[LIBGCC/LIBGCOV PATCH v3 1/2] Add documentation for libgcov

[Alice Suiu <alicesuiu17@xxxxxxxxx>]

From: alicesuiu <alicesuiu17@xxxxxxxxx>

Signed-off-by: Alice Suiu <alicesuiu17@xxxxxxxxx>
---
 README.md | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index e3460cd..5baa2c8 100644
--- a/README.md
+++ b/README.md
@@ -4,8 +4,9 @@ gcc for Unikraft
 This is a port of gcc for Unikraft as an external library.
 
 For the time being we only port libbacktrace and libffi libraries as
-they are needed for the libgo port. You will need newlib for both
-libraries to build. In addition, 
+they are needed for the libgo port and libgcov library because it is needed
+for the PGO (Profile-Guided Optimization) mechanism. You will need newlib
+for all three libraries to build. In addition, 
 
 For libffi you will need the pthread\_embedded external library too.
 
@@ -19,5 +20,36 @@ Note that because of a documented bug in libunwind 
`unw_getcontext`
 leads to a page fault, and in turn `backtrace_full`will also lead to
 one as it uses `_Unwind_Backtrace` which calls `unw_getcontext`.
 
+For libgcov you will need the following libraries that should be added to the
+`Makefile` in this order:
+
+* `pthreads`, e.g `pthread-embedded`
+* `compiler-rt`
+* `libcxx`
+* `libcxxabi`
+* `libunwind`
+* `libc`, e.g. `newlib`
+
+In order to use the `PGO` mechanism, the `-fprofile-generate` and 
`-fprofile-use`
+flags should be added to the `Makefile.uk` of the application running in 
Unikraft.
+
+For example, if you want to use the `PGO` mechanism for the `helloworld`
+application you have to add the following lines in the `Makefile.uk`:
+
+```
+APPHELLOWORLD_CFLAGS-$(CONFIG_OPTIMIZE_PGO_GENERATE)   += -fprofile-generate
+APPHELLOWORLD_CFLAGS-$(CONFIG_OPTIMIZE_PGO_USE)        += -fprofile-use
+```
+
+Several header files should be generated to successfully compile the source 
files
+from the libgcov library. These headers have already been generated in the
+`libgcov/include` directory. If you want to generate again the files, you need
+to download the `GCC` sources and run the following commands:
+
+* in the `libgcc` directory:
+`make auto-target.h gthr-default.h libgcc_tm.h`
+* in the `gcc` directory:
+`make tconfig.h tm.h auto-host.h gcov-iov.h`
+
 Please refer to the `README.md` as well as the documentation in the `doc/`
 subdirectory of the main unikraft repository for further information.
-- 
2.17.1