[llvm-bugs] [Bug 47819] New: Incorrect alignment of common symbol for mixed LTO + native object

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47819

            Bug ID: 47819
           Summary: Incorrect alignment of common symbol for mixed LTO +
                    native object
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: ELF
          Assignee: unassignedbugs at nondot.org
          Reporter: jh7370.2008 at my.bristol.ac.uk
                CC: llvm-bugs at lists.llvm.org, smithp352 at googlemail.com

If a bitcode file defines a common symbol with a larger size, but smaller
alignment, and a native object file defines a common symbol with a smaller
size, but larger alignment, the alignment from smaller symbol is ignored,
unlike the other possible resolution cases of bitcode + bitcode, object +
object, or bitcode + object where the object had the larger alignment.

The problem is because the compiled LTO output symbol unconditionally replaces
the symbol LLD already has internally in this case. Since the LTO output will
only have the alignment of the symbol from the bitcode, it has a smaller
alignment than the one in memory, and so the correct alignment is lost.

Example inputs:

; common.ll
target datalayout =
"e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@b = common global i64 0, align 4

# common2.s
.comm b,8,16

Result: b has size 8, and the .bss section has 4-byte alignment (it should be
size 8 and 16-byte alignment).

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