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Subject: LLVMdev Digest, Vol 113, Issue 74
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Today's Topics:
1. Re: struct alignment question (Reid Kleckner)
2. Re: bit code file incompatibility due to debug info changes
(Bob Wilson)
3. Re: bit code file incompatibility due to debug info changes
(Eric Christopher)
4. Re: bit code file incompatibility due to debug info changes
(Bob Wilson)
5. Re: Null pointers with a non-0 representation (Micah Villmow)
6. Re: Adding line table debug information to LLVM on Windows
(Timur Iskhodzhanov)
----------------------------------------------------------------------
Message: 1
Date: Tue, 19 Nov 2013 12:52:55 -0800
From: Reid Kleckner <rnk at google.com>
To: edA-qa mort-ora-y <eda-qa at disemia.com>
Cc: "<llvmdev at cs.uiuc.edu>" <llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] struct alignment question
Message-ID:
<CACs=ty+vasJ-ErT1bLaBqEmLvNQ+HqEUbNqPgx4MwO05xvzwuQ at mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Maybe. Usually people who want this do something like:
Y = { c, d, e }
X = { a, b, Y }
On Tue, Nov 19, 2013 at 12:04 PM, edA-qa mort-ora-y <eda-qa at
disemia.com>wrote:
> On 19/11/13 08:03, Caldarale, Charles R wrote:
> >> X = { a, b, c, d, e }
> >> Y = { c, d, e }
>
> > In general, no. If a, b, and c were char, and d was an int, using
> > typical C alignments, there would be one slack byte between c and d
> > in X, whereas there would be three in Y. You could probably force
> > what you want with packed structures or by playing with the data
> > layout setting.
>
> Yes, I forgot about the simple scenario. If I force the alignment on
"c"
> should I be okay? That is, if I force it to the maximum alignment for
> the platform will { c, d, e } always have the same layout?
>
>
>
> --
> edA-qa mort-ora-y
> Leaf Creator
>
> Leaf - the language we always wanted
> http://leaflang.org/
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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Message: 2
Date: Tue, 19 Nov 2013 13:28:34 -0800
From: Bob Wilson <bob.wilson at apple.com>
To: Chandler Carruth <chandlerc at google.com>
Cc: LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] bit code file incompatibility due to debug info
changes
Message-ID: <2681F271-BB87-49F2-9CC8-08B129D70349 at apple.com>
Content-Type: text/plain; charset="us-ascii"
On Nov 19, 2013, at 11:01 AM, Bob Wilson <bob.wilson at apple.com> wrote:
>
> On Nov 19, 2013, at 10:58 AM, Chandler Carruth <chandlerc at
google.com> wrote:
>
>>
>> On Tue, Nov 19, 2013 at 10:48 AM, Bob Wilson <bob.wilson at
apple.com> wrote:
>> I don't have a strong opinion on how we fix it, but I think
it's really important to do something and get it done for the 3.4 release.
>>
>> I agree with everything here being a reasonable step for the 3.4
release except:
>>
>> - implement Quentin's previous proposal for back-end diagnostics
(and add a diagnostic handler to Apple's linker), so that we can warn about
out-of-date debug info
>>
>> I don't think this is important for the release. I don't think
open source people have any expectation of stability here, and while it is a
nice-to-have, it doesn't seem necessary, and seems clearly like a
significant extension that shouldn't be cherry-picked late into the release.
>
> OK, that's fair. As long as we can get it on trunk soon, we can
cherry-pick it to an internal release branch.
>
Two questions:
1) Can we reopen the bugzilla PR that I filed and use that to cover the
additional of a version number and code to ignore the debug info when the
version number does not match? Eric has already closed it twice, so I'm
asking this time instead of just reopening it.
2) Who is going to do this work?
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Message: 3
Date: Tue, 19 Nov 2013 13:31:54 -0800
From: Eric Christopher <echristo at gmail.com>
To: Bob Wilson <bob.wilson at apple.com>
Cc: LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] bit code file incompatibility due to debug info
changes
Message-ID:
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> Two questions:
>
> 1) Can we reopen the bugzilla PR that I filed and use that to cover the
> additional of a version number and code to ignore the debug info when the
> version number does not match? Eric has already closed it twice, so
I'm
> asking this time instead of just reopening it.
