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Mehdi Amini via llvm-dev

2017-Mar-07 06:12 UTC

[llvm-dev] [BUG Report] -dead_strip, strips prefix data unconditionally on macOS

> On Mar 6, 2017, at 7:56 PM, James Y Knight via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Oh, that's great that it's possible to implement properly, now.
Does it actually work for
> 
> It'd be cool if LLVM hooked up its generic section handling support to
this feature now, so that the only global symbols that *didn't* get marked
as .alt_entry were those at the beginning of what llvm would consider sections.
> 
> Then apple platforms could behave sanely, like all other platforms do, only
with -f{function,data}-sections defaulted to on instead of off.
What is the advantage of not using -f{function,data}-sections? (i.e. what isn’t
sane about it?)

I’m asking because I was told the only reason not to use it all the time on ELF
is that it makes intermediate object files larger.

Thanks,

— 
Mehdi



> 
> (That is, if you specify -fno-function-sections -fno-data-sections, it
could mark nearly *everything* as .alt_entry -- except the first symbol in the
object file)
> 
> 
> On Mon, Mar 6, 2017 at 2:35 PM, Peter Collingbourne <peter at pcc.me.uk
<mailto:peter at pcc.me.uk>> wrote:
> That is in theory what omitting the .subsections_via_symbols directive is
supposed to do, but in an experiment I ran a year or two ago I found that the
Mach-O linker was still dead stripping on symbol boundaries with this directive
omitted.
> 
> In any case, a more precise approach has more recently (~a few months ago)
become possible. There is a relatively new asm directive called .altentry that,
as I understand it, tells the linker to disregard a given symbol as a section
boundary (LLVM already uses this for aliases pointing into the middle of a
global). So what you would do is to use .altentry on the function symbol, with
an internal symbol appearing before the prefix data to ensure that it is not
considered part of the body of the previous function.
> 
> Peter
> 
> On Mon, Mar 6, 2017 at 11:19 AM, James Y Knight via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> AFAIK, this cannot actually work on Apple platforms, because its object
file format (Mach-O) doesn't use sections to determine the ranges of
code/data to keep together, but instead _infers_ boundaries based on the range
between global symbols in the symbol table.
> 
> So, the symbol pointing to the beginning of @main *necessarily* makes that
be a section boundary.
> 
> I think the best that could be done in LLVM is to not emit the
".subsections_via_symbols" asm directive (effectively disabling dead
stripping on that object) if any prefix data exists. Currently it emits that
flag unconditionally for MachO.
> 
> On Mon, Mar 6, 2017 at 4:40 AM, Moritz Angermann via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Hi,
> 
> I just came across a rather annoying behavior with llvm 3.9. Assuming the
following
> samle code in test.ll:
> 
> ; Lets have some global int x = 4
> @x = global i32 10, align 4
> ; and two strings "p = %d\n" for the prefix data,
> ; as well as "x = %d\n" to print the (global) x value.
> @.str = private unnamed_addr constant [8 x i8] c"x = %d\0A\00",
align 1
> @.str2 = private unnamed_addr constant [8 x i8] c"p = %d\0A\00",
align 1
> 
> ; declare printf, we'll use this later for printf style debugging.
> declare i32 @printf(i8*, ...)
> 
> ; define a main function.
> define i32 @main() prefix i32 123 {
>   ; obtain a i32 pointer to the main function.
>   ; the prefix data is right before that pointer.
>   %main = bitcast i32 ()* @main to i32*
> 
>   ; use the gep, to cmpute the start of the prefix data.
>   %prefix_ptr = getelementptr inbounds i32, i32* %main, i32 -1
>   ; and load it.
>   %prefix_val = load i32, i32* %prefix_ptr
> 
>   ; print that value.
>   %ret = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([8 x i8],
[8 x i8]* @.str2, i32 0, i32 0), i32 %prefix_val)
> 
>   ; similarly let's do the same with the global x.
>   %1 = alloca i32, align 4
>   store i32 0, i32* %1, align 4
>   %2 = load i32, i32* @x, align 4
>   %3 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([8 x i8], [8
x i8]* @.str, i32 0, i32 0), i32 %2)
>   ret i32 0
> }
> 
> gives the following result (expected)
> 
>    $ clang test.ll
>    $ ./a.out
>    p = 123
>    x = 10
> 
> however, with -dead_strip on macOS, we see the following:
> 
>    $ clang test.ll -dead_strip
>    $ ./a.out
>    p = 0
>    x = 10
> 
> Thus I believe we are incorrectly stripping prefix data when linking with
-dead_strip on macOS.
> 
> As I do not have a bugzilla account, and hence cannot post this as a proper
bug report.
> 
> Cheers,
>  Moritz
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 
> 
> 
> -- 
> -- 
> Peter
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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James Y Knight via llvm-dev

2017-Mar-07 19:23 UTC

head link

[llvm-dev] [BUG Report] -dead_strip, strips prefix data unconditionally on macOS

My point was more that it'd be great if symbols and sections and dead-code
stripping, in general, would work more like they do elsewhere. ISTM that'd
make things cleaner generally. No special case for offset aliases would be
needed, nor for features like prefix data. They'd just work by virtue of
generic handling for llvm sections turning into the appropriate symbol
flags. (It's a shame that defining the start of a subsection wasn't made
opt-IN for a symbol in macho, but oh well.)

