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

[Peter Collingbourne via llvm-dev]

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
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>> llvm-dev at lists.llvm.org
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>>
>
>
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>


-- 
-- 
Peter
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