Backport https://codereview.appspot.com/155450044 from the
master Go library. Original description:
I came across this while debugging a GC problem in gccgo.
There is code in assignTo and cvtT2I that handles assignment
to all interface values. It allocates an empty interface even
if the real type is a non-empty interface. The fields are
then set for a non-empty interface, but the memory is recorded
as holding an empty interface. This means that the GC has
incorrect information.
This is extremely unlikely to fail, because the code in the GC
that handles empty interfaces looks like this:
obj = nil;
typ = eface->type;
if(typ != nil) {
if(!(typ->kind&KindDirectIface) || !(typ->kind&KindNoPointers))
obj = eface->data;
In the current runtime the condition is always true--if
KindDirectIface is set, then KindNoPointers is clear--and we
always want to set obj = eface->data. So the question is what
happens when we incorrectly store a non-empty interface value
in memory marked as an empty interface. In that case
eface->type will not be a *rtype as we expect, but will
instead be a pointer to an Itab. We are going to use this
pointer to look at a *rtype kind field. The *rtype struct
starts out like this:
type rtype struct {
size uintptr
hash uint32 // hash of type; avoids computation in hash tables
_ uint8 // unused/padding
align uint8 // alignment of variable with this type
fieldAlign uint8 // alignment of struct field with this type
kind uint8 // enumeration for C
An Itab always has at least two pointers, so on a
little-endian 64-bit system the kind field will be the high
byte of the second pointer. This will normally be zero, so
the test of typ->kind will succeed, which is what we want.
On a 32-bit system it might be possible to construct a failing
case by somehow getting the Itab for an interface with one
method to be immediately followed by a word that is all ones.
The effect would be that the test would sometimes fail and the
GC would not mark obj, leading to an invalid dangling
pointer. I have not tried to construct this test.
I noticed this in gccgo, where this error is much more likely
to cause trouble for a rather random reason: gccgo uses a
different layout of rtype, and in gccgo the kind field happens
to be the low byte of a pointer, not the high byte.
From-SVN: r216489
This variable is unused apparently as a result of local changes.
gccgo accepts this variable declaration, but other frontends may not.
From-SVN: r212873
This revision was committed January 7, 2014. The next
revision deleted runtime/mfinal.c. That will be done in a
subsequent merge.
This merge changes type descriptors to add a zero field,
pointing to a zero value for that type. This is implemented
as a common variable.
* go-gcc.cc (Gcc_backend::implicit_variable): Add is_common and
alignment parameters. Permit init parameter to be NULL.
From-SVN: r211249
The gccgo-specific iword function was checking v.kind, but for
a method value that is always Func. Fix to check v.typ.Kind()
instead.
From-SVN: r202670
This changes the compiler and runtime to not pass a closure
value as the last argument, but to instead pass it via
__go_set_closure and retrieve it via __go_get_closure. This
eliminates the need for function descriptor wrapper functions.
It will make it possible to retrieve the closure value in a
reflect.MakeFunc function.
From-SVN: r202233
This changes the representation of a Go value of function type
from being a pointer to function code (like a C function
pointer) to being a pointer to a struct. The first field of
the struct points to the function code. The remaining fields,
if any, are the addresses of variables referenced in enclosing
functions. For each call to a function, the address of the
function descriptor is passed as the last argument.
This lets us avoid generating trampolines, and removes the use
of writable/executable sections of the heap.
From-SVN: r200181
Update to current version of Go library.
Update testsuite for removed types.
* go-lang.c (go_langhook_init): Omit float_type_size when calling
go_create_gogo.
* go-c.h: Update declaration of go_create_gogo.
From-SVN: r169098
gcc/:
* gcc.c (default_compilers): Add entry for ".go".
* common.opt: Add -static-libgo as a driver option.
* doc/install.texi (Configuration): Mention libgo as an option for
--enable-shared. Mention go as an option for --enable-languages.
* doc/invoke.texi (Overall Options): Mention .go as a file name
suffix. Mention go as a -x option.
* doc/frontends.texi (G++ and GCC): Mention Go as a supported
language.
* doc/sourcebuild.texi (Top Level): Mention libgo.
* doc/standards.texi (Standards): Add section on Go language.
Move references for other languages into their own section.
* doc/contrib.texi (Contributors): Mention that I contributed the
Go frontend.
gcc/testsuite/:
* lib/go.exp: New file.
* lib/go-dg.exp: New file.
* lib/go-torture.exp: New file.
* lib/target-supports.exp (check_compile): Match // Go.
From-SVN: r167407
