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

Tue Mar 7 11:51:32 PST 2017

No -- that's the whole topic of discussion here.. With the appropriate automatic emission of alt_entry symbol flags in llvm, it seems that you ought to be able to get the desired behaviors *even with* the MachO object file format being what it is.

On Mar 7, 2017, at 2:29 PM, Reid Kleckner <rnk at google.com> wrote:
> 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 <mailto: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 <mailto: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 <mailto: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
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>> 
>> 
>> 
>> -- 
>> -- 
>> Peter
>> 
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