Implemented loose checking for potential widened loads

docs/ASAN.rst

 Using AddressSanitizer in Subzero
 =================================
 
 AddressSanitizer is a powerful compile-time tool used to detect and report
 illegal memory accesses. For a full description of the tool, see the original
 `paper
 <https://www.usenix.org/system/files/conference/atc12/atc12-final39.pdf>`_.
 AddressSanitizer is only supported on native builds of .pexe files and cannot be
 used in production.
 
 In Subzero, AddressSanitizer depends on being able to find and instrument calls
 to various functions such as malloc() and free(), and as such the .pexe file
 being translated must not have had those symbols stripped. Subzero will not
 complain if it is told to translate a .pexe file with its symbols stripped, but
 it will not be able to find calls to malloc() and free(), so AddressSanitizer
 will not work correctly in the final executable.
 
 These are the steps to compile hello.c to an instrumented object file::
 
     pnacl-clang -o hello.nonfinal.pexe hello.c
     pnacl-finalize --no-strip-syms -o hello.pexe hello.nonfinal.pexe
     pnacl-sz -fsanitize-address -filetype=obj -o hello.o hello.pexe
 
 The resulting object file must be linked with the Subzero-specific
 AddressSanitizer runtime to work correctly. A .pexe file can be compiled with
 AddressSanitizer and properly linked into a final executable using
 subzero/pydir/szbuild.py with the --fsanitize-address flag, i.e.::
 
     pydir/szbuild.py --fsanitize-address hello.pexe
+
+Handling Wide Loads
+===================
+
+Since AddressSanitizer is implemented only in Subzero, the target .pexe may
+contain widened loads that would cause false positives. To avoid reporting such
+loads as errors, we treat any word-aligned, four byte load as a potentially
+widened load and only check the first byte of the loaded word against shadow
+memory.