I think it's better to pass amount in bytes here, as specified by user
(alignas takes bytes, not bits).
On 09/13/2016 09:00 PM, Robinson, Paul wrote:
I hadn't thought Clang wanted to be *quite* so knowledgeable about
targets, and similarly not so tightly tied to byte-addressable
targets. But if both of those things are actually okay, then it's
fine to set the alignment value here to what would be passed through
to DWARF.
--paulr
*From:*David Blaikie [mailto:dblai...@gmail.com]
*Sent:* Monday, September 12, 2016 6:11 PM
*To:* Robinson, Paul;
reviews+d24426+public+6ee6274d38fdf...@reviews.llvm.org;
vlesc...@accesssoftek.com; echri...@gmail.com; apra...@apple.com;
mehdi.am...@apple.com
*Cc:* cfe-commits (cfe-commits@lists.llvm.org)
*Subject:* Re: [PATCH] D24426: DebugInfo: Pass non-zero alignment to
DIBuilder only if aligment was forced
On Mon, Sep 12, 2016 at 6:01 PM Robinson, Paul <paul.robin...@sony.com
<mailto:paul.robin...@sony.com>> wrote:
The text in the committee draft is different (e.g., the
exhortation about non-default alignment is gone), with an example
to the effect that a value of 8 means the entity's address is a
multiple of 8 (not 2^8). So, alignment is conceived in terms of
address bits, whatever those represent (not always bytes).
Not sure I quite follow. OK, so in an octet addressable world (which
LLVM is - there have been some attempts to support non-octet
addressing, but I don't think any have been near to successful) then
DW_AT_alignment is byte alignment (1 means there are no zero bits in
the address, 2 means there's 1 trailing zero bit in the address, etc).
If Clang is being infested with more target knowledge, okay, but
that means tolerating the weirder targets in these cases.
Dividing by CHAR_BITS makes an assumption that isn't necessarily
correct.
Clang has the knowledge already - it knows the alignment of the types
its allocating, etc. So I'm not sure what infestation you're referring to.
I've sort of lost track of what we're discussing here.
Essentially what I'm suggesting is that Clang should put whatever
number is going to go in the DWARF, into the metadata. I don't believe
the LLVM backends have greater knowledge than the frontend does in
this domain - have I missed something there, are there examples where
that could/would be true?
- David
--paulr
P.S. The committee is hoping to get a draft out for public comment
Real Soon Now.
Looking forward to it :)
*From:*cfe-commits [mailto:cfe-commits-boun...@lists.llvm.org
<mailto:cfe-commits-boun...@lists.llvm.org>] *On Behalf Of *David
Blaikie via cfe-commits
*Sent:* Monday, September 12, 2016 5:12 PM
*To:* reviews+d24426+public+6ee6274d38fdf...@reviews.llvm.org
<mailto:reviews%2bd24426%2bpublic%2b6ee6274d38fdf...@reviews.llvm.org>;
vlesc...@accesssoftek.com <mailto:vlesc...@accesssoftek.com>;
echri...@gmail.com <mailto:echri...@gmail.com>; apra...@apple.com
<mailto:apra...@apple.com>; mehdi.am...@apple.com
<mailto:mehdi.am...@apple.com>
*Cc:* cfe-commits@lists.llvm.org <mailto:cfe-commits@lists.llvm.org>
*Subject:* Re: [PATCH] D24426: DebugInfo: Pass non-zero alignment
to DIBuilder only if aligment was forced
On Mon, Sep 12, 2016 at 5:00 PM Paul Robinson
<paul.robin...@sony.com <mailto:paul.robin...@sony.com>> wrote:
probinson added a subscriber: probinson.
================
Comment at: lib/CodeGen/CGDebugInfo.cpp:3691
@@ -3635,1 +3690,3 @@
+ if (D->hasAttr<AlignedAttr>())
+ AlignInBits = D->getMaxAlignment();
StringRef DeclName, LinkageName;
----------------
dblaikie wrote:
> is max alignment the right thing here? Should it be min
alignment?
> (is alignment in bits the desired thing across all of this
too? It looked like in the backend patch there was some
division by CHAR_BITS, etc?)
I should think bits is the right choice here; seems more the
province of the backend to convert it into the appropriate
addressable units (commonly but not universally chars).
The alternative thinking is that we've a generally sense we want
to make more of this type information opaque to LLVM - so I'm
somewhat inclined to make the frontend do the work of choosing
what to emit and the backend just being as simple as possible.
Hmm, seems like the DWARF spec details I can find:
http://www.dwarfstd.org/ShowIssue.php?issue=140528.1 don't really
specify what the value of DW_AT_alignment is, it's sort of
assumed, by the looks of it? I'm assuming it's bytes, the same as
the byte_size attribute.
https://reviews.llvm.org/D24426
--
Best Regards,
Victor
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