I'm not really sure the bug is needed, but if you want to open a bug
then that's fine. I think there are some holes in what's going to
specified where, but that can be discussed when someone starts working
on it.
>
> 2) Who is going to do this work?
No idea.
-eric
------------------------------
Message: 4
Date: Tue, 19 Nov 2013 13:35:25 -0800
From: Bob Wilson <bob.wilson at apple.com>
To: Eric Christopher <echristo at gmail.com>
Cc: LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] bit code file incompatibility due to debug info
changes
Message-ID: <E49F63C4-F360-4F5D-843D-74EA54A271D8 at apple.com>
Content-Type: text/plain; CHARSET=US-ASCII
On Nov 19, 2013, at 1:31 PM, Eric Christopher <echristo at gmail.com>
wrote:
>> Two questions:
>>
>> 1) Can we reopen the bugzilla PR that I filed and use that to cover the
>> additional of a version number and code to ignore the debug info when
the
>> version number does not match? Eric has already closed it twice, so
I'm
>> asking this time instead of just reopening it.
>
> I'm not really sure the bug is needed, but if you want to open a bug
> then that's fine. I think there are some holes in what's going to
> specified where, but that can be discussed when someone starts working
> on it.
OK. I have reopened pr17982 to cover this issue. I don't intend to specify
much in bugzilla, but we should have something to track the overall problem.
>
>>
>> 2) Who is going to do this work?
>
> No idea.
>
> -eric
------------------------------
Message: 5
Date: Tue, 19 Nov 2013 21:44:23 +0000
From: Micah Villmow <micah.villmow at smachines.com>
To: Clark Smith Cox III <clark.cox at apple.com>
Cc: "llvmdev at cs.uiuc.edu" <llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] Null pointers with a non-0 representation
Message-ID:
<3947CD34E13C4F4AB2D94AD35AE3FE60074E2445 at smi-exchange1.smi.local>
Content-Type: text/plain; charset="us-ascii"
Ahh yeah duh, your right. I misread that.
That doesn't solve this problem however, which is a different example of
what I was trying to show but failed.
kernel void test(local int* a) {
local int*ptr = NULL;
if (a != ptr) *a = 0;
}
from host code:
test(0);
Micah
From: Clark Smith Cox III [mailto:clark.cox at apple.com]
Sent: Tuesday, November 19, 2013 12:40 PM
To: Micah Villmow
Cc: Michael LIAO; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Null pointers with a non-0 representation
On Nov 19, 2013, at 10:52, Micah Villmow <micah.villmow at
smachines.com<mailto:micah.villmow at smachines.com>> wrote:
+llvm-dev
Yes. The question then becomes, does LLVM handle the case where the cast of the
value 0 to a non-void* pointer?
Think of this case:
int *ptr = (int*)0;
based on a strict reading of the spec, ptr itself technically is not the null
pointer constant.
Not true, the literal zero *is* a null pointer constant, which is then converted
to a null pointer.
If ptr points to an object in the local address space in OpenCL, or any address
space where 0 is a valid address and memory exists, does LLVM handle this
correctly?
I don't believe that it does, but I can't say for certain.
Micah
From: Michael LIAO [mailto:michael.hliao at gmail.com]
Sent: Sunday, November 17, 2013 10:46 PM
To: Micah Villmow
Subject: Re: [LLVMdev] Null pointers with a non-0 representation
Hi Micah & Michael
By C standard, section 6.3.2.2, "An integer constant expression with the
value 0, or such an expression cast to type void *, is called a null pointer
constant. If a null pointer constant is converted to a pointer type, the
resulting pointer, called a null pointer, is guaranteed to compare unequal to a
pointer to any object or function." As LLVM IR is modelled after C, I
assume null pointer in LLVM IR is equivalent to 0 as well. The folding from 0 to
null pointer in LLVM follows that behaviour very similar.
- michael
On Sun, Nov 17, 2013 at 1:00 PM, Micah Villmow <micah.villmow at
smachines.com<mailto:micah.villmow at smachines.com>> wrote:
The reason why I suggested looking there is because the same questions came up
as address 0 in some of the OpenCL address spaces are legal addresses. So in
order to solve this problem, they would have to find answers to your questions,
mainly your 'c)' question.
