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

[Jonas Devlieghere via llvm-dev]

Hey Dennis,

Maybe I'm doing something wrong, but I cannot reproduce your issue.
Starting with your example as input.ll:

$ llvm-as input.ll -o input.bc
$ clang -fembed-bitcode=all -x ir input.ll -c -o ll.o
$ clang -fembed-bitcode=all -x ir input.bc -c -o bc.o
$ md5 ll.o
MD5 (ll.o) = 7e9bd15c4dd786a4bb4aec762d4e842e
$ md5 bc.o
MD5 (bc.o) = 7e9bd15c4dd786a4bb4aec762d4e842e

I verified with otool that there's actually an embedded bitcode section.

However, looking at the cc1 invocation, I noticed the
`-emit-llvm-uselists`, that sounds like it might have something to do with
the behavior you're seeing?

Cheers,
Jonas

On Tue, Jan 22, 2019 at 8:21 AM via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> It's good that IR <-> BC produces consistent results.  I think it's not
> good that IR->Obj and BC->Obj produce different results.
>
> The next step would be to look at the dumps from llc to see what is
> different when starting with IR versus BC. That should help track down
> where something gets lost.
>
> --paulr
>
>
>
> *From:* Dennis Frett [mailto:dennis.frett at guardsquare.com]
> *Sent:* Sunday, January 20, 2019 10:18 AM
> *To:* Robinson, Paul
> *Cc:* llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Difference when compiling human readable IR vs
> bitcode with clang frontend
>
>
>
> 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
> <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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