Now, one of the *consequences* of hooking up the infrastructure that was is
that -fno-{function,data}-sections would automatically work. But I was
thinking of that independently of whether a user might want to actually
specify that.

However -- there is actually another possible reason to disable it: when
it's disabled, you avoid inserting relocations for references to local data
and functions. That can sometimes allow the use of shorter instructions to
reference nearby data -- so, in fact, it's possible for
-f{function,data}-sections to make your FINAL binary larger than it would
be without.


On Tue, Mar 7, 2017 at 1:12 AM, Mehdi Amini <mehdi.amini at apple.com>
wrote:
>
> On Mar 6, 2017, at 7:56 PM, James Y Knight via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Oh, that's great that it's possible to implement properly, now.
Does it
> actually work for
>
> It'd be cool if LLVM hooked up its generic section handling support to
> this feature now, so that the only global symbols that *didn't* get
marked
> as .alt_entry were those at the beginning of what llvm would consider
> sections.
>
> Then apple platforms could behave sanely, like all other platforms do,
> only with -f{function,data}-sections defaulted to on instead of off.
>
>
> What is the advantage of not using -f{function,data}-sections? (i.e. what
> isn’t sane about it?)
>
> I’m asking because I was told the only reason not to use it all the time
> on ELF is that it makes intermediate object files larger.
>
> Thanks,
>
> —
> Mehdi
>
>
>
>
>
> (That is, if you specify -fno-function-sections -fno-data-sections, it
> could mark nearly *everything* as .alt_entry -- except the first symbol in
> the object file)
>
>
> On Mon, Mar 6, 2017 at 2:35 PM, Peter Collingbourne <peter at
pcc.me.uk>
> wrote:
>
>> That is in theory what omitting the .subsections_via_symbols directive
is
>> supposed to do, but in an experiment I ran a year or two ago I found
that
>> the Mach-O linker was still dead stripping on symbol boundaries with
this
>> directive omitted.
>>
>> In any case, a more precise approach has more recently (~a few months
>> ago) become possible. There is a relatively new asm directive called
>> .altentry that, as I understand it, tells the linker to disregard a
given
>> symbol as a section boundary (LLVM already uses this for aliases
pointing
>> into the middle of a global). So what you would do is to use .altentry
on
>> the function symbol, with an internal symbol appearing before the
prefix
>> data to ensure that it is not considered part of the body of the
previous
>> function.
>>
>> Peter
>>
>> On Mon, Mar 6, 2017 at 11:19 AM, James Y Knight via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> AFAIK, this cannot actually work on Apple platforms, because its
object
>>> file format (Mach-O) doesn't use sections to determine the
ranges of
>>> code/data to keep together, but instead _infers_ boundaries based
on the
>>> range between global symbols in the symbol table.
>>>
>>> So, the symbol pointing to the beginning of @main *necessarily*
makes
>>> that be a section boundary.
>>>
>>> I think the best that could be done in LLVM is to not emit the
>>> ".subsections_via_symbols" asm directive (effectively
disabling dead
>>> stripping on that object) if any prefix data exists. Currently it
emits
>>> that flag unconditionally for MachO.
>>>
>>> On Mon, Mar 6, 2017 at 4:40 AM, Moritz Angermann via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi,
>>>>
>>>> I just came across a rather annoying behavior with llvm 3.9.
Assuming
>>>> the following
>>>> samle code in test.ll:
>>>>
>>>> ; Lets have some global int x = 4
>>>> @x = global i32 10, align 4
>>>> ; and two strings "p = %d\n" for the prefix data,
>>>> ; as well as "x = %d\n" to print the (global) x
value.
>>>> @.str = private unnamed_addr constant [8 x i8] c"x =
%d\0A\00", align 1
>>>> @.str2 = private unnamed_addr constant [8 x i8] c"p =
%d\0A\00", align 1
>>>>
>>>> ; declare printf, we'll use this later for printf style
debugging.
>>>> declare i32 @printf(i8*, ...)
>>>>
>>>> ; define a main function.
>>>> define i32 @main() prefix i32 123 {
>>>>   ; obtain a i32 pointer to the main function.
>>>>   ; the prefix data is right before that pointer.
>>>>   %main = bitcast i32 ()* @main to i32*
>>>>
>>>>   ; use the gep, to cmpute the start of the prefix data.
>>>>   %prefix_ptr = getelementptr inbounds i32, i32* %main, i32 -1
>>>>   ; and load it.
>>>>   %prefix_val = load i32, i32* %prefix_ptr
>>>>
>>>>   ; print that value.
>>>>   %ret = call i32 (i8*, ...) @printf(i8* getelementptr inbounds
([8 x
>>>> i8], [8 x i8]* @.str2, i32 0, i32 0), i32 %prefix_val)
>>>>
>>>>   ; similarly let's do the same with the global x.
>>>>   %1 = alloca i32, align 4
>>>>   store i32 0, i32* %1, align 4
>>>>   %2 = load i32, i32* @x, align 4
>>>>   %3 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds
([8 x
>>>> i8], [8 x i8]* @.str, i32 0, i32 0), i32 %2)
>>>>   ret i32 0
>>>> }
>>>>
>>>> gives the following result (expected)
>>>>
>>>>    $ clang test.ll
>>>>    $ ./a.out
>>>>    p = 123
>>>>    x = 10
>>>>
>>>> however, with -dead_strip on macOS, we see the following:
>>>>
>>>>    $ clang test.ll -dead_strip
>>>>    $ ./a.out
>>>>    p = 0
>>>>    x = 10
>>>>
>>>> Thus I believe we are incorrectly stripping prefix data when
linking
>>>> with -dead_strip on macOS.
>>>>
>>>> As I do not have a bugzilla account, and hence cannot post this
as a
>>>> proper bug report.
>>>>
>>>> Cheers,
>>>>  Moritz
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>>
>> --
>> --
>> Peter
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Reid Kleckner via llvm-dev