I took a look at LangRef and it does look like there is a 'null'
representation of the null pointer constant, which wasn't part of LLVM back
when I worked on this problem. So maybe you are just running into an issue
where the code hasn't been updated yet because it works on everyones
platform but yours.
Micah
From: Kuperstein, Michael M [mailto:michael.m.kuperstein at
intel.com<mailto:michael.m.kuperstein at intel.com>]
Sent: Sunday, November 17, 2013 3:28 AM
To: Micah Villmow; LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu>
Subject: RE: Null pointers with a non-0 representation
Hi Micah,
Thanks a lot for the reference.
Unfortunately, it looks like the discussion there is dealing with a somewhat
different issue - modeling OpenCL address spaces. It's true that the null
pointer issue may arise in a similar context (e.g. null pointers in different
address spaces may have different internal representations), but it's not
entirely related. Or have I missed something in the thread?
Thanks,
Michael
From: Micah Villmow [mailto:micah.villmow at smachines.com]
Sent: Sunday, November 17, 2013 11:42
To: Kuperstein, Michael M; LLVMdev at cs.uiuc.edu<mailto:LLVMdev at
cs.uiuc.edu>
Subject: RE: Null pointers with a non-0 representation
I ran into the same problem in OpenCL with Local/Private address spaces and I
believe it was discussed here in the past. So I would suggest looking through
the archives as I believe some of your questions are answered there.
This post might be a good starting point:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-October/044101.html
Also the SPIR provisional spec as of last November deals with this issue if I
remember correctly, so maybe you can discuss with Boaz Ouriel if he is still at
Intel Israel about how they went about handling this problem, or look at the
SPIR related posts from last year.
Hope this helps,
Micah
From: llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at
cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Kuperstein,
Michael M
Sent: Sunday, November 17, 2013 1:15 AM
To: LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu>
Subject: [LLVMdev] Null pointers with a non-0 representation
I feel a bit silly asking this, but here goes.
The C spec does not require the null pointer to be implemented as an integer
with the 0 bit-pattern. The implementation may arbitrarily choose any other
(integer, but not necessarily) representation, as long as it's distinct from
any legal pointer. The only requirement is that casting an integer 0 to a
pointer type results in a null pointer.
LLVM IR appears to have similar the casting behavior. That is, if I try to
create a pointer with integer value 0 (e.g. by constructing an inttoptr constant
expression with a i32 0 argument), it gets immediately folded into a null
pointer constant. Furthermore, a PtrToInt from a null pointer results in 0
(which is even "stricter" than the C spec.)
So, that raises a few question:
a) Are those the desired semantics? I guess if clang generates this kind of
IntToPtr instructions (as opposed to directly resolving the cast to a null
pointer) this is unavoidable. But otherwise, why does LLVM treat null pointers
and pointers with the integer value 0 as equivalent?
It's completely natural for most platforms, but isn't necessarily the
right thing to do.
b) Assuming this is really desirable, should the fact that casting an
integer 0 to a pointer type results in a null pointer be documented in the
langref (as part of the inttoptr/ptrtoint documentation)? Or is it too
"low-level"?
c) Let's say I want to create a pointer with the numeric value 0,
distinct from the null pointer, because 0 really does represent a valid pointer
on my platform. Should this possible at the IR level? If it should be, then how?
Anyone has any inputs, except laughing me out of the (virtual) room? :-)
Thanks,
Michael
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Message: 6
Date: Mon, 18 Nov 2013 21:14:17 +0400
From: Timur Iskhodzhanov <timurrrr at google.com>
To: Jo?o Matos <ripzonetriton at gmail.com>, David Blaikie
<dblaikie at gmail.com>, Eric Christopher <echristo at gmail.com>
Cc: LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] Adding line table debug information to LLVM on
Windows
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<CAH38oYiu7oLYH494zNo7yUH+BH8XWFJLBoQi5BedaUDpLUSm8w at mail.gmail.com>
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I wrote some more lit tests for my patch and realized I was generating
some redundant info. This is fixed now. Attached is a new version
of the prototype patch with some more tests.
2013/11/15 Jo?o Matos <ripzonetriton at
gmail.com>:> Hi Timur,
>
> There's also a pending patch adding CodeView support in Phab:
> http://llvm-reviews.chandlerc.com/D165
I haven't looked at your patch yet, but based on the very low phab
review number, I'm pretty sure there are good reasons this wasn't
committed.
