[llvm-bugs] [Bug 49090] New: Clang on Windows with "--target=x86_64-elf " seems to compile with "-ffunction-sections"

Mon Feb 8 08:04:24 PST 2021

https://bugs.llvm.org/show_bug.cgi?id=49090

            Bug ID: 49090
           Summary: Clang on Windows with "--target=x86_64-elf " seems to
                    compile with "-ffunction-sections"
           Product: clang
           Version: 11.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: rianquinn at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

For some reason, when you cross compiler for "--target=x86_64-elf" on Windows
using Clang 11, and then link using ld.lld on Windows, you get an ELF file that
appears to have "-ffunction-sections" turned on, even though I have not turned
it on. In fact, if I turn it off, nothing changes. 

Here is an example:
```
[/c/working/build/ext_cross_compile/bin]: llvm-readelf.exe -lW example_default 

Elf file type is EXEC (Executable file)
Entry point 0x328000000c80
There are 4 program headers, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz
  Flg Align
  LOAD           0x001000 0x0000328000000000 0x0000328000000000 0x007228
0x007228 R E 0x1000
  LOAD           0x009000 0x0000328000008000 0x0000328000008000 0x0004f0
0x0004f0 R   0x1000
  LOAD           0x00a000 0x0000328000009000 0x0000328000009000 0x000008
0x000010 RW  0x1000
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000
0x000000 RW  0x0

 Section to Segment mapping:
  Segment Sections...
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```

I am not sure if this is a bug or not, but it seems like something strange is
going on here. This is the toolchain file that we use to cross compile on
Windows for reference:
```
set(CMAKE_SYSTEM_NAME Generic)
set(CMAKE_CXX_COMPILER_WORKS 1)

string(CONCAT HYPERVISOR_EXT_CXX_FLAGS
    "--target=x86_64-elf "
    "-ffreestanding "
    "-mno-mmx "
    "-mno-sse "
    "-mno-sse2 "
    "-mno-sse3 "
    "-mno-ssse3 "
    "-mno-sse4.1 "
    "-mno-sse4.2 "
    "-mno-sse4 "
    "-mno-avx "
    "-mno-aes "
    "-mno-sse4a "
    "-mcmodel=large "
    "-std=c++20 "
)

string(CONCAT HYPERVISOR_EXT_LINK_FLAGS
    "-static "
    "-nostdlib "
    "-z noexecstack "
    "-T ${CMAKE_BINARY_DIR}/toolchain/x64/ext.ld "
)

set(CMAKE_ASM_COMPILE_OBJECT
    "<CMAKE_CXX_COMPILER> ${HYPERVISOR_EXT_CXX_FLAGS} <DEFINES> <INCLUDES>
<FLAGS> -o <OBJECT> -c <SOURCE>")

set(CMAKE_CXX_COMPILE_OBJECT
    "<CMAKE_CXX_COMPILER> ${HYPERVISOR_EXT_CXX_FLAGS} <DEFINES> <INCLUDES>
<FLAGS> -o <OBJECT> -c <SOURCE>")

set(CMAKE_CXX_LINK_EXECUTABLE
    "${HYPERVISOR_CXX_LINKER} ${HYPERVISOR_EXT_LINK_FLAGS}
<CMAKE_CXX_LINK_FLAGS> <LINK_FLAGS> <OBJECTS> -o <TARGET> <LINK_LIBRARIES>"
)
```

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