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

Jonas Devlieghere via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 22 17:08:17 PST 2019

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 x8664-apple-macosx10.13.0 -emit-obj -fembed- bitcode=all -x ir test.ll -o testll.o # Compile IR -> BC -> Obj. clang -cc1 -triple x8664-apple-macosx10.13.0 -emit-llvm-bc -fblocks - fencode-extended-block-signature -x ir test.ll -o test.bc clang -cc1 -triple x8664-apple-macosx10.13.0 -emit-obj -fembed- bitcode=all -x ir test.bc -o testbc.o # Extract and disassemble embedded bitcode from both scenarios. _segedit testbc.o -extract _LLVM bitcode bcbc.bc _segedit testll.o -extract _LLVM bitcode llbc.bc llvm-dis bcbc.bc llvm-dis llbc.bc # Diff both IR files to show that only bcbc.ll contains "llvm.compiler.used" diff bcbc.ll llbc.ll - Dennis Frett

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