> Does your patch provide just a subset of the CodeView debug info provided
in
> the other patch?
Yes. I prefer small incremental changes.
Also, the file:line debug info is much much more important for me atm
than the other types of debug info.
> Looking at the patch, I think the approach the other patch took of
> abstracting the emission of debug information is a bit cleaner and it will
> probably make life easier when adding more debug formats in the future.
Why wasn't the "abstract the emission of DI" part of that patch
reviewed/committed separately?
> On Fri, Nov 15, 2013 at 4:39 PM, Timur Iskhodzhanov <timurrrr at
google.com>
> wrote:
>>
>> 2013/11/14 Timur Iskhodzhanov <timurrrr at google.com>:
>> > Hi David, Eric, LLVM devs,
>> >
>> > You've probably heard about AddressSanitizer (ASan) and other
>> > sanitizers based on LLVM. One of the things that makes ASan
>> > not as awesome on Windows as it is on Linux
>> > is the symbolization of the stacks.
>> >
>> > Currently, ASan runtime on Windows uses
>> > CaptureStackBackTrace/SymFromAddr/SymGetLineFromAddr64
>> > to unwind and symbolize stacks. This works like a charm
>> > in-process for stack frames built with CL, but yields
>> > "function+0x0ff537" for frames built with Clang.
>> >
>> > I came up with a prototype which emits "old-style debug
info" COFF
>> > sections that are sufficient to get function name / filename /
>> > linenumber information. That's pretty much everything
that's required
>> > for ASan to work beautifully in terms of the completeness of error
>> > reports.
>> >
>> > Attached is a prototype patch which I've tried on some simple
tests,
>> > including some more complex ones with weird #line constructions.
>> > It also works just great on ASan/Win tests without any
link/run-time
>> > warnings (I had a bunch of those during development, so I can tell
it
>> > works rather than fails silently).
>> >
>> > I didn't have time to work on threading the command-line flags
into
>> > the AsmPrinter yet, so currently it just replaces DWARF entirely.
>> > Of course, this should be fixed before this lands into trunk.
>> > Currently, one can try this patch by using "clang-cl -Xclang
-g ...".
>> > Eventually we should have some dedicated flag for clang-cl.
>> >
>> > Can you please take a look at the patch and suggest a good path
forward?
>> >
>> > I'm very unfamiliar with LLVM CodeGen/MC, so I'm pretty
sure I made a
>> > few weird things in the code... I've also put a few TODOs
with
>> > questions and suggestions.
>> >
>> > Some general questions:
>> > 1) Threading flags from the driver down to CodeGen.
>> > How do we do that? Should we support all 4 combinations
>> > of no-info/DWARF/CVLT/both?
>> > How about "-Zmlt" as the clang-cl flag name?
("minimal line tables")
>> >
>> > 2) Am I right that DWARF is pretty much the only debug info format
>> > supported by LLVM/AsmPrinter right now? Do we want to take
>> > an effort to come up with a generic debuginfogenerator interface
>> > to share between DwarfDebug and WinCodeViewLineTables?
>> > Then AsmPrinter should just hold a
SmallVector<DebugInfoEmitter*>
>> > rather than a pair of DD/DE pointers.
>> >
>> > 3) How would you suggest to write WinCodeViewLineTables tests
>> > given that dumpbin is not available everywhere except Windows?
>> > // Yeah, I should have looked at the DwarfDebug LIT tests and
>> > // written some; but the prototype development went faster
>> > // than I expected...
>>
>> I found the MCAsmStreamer being used by llc which gives a decent text
>> format.
>> I wrote a simple x86+x86_64 .ll test and FileCheck expectations for
>> the llc asm output.
>> Is this the right approach to write tests?
>> If so, I'll convert my remaining C program test cases into such an
>> .ll+llc tests.
>>
>> > Can you suggest ways to split this patch so it's easier
>> > to review part-by-part before this hits trunk?
>>
>> Attached is an updated patch with a new test and a few minor things
>> improved.
>> I also removed the "TODO: test on X64" as I did try it on x64
and no
>> changes were required.
>>
>> Looking forward to your feedback!
>>
>> > Thanks!
>> > --
>> > Timur Iskhodzhanov,
>> > Google
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
>
>
> --
> Jo?o Matos
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