[llvm-dev] Difference when compiling human readable IR vs bitcode with clang frontend

[Dennis Frett via llvm-dev]

Going from IR <-> BC does not seem to create a difference.
IR -> BC -> IR -> BC, either with clang frontent or by using llvm-as and llvm-dis yields identical bc files.

I have only been able to reproduce this issue when emitting to an object file.


> On 18 Jan 2019, at 18:09, <paul.robinson at sony.com> <paul.robinson at sony.com> wrote:
> 
> 
> 
>> -----Original Message-----
>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
>> Dennis Frett via llvm-dev
>> Sent: Friday, January 18, 2019 4:15 AM
>> To: llvm-dev at lists.llvm.org
>> Subject: [llvm-dev] Difference when compiling human readable IR vs bitcode
>> with clang frontend
>> 
>> We've noticed a difference in the embedded bitcode when compiling human
>> readable IR to an object directly vs first compiling IR to BC and then an
>> object through clang -cc1.
>> If the original IR file contained an "llvm.compiler.used" gv, it will be
>> preserved when compiling IR -> BC -> Obj.
>> When compiling IR -> Obj directly, it will be removed.
>> 
>> This difference does not exist for the "llvm.used" gv however, it is
>> always preserved.
>> This questions seems related to the following lit test in LLVM:
>> https://github.com/llvm-
>> mirror/llvm/blob/master/test/Transforms/GlobalOpt/compiler-used.ll.
>> 
>> Is this somehow expected behaviour?
> 
> I am curious what happens if you do IR -> BC -> IR -> BC; I'd expect
> the IR to more-or-less match (the differences all being due to one
> is hand-written and one is a disassembly) and the two BC files should
> be identical.
> 
> If not (and I'm am guessing they aren't, which is why you see some
> differences in the compiled object file) that's a bug.
> --paulr
> 
>> 
>> Reproduce:
>> Source taken from the lit test.
>> 
>> define void @foo() {
>>  ret void
>> }
>> 
>> @llvm.used = appending global [1 x i8*] [i8* bitcast (void ()* @foo to
>> i8*)], section "llvm.metadata"
>> @llvm.compiler.used = appending global [1 x i8*] [i8* bitcast (void ()*
>> @foo to i8*)], section "llvm.metadata"
>> 
>> 
>> # Compile IR -> Obj directly.
>> clang -cc1 -triple x86_64-apple-macosx10.13.0 -emit-obj -fembed-
>> bitcode=all -x ir test.ll -o test_ll.o
>> 
>> # Compile IR -> BC -> Obj.
>> clang -cc1 -triple x86_64-apple-macosx10.13.0 -emit-llvm-bc -fblocks -
>> fencode-extended-block-signature -x ir test.ll  -o test.bc
>> clang -cc1 -triple x86_64-apple-macosx10.13.0 -emit-obj -fembed-
>> bitcode=all -x ir test.bc -o test_bc.o
>> 
>> # Extract and disassemble embedded bitcode from both scenarios.
>> segedit test_bc.o -extract __LLVM __bitcode bc_bc.bc
>> segedit test_ll.o -extract __LLVM __bitcode ll_bc.bc
>> llvm-dis bc_bc.bc
>> llvm-dis ll_bc.bc
>> 
>> # Diff both IR files to show that only bc_bc.ll contains
>> "llvm.compiler.used"
>> diff bc_bc.ll ll_bc.ll
>> 
>> - Dennis Frett

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