Move the complex address expression out of DIVariable and into an extra

argument of the llvm.dbg.declare/llvm.dbg.value intrinsics.

Previously, DIVariable was a variable-length field that has an optional
reference to a Metadata array consisting of a variable number of
complex address expressions. In the case of OpPiece expressions this is
wasting a lot of storage in IR, because when an aggregate type is, e.g.,
SROA'd into all of its n individual members, the IR will contain n copies
of the DIVariable, all alike, only differing in the complex address
reference at the end.

By making the complex address into an extra argument of the
dbg.value/dbg.declare intrinsics, all of the pieces can reference the
same variable and the complex address expressions can be uniqued across
the CU, too.
Down the road, this will allow us to move other flags, such as
"indirection" out of the DIVariable, too.

The new intrinsics look like this:
declare void @llvm.dbg.declare(metadata %storage, metadata %var, metadata %expr)
declare void @llvm.dbg.value(metadata %storage, i64 %offset, metadata %var, metadata %expr)

This patch adds a new LLVM-local tag to DIExpressions, so we can detect
and pretty-print DIExpression metadata nodes.

What this patch doesn't do:

This patch does not touch the "Indirect" field in DIVariable; but moving
that into the expression would be a natural next step.

http://reviews.llvm.org/D4919
rdar://problem/17994491

Thanks to dblaikie and dexonsmith for reviewing this patch!

Note: I accidentally committed a bogus older version of this patch previously.
llvm-svn: 218787

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Support/Dwarf.cpp b/llvm/lib/Support/Dwarf.cpp
index 5237ac22025..1a8f4a1ec2e 100644
--- a/llvm/lib/Support/Dwarf.cpp
+++ b/llvm/lib/Support/Dwarf.cpp
@@ -82,6 +82,7 @@ const char *llvm::dwarf::TagString(unsigned Tag) {
   case DW_TAG_hi_user:                   return "DW_TAG_hi_user";
   case DW_TAG_auto_variable:             return "DW_TAG_auto_variable";
   case DW_TAG_arg_variable:              return "DW_TAG_arg_variable";
+  case DW_TAG_expression:                return "DW_TAG_expression";
   case DW_TAG_rvalue_reference_type:     return "DW_TAG_rvalue_reference_type";
   case DW_TAG_template_alias:            return "DW_TAG_template_alias";
   case DW_TAG_coarray_type:              return "DW_TAG_coarray_type";