I'm trying to tailor compiler flags of clang-11 to the apple-m1 CPU, which clang-11 doesn't know about yet.
The output of /usr/bin/clang -E - -mcpu=apple-m1 -### on macOS outputs a command with flags like these in it: "-target-feature" "+zcz"
From that you can infer the following features of the CPU:
armv8.5a+fp-armv8+neon+crc+crypto+dotprod+fp16fml+ras+lse+rdm+rcpc+zcm+zcz+fullfp16+sm4+sha3+sha2+aes
However, out of these, +fp-armv8+neon+zcm+zcz+fullfp16 are not recognised to be valid by any clang compiler:
$ cc -march=armv8.5a+zcz test.c
clang-11: error: the clang compiler does not support '-march=armv8.5a+zcz'
How can I tell clang to optimise for those target flags?
For example, when I run clang with clang ... -Wno-error -Werror, I don't want to it to show -Wno-error, since it's being overridden. I could use -v or -### to print out what the driver's executing, but that doesn't help because it still shows -Wno-error -Werror at the end. How might I show the computed compiler flags?
The -cc1 flag -compiler-options-dump appears to be the best option at the moment.
I am working on the llvm project. Recently I tryed to compiler one of my .c files using clang command line into an .s file by using the next command:
clang --target=arch -S -O0 select.c -o select.s
and it crashed in the backend in the function ARCHInstrInfo::storeRegToStackSlot with the backtrace of the stack.
However when I tryed to do it in steps:
clang -O0 -emit-llvm select.c -c -o select.bc
llc -filetype=asm -march=arch ./select.bc -o ./select.s -print-after-all -debug-only isel
it succeeded !! (?)
How can I see how the clang is calling to the backend (llc) ?
I tryed to run the clang with -v flag but it didn't printed how it is calling to the backend...
So the first one that sticks out is that llc defaults to O2 rather than O0 so you might want to look there first.
I'm working on an LLVM backend for a new architecture and we need to have position independent executables. I can pass '-fPIE' on the clang command line, but I don't see any indication of this show up in the resulting LLVM IR. For example, if I run:
clang -v -emit-llvm -fPIC -O0 -S global_dat.c -o global_dat_x86_pic.ll
And then take a look at the resulting global_dat_x86_pic.ll file, I see the following near the bottom:
!0 = !{i32 1, !"PIC Level", i32 2}
Ok, makes sense.
However if I run:
clang -v -emit-llvm -fPIE -O0 -S global_dat.c -o global_dat_x86_pie.ll
I see that the two .ll files are identical. Near the bottom of global_cat_x86_pie.ll I see:
!0 = !{i32 1, !"PIC Level", i32 2}
Which is identical to the case where I ran with -fPIE. There's no indication of "PIE Level" in the .ll file. If this .ll file were passed on to llc how would llc know that -fPIE had been set on the clang command line?
I have run in gdb and see that in fact in the second case with -fPIE on the clang commandline there is an Opts.PIELevel (in $LLVM_HOME/tools/clang/lib/Frontend/CompilerInvocation.cpp) that gets set to 2 (in fact, both Opts.PIELevel and Opts.PICLevel are set to 2 in that case whereas in -fPIC is passed to clang only Opts.PICLevel is set to 2)
This depends on your default target triple, which I can't tell from your question. You can see what happens if you cut off the default architecture (or run a native clang on an arch that does support PIE)
For example, bare X86-64 shows this,
$ clang -c hello.c -target x86_64 -fPIE -emit-llvm -###
if you run that command, you'll find "-pie-level" "2" in the output, which is how llc (well, it's internal equivalent) knows about it.
The key here is that you'll have to arrange for your backend to do something with this flag. Certain platforms (like Darwin) just ignore it. If you happen to be experiment on an OSX host, you won't see -pie-level in the bogbrush output.
I am adding some pthreads code into my Linux application that I'm building with autotools. I was getting an error about not linking in libpthreads. So I want to specify the pthreads dependency and compiler/linker flags in autotools.
I found some references that say use an ACX_PTHREAD macro. GNU provides an AX_PTHREAD macro. Both are very similar in concept. But I've tried both (on Ubuntu 13.04 64-bit), and found that they set -pthread in $PTHREAD_CFLAGS, but for some reason they don't set the -lpthread linker flag in $PTHREAD_LIBS.
The build fails. When I run make, I get:
...
/bin/sh ../libtool --tag=CXX --mode=link g++ -g -O2 -o myapp main.o ... -lconfuse -llog4cpp -lnsl -lpopt -lfuse -L/usr/local/lib -lrt
libtool: link: g++ -g -O2 -o .libs/myapp main.o ... -lconfuse -llog4cpp -lnsl /usr/lib/x86_64-linux-gnu/libpopt.so -lfuse -L/usr/local/lib -lrt
/usr/bin/ld: app-fuse.o: undefined reference to symbol 'pthread_kill##GLIBC_2.2.5'
/usr/bin/ld: note: 'pthread_kill##GLIBC_2.2.5' is defined in DSO /lib/x86_64-linux-gnu/libpthread.so.0 so try adding it to the linker command line
/lib/x86_64-linux-gnu/libpthread.so.0: could not read symbols: Invalid operation
collect2: error: ld returned 1 exit status
...
