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

[David Blaikie via llvm-commits]

Should we assert/fail verification on this? (probably a separate question
from this patch)


On Fri, Dec 9, 2016 at 11:08 AM Adrian Prantl <aprantl at apple.com> wrote:

> On Dec 9, 2016, at 11:01 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Fri, Dec 9, 2016 at 10:43 AM Adrian Prantl via Phabricator <
> reviews at reviews.llvm.org> wrote:
>
> 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.
>
>
> Ah, thanks - and what happens if there's overlap? Is there some error
> handling?
>
>
> DwarfDebug::buildLocationList() collects looks at the fragments and and
> then chops up their ranges into the smallest continuous region that is
> covered by a set of non-overlapping fragments. So what will likely happen
> is that it will produce a zero-sized region for the first overlapping
> fragment that will then be silently dropped.
>
> -- adrian
>
>
>
>
>
>
> ================
> 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
>
>
>
>
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