Using valgrind to analyze EHC programs

Valgrind is a great framework for dynamic analysis of binaries. The standard 
distribution includes a tool to check for memory errors and a tool for measuring
the cache behaviour of a executable. 

Valgrind simulates the CPU and memory of the platform in order to control and
analyze the program. This works quite well, but breaks when using the Boehm GC.
This has two reasons:

Boehm GC will walk the stack, while Valgrind uses some stackspace for itself.
When the GC tries to touch the pages that are mapped by Valgrind itself, Valgrind
exits the program with a SEGFAULT. In order to fix this, we need to tell the 
GC where the real stackbottom is (1)

Another problem arises on the linux platform. sbrk() is a system call to enlarge
the data segment of the program (heap). The garbage collector probably tries to
extend the heap in the mmap()'ed segment used by Valgrind. Thus valgrind refuses
to extend the heap, leading to a GC termination.(2)

Furthermore, the boehm gc does a lot of fiddling with uninitialized values and
pointer magic. This leads to many warnings in Valgrind which are not interesting.

Solutions:
1) In $EHC_HOME/src/rts/llvm-gc.cc the following piece of code is commented in
   llvmgc_init()
  
//#include <unistd.h>
...  
void llvmgc_init()
{
  #if USE_BOEHM_GC
    // Find the 'cold' stack bottom. Useful when runned under control ofvalgrind.
    // The environment pointer passed to main is at the bottom of the stack, so
    // we use this as 'cold' stack bottom.
    //
    //GC_stackbottom = (char*) __environ;
...

  Uncomment the last line and the include to set the correct stack bottom.

2) In $EHC_HOME/extlibs/bgc/files.mk you see the following make target:

$(EXTLIBS_BGC_INS_FLAG): $(EXTLIBS_BGC_ARCHIVE) $(EXTLIBS_BGC_MKF)
  mkdir -p $(BLDABS_EXTLIBS_BGC_PREFIX) && \
  cd $(BLDABS_EXTLIBS_BGC_PREFIX) && \
  tar xfz $(EXTLIBS_BGC_ARCHIVE) && \
  cd $(EXTLIBS_BGC_NAME) && \
  ./configure --prefix=$(INSABS_EXTLIBS_BGC_PREFIX) --disable-threads && \
  make && \
  make install && \
  touch $@

  Change the ./configure call to the following:

  ./configure --prefix=$(INSABS_EXTLIBS_BGC_PREFIX) --disable-threads CFLAGS=-DUSE_MMAP && \

  This way you configure Boehm GC to use mmap() instead of sbrk()

3) Use the suppression file ($EHC_HOME/boehm-gc.suppressions) to filter out the most garbage collection
   noise

   valgrind --tool=<toolname> --suppressions=$EHC_HOME/boehm-gc.suppressions <EXE>