[PATCH] D27550: Fix LLVM's use of DW_OP_bit_piece in DWARF expressions.

[Adrian Prantl via Phabricator via llvm-commits]

aprantl marked 4 inline comments as done.
aprantl added a comment.

In https://reviews.llvm.org/D27550#618335, @dblaikie wrote:

> Not quite following all the changes here, but they look roughly plausible.
>
> Is there some code I couldn't find that sorts all the fragments before emitting them? (the old scheme wouldn't've needed this, since each bit_piece could specify where it appears - but maybe we were already sorting them anyway so pieces were linearly increasing in position? - the new scheme would require such sorting because the positions are implicit relative to prior pieces)


Yes, DebugLocEntry::sortUniqueValues() (and possibly others locations) is sorting the fragments by offset.



================
Comment at: lib/CodeGen/AsmPrinter/DwarfUnit.cpp:505
   }
+  Expr.AddExpression(ArrayRef<uint64_t>(DIExpr));
+  Expr.finalize();
----------------
dblaikie wrote:
> makeArrayRef?
Nice. Didn't know we had that.


================
Comment at: test/DebugInfo/AArch64/frameindices.ll:9
+; CHECK: DW_AT_location [DW_FORM_block1]    (<0x0c> 93 01 91 51 93 0f 93 01 91 4a 93 07 )
+;  -- piece 0x00000001, fbreg -47, piece 0x0000000f, piece 0x00000001, fbreg -54, piece 0x00000007 ------^
 ; CHECK: DW_AT_abstract_origin {{.*}} "p1"
----------------
dblaikie wrote:
> Took me a bit to go through both these examples (keeping both the DW_OP_bit_piece/piece interpretations in my head, etc) - but yes, great to see this, as it looks good/right/etc :)
Thanks, from past experience,  I really appreciate someone double-checking this!


================
Comment at: test/DebugInfo/MIR/X86/bit-piece-dh.mir:1
+# RUN: llc -filetype=obj -o - %s | llvm-dwarfdump - | FileCheck %s
+# CHECK: .debug_info contents:
----------------
dblaikie wrote:
> what's 'dh' stand for in the title of this test?
> 
> What codepath does this test? (looks like the expression isn't in DIExpression - it's created in some MI/CodeGen layer?)
The 8-bit x86 register "dh" hi-part of (r)dx.

```
0    8   16    32       64
+-----------------------+
|              rdx      |
|        edx    |       |
|    dx   |     |       |
| dl | dh |     |       |
+----+----+-----+-------+
```

This is testing DwarfExpression.cpp:111 (Walk up the super-register chain until we find a valid number and emit a DW_OP_bit_piece for it).


================
Comment at: test/DebugInfo/X86/dw_op_minus_direct.ll:9-12
+; CHECK: Beginning address offset: 0x0000000000000000
+; CHECK:    Ending address offset: 0x0000000000000004
+; CHECK:     Location description: 50 10 01 1c 93 04
+;                                  rax, constu 0x00000001, minus, piece 0x00000004
----------------
dblaikie wrote:
> This is test coverage for DwarfExpression.cpp:244-ish? Probably better as a separate commit?
It tests a bugfix that fell out of this rewrite/factoring. The code in trunk would have crashed when it encountered a DW_OP_minus without a DW_OP_deref. I would leave it in here.


https://reviews.llvm.org/D27550