[llvm-bugs] [Bug 35197] New: [LTO] undefined reference to global variable that was used only in asm

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35197

            Bug ID: 35197
           Summary: [LTO] undefined reference to global variable that was
                    used only in asm
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: ilia.taraban at intel.com
                CC: llvm-bugs at lists.llvm.org

This test fails at linkage with "undefined reference" with LTO:

================= nice.c ==============
int f[2];

int
main()
{
    __asm {
        mov     f[rax], rax
    }
    return 0;
}

=======================================

>>> clang -v
clang version 6.0.0 (trunk 317339)
Target: x86_64-unknown-linux-gnu
Thread model: posix
...

>>> clang -c -O0 -fasm-blocks nice.c 
>>> clang -O1        nice.o

But fails with -flto
>>> clang -c -O0 -fasm-blocks nice.c -flto
>>> clang -O1 -flto  nice.o
/tmp/lto-llvm-61977c.o:ld-temp.o:function main: error: undefined reference to
'f'
clang-6.0: error: linker command failed with exit code 1 (use -v to see
invocation)

>>> clang -O1 -flto  nice.o  -Wl,-plugin-opt,-opt-bisect-limit=1
BISECT: running pass (1) Dead Global Elimination on module (ld-temp.o)
...
BISECT: NOT running pass (46) X86 LEA Fixup on function (main)
/tmp/lto-llvm-7d6d61.o:ld-temp.o:function main: error: undefined reference to
'f'
clang-6.0: error: linker command failed with exit code 1 (use -v to see
invocation)


If we look at nice.o, we'll see that we still have variable 'f' before linkage
stage:

================= nice.o.ll ===========
@f = common global [2 x i32] zeroinitializer, align 4

; Function Attrs: noinline nounwind optnone uwtable
define i32 @main() #0 {
entry:
  %retval = alloca i32, align 4
  store i32 0, i32* %retval, align 4
  call void asm sideeffect inteldialect "mov f[rax], rax",
"~{dirflag},~{fpsr},~{flags}"() #1, !srcloc !2
  ret i32 0
}
=======================================

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