When I pass --coverage to clang (which I build from trunk together with llvm, projects/compiler-rt and clang/tools/extra), I get a link error. It seems that  /opt/llvm/r181582/lib/clang/3.4/lib/linux/libclang_rt.profile-x86_64.a does not get included. It is not a general problem with it not finding the compiler-rt installation though, as -fsantize=undefined workes fine.

The following transcript shows a hello world program, the link failure that is printed when only --coverage is specified, that linking succeeds if libclang_rt.profile-x86_64.a is manually specified, and that -fsantitize=undefined manages to find libclang_rt.ubsan-x86_64.a on its own.

$ cat hello.c 
#include <stdio.h>

int main(int argc __attribute__((unused)), char* argv[] __attribute__((unused)))
{
	printf("Hello world\n");
	return 0;
}
$ 
$ 
$ /opt/llvm/r181582/bin/clang -v --coverage hello.c clang version 3.4 (trunk 181582)
Target: x86_64-unknown-linux-gnu
Thread model: posix
 "/opt/llvm/r181582/bin/clang" -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -main-file-name hello.c -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -target-linker-version 2.23.2 -v -femit-coverage-notes -femit-coverage-data -resource-dir /opt/llvm/r181582/bin/../lib/clang/3.4 -internal-isystem /usr/local/include -internal-isystem /opt/llvm/r181582/bin/../lib/clang/3.4/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdebug-compilation-dir /home/magnus/src/clang/coverage -ferror-limit 19 -fmessage-length 80 -mstackrealign -fobjc-runtime=gcc -fobjc-default-synthesize-properties -fdiagnostics-show-option -fcolor-diagnostics -backend-option -vectorize-loops -o /tmp/hello-m48ncx.o -x c hello.c
clang -cc1 version 3.4 based upon LLVM 3.4svn default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /opt/llvm/r181582/bin/../lib/clang/3.4/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
 "/usr/bin/ld" -z relro --hash-style=gnu --build-id --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.7/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.7 -L/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.7/../../.. -L/lib -L/usr/lib /tmp/hello-m48ncx.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.7/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crtn.o /opt/llvm/r181582/bin/../lib/libprofile_rt.a
/tmp/hello-m48ncx.o: In function `__llvm_gcov_writeout':
hello.c:(.text+0x65): undefined reference to `llvm_gcda_start_file'
hello.c:(.text+0x7b): undefined reference to `llvm_gcda_emit_function'
hello.c:(.text+0x8d): undefined reference to `llvm_gcda_emit_arcs'
hello.c:(.text+0x92): undefined reference to `llvm_gcda_end_file'
/tmp/hello-m48ncx.o: In function `__llvm_gcov_init':
hello.c:(.text+0xd5): undefined reference to `llvm_gcov_init'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
$ 
$ 
$ /opt/llvm/r181582/bin/clang --coverage -v hello.c /opt/llvm/r181582/lib/clang/3.4/lib/linux/libclang_rt.profile-x86_64.a
clang version 3.4 (trunk 181582)
Target: x86_64-unknown-linux-gnu
Thread model: posix
 "/opt/llvm/r181582/bin/clang" -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -main-file-name hello.c -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -target-linker-version 2.23.2 -v -femit-coverage-notes -femit-coverage-data -resource-dir /opt/llvm/r181582/bin/../lib/clang/3.4 -internal-isystem /usr/local/include -internal-isystem /opt/llvm/r181582/bin/../lib/clang/3.4/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdebug-compilation-dir /home/magnus/src/clang/coverage -ferror-limit 19 -fmessage-length 80 -mstackrealign -fobjc-runtime=gcc -fobjc-default-synthesize-properties -fdiagnostics-show-option -fcolor-diagnostics -backend-option -vectorize-loops -o /tmp/hello-JKAJNW.o -x c hello.c
clang -cc1 version 3.4 based upon LLVM 3.4svn default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /opt/llvm/r181582/bin/../lib/clang/3.4/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
 "/usr/bin/ld" -z relro --hash-style=gnu --build-id --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.7/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.7 -L/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.7/../../.. -L/lib -L/usr/lib /tmp/hello-JKAJNW.o /opt/llvm/r181582/lib/clang/3.4/lib/linux/libclang_rt.profile-x86_64.a -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.7/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crtn.o /opt/llvm/r181582/bin/../lib/libprofile_rt.a
$ 
$ 
$ ./a.out 
Hello world
$ 
$ 
$ /opt/llvm/r181582/bin/clang -v -fsanitize=undefined hello.c clang version 3.4 (trunk 181582)
Target: x86_64-unknown-linux-gnu
Thread model: posix
 "/opt/llvm/r181582/bin/clang" -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -main-file-name hello.c -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -target-linker-version 2.23.2 -v -resource-dir /opt/llvm/r181582/bin/../lib/clang/3.4 -internal-isystem /usr/local/include -internal-isystem /opt/llvm/r181582/bin/../lib/clang/3.4/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdebug-compilation-dir /home/magnus/src/clang/coverage -ferror-limit 19 -fmessage-length 80 -fsanitize=alignment,bool,bounds,enum,float-cast-overflow,float-divide-by-zero,integer-divide-by-zero,null,object-size,return,shift,signed-integer-overflow,unreachable,vla-bound,vptr -mstackrealign -fobjc-runtime=gcc -fobjc-default-synthesize-properties -fdiagnostics-show-option -fcolor-diagnostics -backend-option -vectorize-loops -o /tmp/hello-rF6z0N.o -x c hello.c
clang -cc1 version 3.4 based upon LLVM 3.4svn default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /opt/llvm/r181582/bin/../lib/clang/3.4/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
 "/usr/bin/ld" -whole-archive /opt/llvm/r181582/bin/../lib/clang/3.4/lib/linux/libclang_rt.san-x86_64.a -no-whole-archive -z relro --hash-style=gnu --build-id --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.7/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.7 -L/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.7/../../.. -L/lib -L/usr/lib /tmp/hello-rF6z0N.o -lpthread -ldl -whole-archive /opt/llvm/r181582/bin/../lib/clang/3.4/lib/linux/libclang_rt.ubsan-x86_64.a -no-whole-archive -lpthread -ldl -export-dynamic -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.7/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crtn.o
$ 
$ 
$ ./a.out 
Hello world
$