[llvm-dev] ThinLTO Bug ?

[Xin Tong via llvm-dev]

Hi

I have encountered a module verification failure when running 2 files
with thin-lto (I hit this bug in the link step in LTO on a really
large program and I reduced to this test case).


~/llvmfb/build-debug/bin/llvm-lto2 run -o c a.o  b.o -r a.o,c,px  -r
b.o,a,px -r b.o,b,px  -r a.o,gv -r b.o,gv
Called function is not the same type as the call!
%2 = call i1 bitcast (i1 (%struct.TA*)* @b to i1 (%0*)*)(%struct.TA* %0)
in function a
LLVM ERROR: Broken function found, compilation aborted!


My understanding why this is happening is roughly as follow. I would
like to check with people whether this is a real bug. And also would
like to help fixing it with other people’s help if it’s a real bug. I
am quite new to this part of LLVM.

1. DICompositeType “SHARED” in a.ll is ODRed with the one in b.ll at load time.
2. %struct.TA.0 (renamed from %struct.TA at load time) = type opaque
and i1 (%struct.TA.0*)* @b  in b.ll gets pulled into the destination
module when we are merging in a.ll.
3. We only found out that %struct.TA.0 maps to %struct.TA when we link
in b.ll because of the @gv global variable.
4. Then we have a problem, because declare i1 @b(%struct.TA.0*) in
a.ll should have been remapped due to what we found in #3, but was
not.  This eventually leads to the IR verifier failure.

Thanks
-Xin


Commands to reproduce with ToT LLVM:
===============================

~/llvm/build-debug/bin/opt -module-summary -o a.o a.ll
~/llvm/build-debug/bin/opt -module-summary -o b.o b.ll
~/llvm/build-debug/bin/llvm-lto2 run -o c a.o b.o -r a.o,c,px -r
b.o,a,px -r b.o,b,px -r a.o,gv -r b.o,gv



[~/thinltobug]$ cat a.ll

target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

target triple = "x86_64-unknown-linux-gnu"

%struct.TA = type {}



@gv = dso_local global %struct.TA * zeroinitializer

define i32 @c() !dbg !6 {

  bitcast %struct.TA ** @gv to i8*

  unreachable

}





!llvm.module.flags = !{!0, !1}

!llvm.dbg.cu = !{!2}

!0 = !{i32 2, !"Debug Info Version", i32 3}

!1 = !{i32 1, !"ThinLTO", i32 0}

!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3,
retainedTypes: !4)

!3 = !DIFile(filename: "f2", directory: "")

!4 = !{!5}

!5 = !DICompositeType(tag: DW_TAG_class_type, file: !3, flags:
DIFlagFwdDecl, identifier: "SHARED")

!6 = distinct !DISubprogram(unit: !2)



[~/thinltobug]$ cat b.ll

target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

target triple = "x86_64-unknown-linux-gnu"



%struct.TA = type opaque



@gv = external dso_local global %struct.TA *



define i1 @b(%struct.TA*) {

  unreachable

}



define i1 @a(%struct.TA* %a) {

  call i1 @b(%struct.TA* %a)

  unreachable

}



!llvm.module.flags = !{!0, !1}

!llvm.dbg.cu = !{!2}

!0 = !{i32 2, !"Debug Info Version", i32 3}

!1 = !{i32 1, !"ThinLTO", i32 0}

!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3,
retainedTypes: !4)

!3 = !DIFile(filename: "f1", directory: "")

!4 = !{!5, !9}

!5 = !DICompositeType(tag: DW_TAG_class_type, file: !3,
templateParams: !6, scope: !8)

!6 = !{!7}



; The reference to @b and struct.TA.0 (renamed)  that will be loaded in %a.o



!7 = !DITemplateValueParameter(value: i1 (%struct.TA*)* @b)

!8 = distinct !DISubprogram(unit: !2)



!9 = !DICompositeType(tag: DW_TAG_array_type, identifier: "SHARED", scope: !8)