2017-Mar-07 19:29 UTC

head link

[llvm-dev] [BUG Report] -dead_strip, strips prefix data unconditionally on macOS

I mean, yes, it would be great if MachO were a better object file format.
But it has a limited number of sections, so .subsections_via_symbols and
atoms work around that problem. The only way to do better in the long term
is to use a better object file format, either new or existing. Every so
often somebody takes a look at that, but so far Apple has continued to
extend MachO as necessary to support their use cases.

On Tue, Mar 7, 2017 at 11:23 AM, James Y Knight via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> My point was more that it'd be great if symbols and sections and
dead-code
> stripping, in general, would work more like they do elsewhere. ISTM
that'd
> make things cleaner generally. No special case for offset aliases would be
> needed, nor for features like prefix data. They'd just work by virtue
of
> generic handling for llvm sections turning into the appropriate symbol
> flags. (It's a shame that defining the start of a subsection wasn't
made
> opt-IN for a symbol in macho, but oh well.)
>
> Now, one of the *consequences* of hooking up the infrastructure that was
> is that -fno-{function,data}-sections would automatically work. But I was
> thinking of that independently of whether a user might want to actually
> specify that.
>
> However -- there is actually another possible reason to disable it: when
> it's disabled, you avoid inserting relocations for references to local
data
> and functions. That can sometimes allow the use of shorter instructions to
> reference nearby data -- so, in fact, it's possible for
> -f{function,data}-sections to make your FINAL binary larger than it would
> be without.
>
>
> On Tue, Mar 7, 2017 at 1:12 AM, Mehdi Amini <mehdi.amini at
apple.com> wrote:
>
>>
>> On Mar 6, 2017, at 7:56 PM, James Y Knight via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> Oh, that's great that it's possible to implement properly, now.
Does it
>> actually work for
>>
>> It'd be cool if LLVM hooked up its generic section handling support
to
>> this feature now, so that the only global symbols that *didn't* get
marked
>> as .alt_entry were those at the beginning of what llvm would consider
>> sections.
>>
>> Then apple platforms could behave sanely, like all other platforms do,
>> only with -f{function,data}-sections defaulted to on instead of off.
>>
>>
>> What is the advantage of not using -f{function,data}-sections? (i.e.
what
>> isn’t sane about it?)
>>
>> I’m asking because I was told the only reason not to use it all the
time
>> on ELF is that it makes intermediate object files larger.
>>
>> Thanks,
>>
>> —
>> Mehdi
>>
>>
>>
>>
>>
>> (That is, if you specify -fno-function-sections -fno-data-sections, it
>> could mark nearly *everything* as .alt_entry -- except the first symbol
in
>> the object file)
>>
>>
>> On Mon, Mar 6, 2017 at 2:35 PM, Peter Collingbourne <peter at
pcc.me.uk>
>> wrote:
>>
>>> That is in theory what omitting the .subsections_via_symbols
directive
>>> is supposed to do, but in an experiment I ran a year or two ago I
found
>>> that the Mach-O linker was still dead stripping on symbol
boundaries with
>>> this directive omitted.
>>>
>>> In any case, a more precise approach has more recently (~a few
months
>>> ago) become possible. There is a relatively new asm directive
called
>>> .altentry that, as I understand it, tells the linker to disregard a
given
>>> symbol as a section boundary (LLVM already uses this for aliases
pointing
>>> into the middle of a global). So what you would do is to use
.altentry on
>>> the function symbol, with an internal symbol appearing before the
prefix
