Re: [fpc-devel] Can FPC at least give a hint/warning?
Gennady Agranov wrote:
> Hi,
>
Or what are the conditions when you want to emit the hint?
>
> I thought that hint could come from some layer that actually allocates
> the record - on the stack or on the heap - this layer may actually know
> the "effective" alignment
>
> Heap allocation should be aligned to the largest required alignment (is
> it 8?) - i.e. it should relatively simple to validate that offset of
> some "double" field is properly aligned
The compiler does not know where a pointer comes from when it is used.
It may be the result of a getmem call, of getting the address of an
unaligned field, or of manual pointer arithmetic. Additionally, the
memory manager may have been overridden and may return anything it wants.
> I actually do not know how stack allocation works - i.e. when and how
> compiler aligns double, extended or record values (packed or unpacked)
The guaranteed alignment of the start of a record declared with
{$packrecords X} is at most a multiple of X, regardless of the type of
its fields. The reasoning is that aligning a record when you don't
guarantee the same alignment in the record itself, is kind of misleading
(even if it helps initially to get some field aligned, any change to the
record could break this) and probably wasteful.
> So if in your opinion this hint would report too many false positives -
> case is closed :)
It's not only about false positives, but also about the fact that it
would be impossible to get rid of this hint/warning when you have
determined that the caller can deal with the unaligned data.
Jonas
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Re: [fpc-devel] Can FPC at least give a hint/warning?
Hi, Or what are the conditions when you want to emit the hint? I thought that hint could come from some layer that actually allocates the record - on the stack or on the heap - this layer may actually know the "effective" alignment Heap allocation should be aligned to the largest required alignment (is it 8?) - i.e. it should relatively simple to validate that offset of some "double" field is properly aligned I actually do not know how stack allocation works - i.e. when and how compiler aligns double, extended or record values (packed or unpacked) So if in your opinion this hint would report too many false positives - case is closed :) >>> if the bug is closed - is there any way to add a note to it? Yes, I found this button after I have went to the URL again and inspected the available UI more thorough :( I did not want to send another e-mail to FPC list - that I have found it :) Sorry about it, Gennady ___ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
Re: [fpc-devel] Can FPC at least give a hint/warning?
Am 07.11.2016 um 03:42 schrieb Gennady Agranov:
> Hi,
>
> I have submitted a bug - http://bugs.freepascal.org/view.php?id=30872
> (misaligned data exception on
> ARMv7)
>
> And this big was resolved as "won't fix"
>
> 1. I understand that one has to work really hard to encounter this bug - and
> there are easy
> workarounds, but it there are also different ways to resolve this bug - e.g.
> just giving a hint
> would suffice - IMHO
While this is possible, if you - as said below - use packrecords 1, you will
get a hint on every use
of such a cardinal as the compiler normally does not know the final address of
a variable in a
program but only its alignment requirements. So even if a cardinal is the first
element of a such a
record, the compiler cannot assume that this field is actually properly aligned
because packrecords
1 relaxes also the alignment requirements of the whole record, so the record
itself might be start
at an unaligned address. This makes imo such a hint useless. Or what are the
conditions when you
want to emit the hint?
>
> 2. All our units use {$PACKRECORDS 1} in order to be layout compatible with
> Delphi and we always can
> add padding or change the field order - if only compiler warned us that field
> is not properly
> aligned for the target platform...
>
> 3. And a separate question - if the bug is closed - is there any way to add a
> note to it?
You can reopen it to add the note.
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Re: [fpc-devel] Can FPC at least give a hint/warning?
On 08/11/16 10:26, Marco van de Voort wrote: In our previous episode, Mark Morgan Lloyd said: The Solaris and C communities appeared to take the line that a correct compiler "could not" generate unaligned accesses, although they held off clarifying whether "could" in that context means "if it does it's an error" or "it must rearrange data to avoid an error". I believe that Solaris and in some cases Linux can handle unaligned accesses in the kernel, although there's obviously a significant performance penalty. I can remember the early powerpc ports where I came into trouble with the NetBSD port because the Linux kernel used emulation to fix unaligned float loads/stores, while NetBSD didn't. I believe the Linux ARM kernel can intercept some alignment traps, the SPARC kernel can do something similar but only for kernel code (I don't know whether that will change with the current push to get Debian onto SPARC-64, it could be an artifact of the kernel being 64-bit but the userspace 32-bit). -- Mark Morgan Lloyd markMLl .AT. telemetry.co .DOT. uk [Opinions above are the author's, not those of his employers or colleagues] ___ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
Re: [fpc-devel] Can FPC at least give a hint/warning?
In our previous episode, Mark Morgan Lloyd said: > The Solaris and C communities appeared to take the line that a correct > compiler "could not" generate unaligned accesses, although they held off > clarifying whether "could" in that context means "if it does it's an > error" or "it must rearrange data to avoid an error". > > I believe that Solaris and in some cases Linux can handle unaligned > accesses in the kernel, although there's obviously a significant > performance penalty. I can remember the early powerpc ports where I came into trouble with the NetBSD port because the Linux kernel used emulation to fix unaligned float loads/stores, while NetBSD didn't. ___ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
Re: [fpc-devel] Can FPC at least give a hint/warning?
On 08/11/16 06:30, Dennis wrote: Gennady Agranov wrote: Hi, I have submitted a bug - http://bugs.freepascal.org/view.php?id=30872 (misaligned data exception on ARMv7) And this big was resolved as "won't fix" 1. I understand that one has to work really hard to encounter this bug - and there are easy workarounds, but it there are also different ways to resolve this bug - e.g. just giving a hint would suffice - IMHO a compiler warning or hint would be nice. Alignment was a major issue back when SPARC was still a realistically viable platform. The Solaris and C communities appeared to take the line that a correct compiler "could not" generate unaligned accesses, although they held off clarifying whether "could" in that context means "if it does it's an error" or "it must rearrange data to avoid an error". I believe that Solaris and in some cases Linux can handle unaligned accesses in the kernel, although there's obviously a significant performance penalty. Working on the principle that the FPC community would do well to avoid antagonising people with expectations derived from experience with other compilers and languages, I'd suggest that a warning when potentially-misaligned code is generated is in order. -- Mark Morgan Lloyd markMLl .AT. telemetry.co .DOT. uk [Opinions above are the author's, not those of his employers or colleagues] ___ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
Re: [fpc-devel] Can FPC at least give a hint/warning?
Gennady Agranov wrote: Hi, I have submitted a bug - http://bugs.freepascal.org/view.php?id=30872 (misaligned data exception on ARMv7) And this big was resolved as "won't fix" 1. I understand that one has to work really hard to encounter this bug - and there are easy workarounds, but it there are also different ways to resolve this bug - e.g. just giving a hint would suffice - IMHO a compiler warning or hint would be nice. Dennis ___ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
[fpc-devel] Can FPC at least give a hint/warning?
Hi,
I have submitted a bug - http://bugs.freepascal.org/view.php?id=30872
(misaligned data exception on ARMv7)
And this big was resolved as "won't fix"
1. I understand that one has to work really hard to encounter this bug -
and there are easy workarounds, but it there are also different ways to
resolve this bug - e.g. just giving a hint would suffice - IMHO
2. All our units use {$PACKRECORDS 1} in order to be layout compatible
with Delphi and we always can add padding or change the field order - if
only compiler warned us that field is not properly aligned for the
target platform...
3. And a separate question - if the bug is closed - is there any way to
add a note to it?
Thank you very much,
Gennady
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