Hello,
This commit adds comments to fields in the cgraph_thunk_info structure
declaration from cgraph.h. They will hopefully answer questions that
people like myself can ask while discovering the thunk machinery. I
also made an assertion stricter in cgraph_node::create_thunk.
I'm adding Nathan in copy as we discussed this thunk matter at this
year's Cauldron. :-)
Bootsrapped and regtested on x86_64-linux. Ok to commit? Thank you in
advance!
gcc/
* cgraph.h (cgraph_thunk_info): Add comments, reorder fields.
* cgraph.c (cgraph_node::create_thunk): Adjust comment, make
assert for VIRTUAL_* arguments stricter.
---
gcc/cgraph.c | 10 +++++++---
gcc/cgraph.h | 36 +++++++++++++++++++++++++++++-------
2 files changed, 36 insertions(+), 10 deletions(-)
diff --git a/gcc/cgraph.c b/gcc/cgraph.c
index 69aa6c5bce2..20ab418d410 100644
--- a/gcc/cgraph.c
+++ b/gcc/cgraph.c
@@ -603,7 +603,7 @@ cgraph_node::create_same_body_alias (tree alias, tree decl)
/* Add thunk alias into callgraph. The alias declaration is ALIAS and it
aliases DECL with an adjustments made into the first parameter.
- See comments in thunk_adjust for detail on the parameters. */
+ See comments in struct cgraph_thunk_info for detail on the parameters. */
cgraph_node *
cgraph_node::create_thunk (tree alias, tree, bool this_adjusting,
@@ -619,8 +619,12 @@ cgraph_node::create_thunk (tree alias, tree, bool
this_adjusting,
node->reset ();
else
node = cgraph_node::create (alias);
- gcc_checking_assert (!virtual_offset
- || wi::eq_p (virtual_offset, virtual_value));
+
+ /* Make sure that if VIRTUAL_OFFSET is in sync with VIRTUAL_VALUE. */
+ gcc_checking_assert (virtual_offset
+ ? wi::eq_p (virtual_offset, virtual_value)
+ : virtual_value == 0);
+
node->thunk.fixed_offset = fixed_offset;
node->thunk.this_adjusting = this_adjusting;
node->thunk.virtual_value = virtual_value;
diff --git a/gcc/cgraph.h b/gcc/cgraph.h
index 57cdaa45681..372ac9f01aa 100644
--- a/gcc/cgraph.h
+++ b/gcc/cgraph.h
@@ -629,18 +629,40 @@ extern const char * const cgraph_availability_names[];
extern const char * const ld_plugin_symbol_resolution_names[];
extern const char * const tls_model_names[];
-/* Information about thunk, used only for same body aliases. */
+/* Sub-structure of cgraph_node. Holds information about thunk, used only for
+ same body aliases.
+
+ Thunks are basically wrappers around methods which are introduced in case
+ of multiple inheritance in order to adjust the value of the "this" pointer
+ or of the returned value. */
struct GTY(()) cgraph_thunk_info {
- /* Information about the thunk. */
- HOST_WIDE_INT fixed_offset;
- HOST_WIDE_INT virtual_value;
- tree alias;
+ /* Set to true when alias node (the cgraph_node to which this struct belong)
+ is a thunk. Access to any other fields is invalid if this is false. */
+ bool thunk_p;
+
+ /* Nonzero for a "this" adjusting thunk and zero for a result adjusting
+ thunk. */
bool this_adjusting;
+
+ /* If true, this thunk is what we call a virtual thunk. In this case, after
+ the FIXED_OFFSET based adjustment is done, add to the result the offset
+ found in the vtable at: vptr + VIRTUAL_VALUE. */
bool virtual_offset_p;
+
+ /* ??? True for special kind of thunks, seems related to instrumentation. */
bool add_pointer_bounds_args;
- /* Set to true when alias node is thunk. */
- bool thunk_p;
+
+ /* Offset used to adjust "this". */
+ HOST_WIDE_INT fixed_offset;
+
+ /* Offset in the virtual table to get the offset to adjust "this". Valid iff
+ VIRTUAL_OFFSET_P is true. */
+ HOST_WIDE_INT virtual_value;
+
+ /* Thunk target, i.e. the method that this thunk wraps. Depending on the
+ TARGET_USE_LOCAL_THUNK_ALIAS_P macro, this may have to be a new alias. */
+ tree alias;
};
/* Information about the function collected locally.
--
2.14.1