>>> data to ensure that it is not considered part of the body of the
previous
>>> function.
>>>
>>> Peter
>>>
>>> On Mon, Mar 6, 2017 at 11:19 AM, James Y Knight via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> AFAIK, this cannot actually work on Apple platforms, because
its object
>>>> file format (Mach-O) doesn't use sections to determine the
ranges of
>>>> code/data to keep together, but instead _infers_ boundaries
based on the
>>>> range between global symbols in the symbol table.
>>>>
>>>> So, the symbol pointing to the beginning of @main *necessarily*
makes
>>>> that be a section boundary.
>>>>
>>>> I think the best that could be done in LLVM is to not emit the
>>>> ".subsections_via_symbols" asm directive (effectively
disabling dead
>>>> stripping on that object) if any prefix data exists. Currently
it emits
>>>> that flag unconditionally for MachO.
>>>>
>>>> On Mon, Mar 6, 2017 at 4:40 AM, Moritz Angermann via llvm-dev
<
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I just came across a rather annoying behavior with llvm
3.9. Assuming
>>>>> the following
>>>>> samle code in test.ll:
>>>>>
>>>>> ; Lets have some global int x = 4
>>>>> @x = global i32 10, align 4
>>>>> ; and two strings "p = %d\n" for the prefix data,
>>>>> ; as well as "x = %d\n" to print the (global) x
value.
>>>>> @.str = private unnamed_addr constant [8 x i8] c"x =
%d\0A\00", align 1
>>>>> @.str2 = private unnamed_addr constant [8 x i8] c"p =
%d\0A\00", align
>>>>> 1
>>>>>
>>>>> ; declare printf, we'll use this later for printf style
debugging.
>>>>> declare i32 @printf(i8*, ...)
>>>>>
>>>>> ; define a main function.
>>>>> define i32 @main() prefix i32 123 {
>>>>>   ; obtain a i32 pointer to the main function.
>>>>>   ; the prefix data is right before that pointer.
>>>>>   %main = bitcast i32 ()* @main to i32*
>>>>>
>>>>>   ; use the gep, to cmpute the start of the prefix data.
>>>>>   %prefix_ptr = getelementptr inbounds i32, i32* %main, i32
-1
>>>>>   ; and load it.
>>>>>   %prefix_val = load i32, i32* %prefix_ptr
>>>>>
>>>>>   ; print that value.
>>>>>   %ret = call i32 (i8*, ...) @printf(i8* getelementptr
inbounds ([8 x
>>>>> i8], [8 x i8]* @.str2, i32 0, i32 0), i32 %prefix_val)
>>>>>
>>>>>   ; similarly let's do the same with the global x.
>>>>>   %1 = alloca i32, align 4
>>>>>   store i32 0, i32* %1, align 4
>>>>>   %2 = load i32, i32* @x, align 4
>>>>>   %3 = call i32 (i8*, ...) @printf(i8* getelementptr
inbounds ([8 x
>>>>> i8], [8 x i8]* @.str, i32 0, i32 0), i32 %2)
>>>>>   ret i32 0
>>>>> }
>>>>>
>>>>> gives the following result (expected)
>>>>>
>>>>>    $ clang test.ll
>>>>>    $ ./a.out
>>>>>    p = 123
>>>>>    x = 10
>>>>>
>>>>> however, with -dead_strip on macOS, we see the following:
>>>>>
>>>>>    $ clang test.ll -dead_strip
>>>>>    $ ./a.out
>>>>>    p = 0
>>>>>    x = 10
>>>>>
>>>>> Thus I believe we are incorrectly stripping prefix data
when linking
>>>>> with -dead_strip on macOS.
>>>>>
>>>>> As I do not have a bugzilla account, and hence cannot post
this as a
>>>>> proper bug report.
>>>>>
>>>>> Cheers,
>>>>>  Moritz
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>>>
>>> --
>>> --
>>> Peter
>>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>
> _______________________________________________
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> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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llvm dev - Mar 2017 - [BUG Report] -dead_strip, strips prefix data unconditionally on macOS

[llvm-dev] [BUG Report] -dead_strip, strips prefix data unconditionally on macOS

[llvm-dev] [BUG Report] -dead_strip, strips prefix data unconditionally on macOS

[llvm-dev] [BUG Report] -dead_strip, strips prefix data unconditionally on macOS