In this case, the ./configure step shows:
...
checking for the pthreads library -lpthreads... no
checking whether pthreads work without any flags... no
checking whether pthreads work with -Kthread... no
checking whether pthreads work with -kthread... no
checking for the pthreads library -llthread... no
checking whether pthreads work with -pthread... yes
checking for joinable pthread attribute... PTHREAD_CREATE_JOINABLE
checking if more special flags are required for pthreads... no
checking for PTHREAD_PRIO_INHERIT... yes
...
I notice it checks for -lpthreads, but shouldn't it be checking for -lpthread?
I've found that I can use:
AC_CHECK_LIB(pthread, pthread_create, [PTHREAD_LIBS+=-lpthread])
and then the build succeeds. But I assume this isn't the best way to make it work on the widest variety of platforms.
I see Ubuntu also has a package libpthread-stubs0-dev. But I'm not sure what it's for.
What is the "right way" to use pthreads with autotools?
Thanks to Peter Simons who asked on the autoconf mailing list, we have a somewhat official answer:
Compiler flags and linker flags are not mutually-exclusive sets, not
least because linking is typically done via the compiler frontend (cc)
and not by invoking the linker (ld) directly. Any flags that you can
use in the compile step (e.g. -O2, -DFOO, -I/tmp/include) will
generally be accepted in the linking step, even if it's not applicable
then. (The reverse may not be true, e.g. -lfoo.)
Given that, it's a lot less error-prone to use PTHREAD_CFLAGS (and
other CFLAGS variables) when linking, rather than duplicating the
applicable flags into PTHREAD_LIBS/LDFLAGS/etc. variables and not
using any CFLAGS variables then.
So just use PTHREAD_CFLAGS for your linker, too.
I bumped into this same issue when I added a first C++ source to an otherwise working C project (a shared library). Adding this C++ file caused libtool to switch from linking with gcc to linking with g++. Seems that linking with gcc a '-pthread' is enough to add the dynamic dependency to libpthread, but when linking with g++, it is not.
I tried with the above patch to a local ax_pthread.m4, but this didn't help. Passing '-lpthread' to g++ would fix the issue.
Edit: for some reason, ax_pthread.m4 forces C as the test language even if the AC_LANG is set as C++. This patch makes things work for me:
--- m4/ax_pthread.m4_orig 2013-06-15 20:03:36.000000000 +0300
+++ m4/ax_pthread.m4 2013-06-15 20:03:51.000000000 +0300
## -87,7 +87,6 ##
AU_ALIAS([ACX_PTHREAD], [AX_PTHREAD])
AC_DEFUN([AX_PTHREAD], [
AC_REQUIRE([AC_CANONICAL_HOST])
-AC_LANG_PUSH([C])
ax_pthread_ok=no
# We used to check for pthread.h first, but this fails if pthread.h
## -313,5 +312,4 ##
ax_pthread_ok=no
$2
fi
-AC_LANG_POP
])dnl AX_PTHREAD
It seems that the AX_PTHREAD macro is finding the -pthread compiler flag, and using that. But it looks as though for that particular flag, it should be specified to the linker as well (it apparently does the equivalent of -lpthread in the linker). I modified the macro as follows, so that the -pthread flag is specified as a linker flag too:
diff --git a/m4/ax_pthread.m4 b/m4/ax_pthread.m4
index 6d400ed..f426654 100644
--- a/m4/ax_pthread.m4
+++ b/m4/ax_pthread.m4
## -172,6 +172,12 ## for flag in $ax_pthread_flags; do
AC_MSG_CHECKING([whether pthreads work without any flags])
;;
+ -pthread)
+ AC_MSG_CHECKING([whether pthreads work with $flag])
+ PTHREAD_CFLAGS="$flag"
+ PTHREAD_LIBS="$flag"
+ ;;
+
-*)
AC_MSG_CHECKING([whether pthreads work with $flag])
PTHREAD_CFLAGS="$flag"
I guess I should submit this to the macro authors.
Expanding on a suggestion above (https://stackoverflow.com/a/20253318/221802) with exact script, this error went away for me after updating my PKbuild.sh script with pthread args:
./bootstrap && \
CPPFLAGS=" -g3 -Wall -pthread "\
CFLAGS=" -pthread -g3 -Wall "\
LDFLAGS=" -lpthread "\
./configure --enable-maintainer-mode \
--enable-debug \
--prefix=/usr \
--mandir=/usr/share/man \
--enable-pie \
--prefix=/usr \
--enable-library \
--enable-test \
......... [and so on]
I used the advice from another post: autoconf with -pthread
Here they mentioned you could download this file:
http://svn.sleuthkit.org/repos/sleuthkit/trunk/configure.ac
Place it into your m4 directory.
Then include this in your configure.ac:
ACX_PTHREAD
Finally, add this to your Makefile.am:
bin_PROGRAMS = main
main_SOURCES = main.c
main_CFLAGS = $(PTHREAD_CFLAGS)
main_LDADD = $(PTHREAD_LIBS)