to reveal whether binutils
(specifically, the assembler) supports emitting those instruction sets.
This information provides additional opportunities to blacklist sub-
configurations that are unsupported by the environment. Thanks to Devin
Matthews for pointing me towards a similar solution in TBLIS as an
example.
- Various other cleanups in configure.
- Reorganized the detection code in the 'build' directory, bringing the
"auto-detect" configuration detection, libmemkind detection, and new
instruction set detection codes into a single new subdirectory named
'detect'.
Details:
- Fixed a failure to observe the value of CC when selecting the compiler
in configure. Thanks to Devangi Parikh for reporting this bug.
- The semantics now also work for the CC environment variable. That is,
if CC is set prior to running configure, that value is used, but will
be overridden by specifying the CC= argument to configure. If the CC
environment variable is not set, the CC= value is used. If neither the
environment variable nor CC= are specified, then the choice is made
internally to configure: first attempting to find gcc, then clang, and
then cc.
