gcov: clang support

LLVM uses profiling data that's deliberately similar to GCC, but has a
very different way of exporting that data.  LLVM calls llvm_gcov_init()
once per module, and provides a couple of callbacks that we can use to
ask for more data.

We care about the "writeout" callback, which in turn calls back into
compiler-rt/this module to dump all the gathered coverage data to disk:

   llvm_gcda_start_file()
     llvm_gcda_emit_function()
     llvm_gcda_emit_arcs()
     llvm_gcda_emit_function()
     llvm_gcda_emit_arcs()
     [... repeats for each function ...]
   llvm_gcda_summary_info()
   llvm_gcda_end_file()

This design is much more stateless and unstructured than gcc's, and is
intended to run at process exit.  This forces us to keep some local
state about which module we're dealing with at the moment.  On the other
hand, it also means we don't depend as much on how LLVM represents
profiling data internally.

See LLVM's lib/Transforms/Instrumentation/GCOVProfiling.cpp for more
details on how this works, particularly GCOVProfiler::emitProfileArcs(),
GCOVProfiler::insertCounterWriteout(), and GCOVProfiler::insertFlush().

[[email protected]: coding-style fixes]
Link: http:https://lkml.kernel.org/r/[email protected]
Signed-off-by: Greg Hackmann <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: Tri Vo <[email protected]>
Co-developed-by: Nick Desaulniers <[email protected]>
Co-developed-by: Tri Vo <[email protected]>
Tested-by: Trilok Soni <[email protected]>
Tested-by: Prasad Sodagudi <[email protected]>
Tested-by: Tri Vo <[email protected]>
Tested-by: Daniel Mentz <[email protected]>
Tested-by: Petri Gynther <[email protected]>
Reviewed-by: Peter Oberparleiter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/gcov/Kconfig

@@ -53,6 +53,7 @@ config GCOV_PROFILE_ALL
 choice
  prompt "Specify GCOV format"
  depends on GCOV_KERNEL
+ depends on CC_IS_GCC
  ---help---
  The gcov format is usually determined by the GCC version, and the
  default is chosen according to your GCC version. However, there are
@@ -62,7 +63,7 @@ choice
 
 config GCOV_FORMAT_3_4
  bool "GCC 3.4 format"
- depends on CC_IS_GCC && GCC_VERSION < 40700
+ depends on GCC_VERSION < 40700
  ---help---
  Select this option to use the format defined by GCC 3.4.
 

kernel/gcov/Makefile

@@ -4,3 +4,4 @@ ccflags-y := -DSRCTREE='"$(srctree)"' -DOBJTREE='"$(objtree)"'
 obj-y := base.o fs.o
 obj-$(CONFIG_GCOV_FORMAT_3_4) += gcc_base.o gcc_3_4.o
 obj-$(CONFIG_GCOV_FORMAT_4_7) += gcc_base.o gcc_4_7.o
+obj-$(CONFIG_CC_IS_CLANG) += clang.o

kernel/gcov/base.c

@@ -64,7 +64,7 @@ static int gcov_module_notifier(struct notifier_block *nb, unsigned long event,
 
  /* Remove entries located in module from linked list. */
  while ((info = gcov_info_next(info))) {
- if (within_module((unsigned long)info, mod)) {
+ if (gcov_info_within_module(info, mod)) {
  gcov_info_unlink(prev, info);
  if (gcov_events_enabled)
  gcov_event(GCOV_REMOVE, info);