[Lldb-commits] [PATCH] D32375: [DWARF] Fix lookup in the abstract origins of inlined blocks/functions

Sean Callanan via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Fri Apr 21 14:34:35 PDT 2017

spyffe created this revision.
spyffe added a project: LLDB.

LLDB uses `clang::DeclContext`s for lookups, and variables get put into the `DeclContext` for their //abstract origin//.  (The abstract origin is a DWARF pointer that indicates the unique definition of inlined code.)  When the expression parser is looking for variables, it locates the `DeclContext` for the current context.  This needs to be done carefully, though, e.g.:

  __attribute__ ((always_inline)) void f(int a) {
      int b = a * 2;
  void g() {

Here, if we're stopped in the inlined copy of `f`, we have to find the `DeclContext` corresponding to the definition of `f` – its abstract origin.  Clang doesn't allow multiple functions with the same name and arguments to exist.  It also means that any variables we see must be placed in the appropriate `DeclContext`.

[Bug 1]: When stopped in an inline block, the function `GetDeclContextDIEContainingDIE` for that block doesn't properly construct a DeclContext for the abstract origin for inlined subroutines.  That means we get duplicated function `DeclContext`s, but function arguments only get put in the abstract origin's `DeclContext`, and as a result when we try to look for them in nested contexts they aren't found.

[Bug 2]: When stopped in an inline block, the DWARF (for space reasons) doesn't explicitly point to the abstract origin for that block.  This means that the function `GetClangDeclContextForDIE` returns a different `DeclContext` for each place the block is inlined.  However, any variables defined in the block have abstract origins, so they will only get placed in the `DeclContext` for their abstract origin.

In this fix, I've introduced a test covering both of these issues, and fixed them.

Bug 1 could be resolved simply by making sure we look up the abstract origin for inlined functions when looking up their DeclContexts on behalf of nested blocks.

For Bug 2, I've implemented an algorithm that makes the `DeclContext` for a block be the containing `DeclContext` for the closest entity we would find during lookup that has an abstract origin pointer.  That means that in the following situation:

  { // block 1
    int a;
    { // block 2
      int b;

if we looked up the `DeclContext` for block 2, we'd find the block containing the abstract origin of `b`, and lookup would proceed correctly because we'd see `b` and `a`.  However, in the situation

  { // block 1
    int a;
    { // block 2

since there isn't anything to look up in block 2, we can't determine its abstract origin (and there is no such pointer in the DWARF for blocks).  However, we can walk up the parent chain and find `a`, and its abstract origin lives in the abstract origin of block 1.  So we simply say that the `DeclContext` for block 2 is the same as the `DeclContext` for block 1, which contains `a`.  Lookups will return the same results.




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