On Fri, May 13, 2016 at 6:51 PM, Uros Bizjak <ubiz...@gmail.com> wrote:
> On Fri, May 13, 2016 at 9:07 AM, Uros Bizjak <ubiz...@gmail.com> wrote:
>> On Fri, May 13, 2016 at 1:20 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>
>>>>> testsuite/gcc.target/i386/pr61599-{1,2}.c testcases expose a failure
>>>>> with -mcmodel -fpic, where:
>>>>>
>>>>> /tmp/ccfpoxHY.o: In function `bar':
>>>>> pr61599-2.c:(.text+0xe): relocation truncated to fit: R_X86_64_PC32
>>>>> against symbol `a' defined in LARGE_COMMON section in /tmp/ccKTKST2.o
>>>>> collect2: error: ld returned 1 exit status
>>>>> compiler exited with status 1
>>>>>
>>>>> CM_MEDIUM_PIC code model assumes that code+got/plt fits in a 31 bit
>>>>> region, data is unlimited. Based on these assumptions, code should be
>>>>> accessed via R_X86_64_GOT64.
>>>>>
>>>>> Attached patch uses UNSPEC_GOT instead of UNSPEC_GOTPCREL also for
>>>>> CM_MEDIUM_PIC.
>>>>>
>>>>> 2016-05-12 Uros Bizjak <ubiz...@gmail.com>
>>>>>
>>>>> PR target/61599
>>>>> * config/i386/i386.c (legitimize_pic_address): Do not use
>>>>> UNSPEC_GOTPCREL for CM_MEDIUM_PIC code model.
>>>>>
>>>>> Patch was bootstrapped on x86_64-linux-gnu and regression tested with
>>>>> -mcmodel=medium -fpic.
>>>>>
>>>>> Jakub, H.J., do you have any comments on the patch?
>>>>
>>>>
>>>> I prefer this patch.
>
>> Yes, your patch is more precise.
>
> OTOH, are we sure there is no linker bug here? We have:
>
> $ objdump -dr pr61599-1a.o
>
> pr61599-1a.o: file format elf64-x86-64
>
>
> Disassembly of section .text:
>
> 0000000000000000 <main>:
> 0: 55 push %rbp
> 1: 48 89 e5 mov %rsp,%rbp
> 4: 48 8d 05 00 00 00 00 lea 0x0(%rip),%rax # b <main+0xb>
> 7: R_X86_64_GOTPC32 _GLOBAL_OFFSET_TABLE_-0x4
> b: b8 00 00 00 00 mov $0x0,%eax
> 10: e8 00 00 00 00 callq 15 <main+0x15>
> 11: R_X86_64_PLT32 bar-0x4
> 15: 89 c2 mov %eax,%edx
> 17: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # 1e <main+0x1e>
> 1a: R_X86_64_GOTPCREL c-0x4
> 1e: 0f b6 80 e1 00 00 00 movzbl 0xe1(%rax),%eax
> 25: 0f be c0 movsbl %al,%eax
> 28: 01 d0 add %edx,%eax
> 2a: 5d pop %rbp
> 2b: c3 retq
>
> $ objdump -dr pr61599-2a.o
>
> pr61599-2a.o: file format elf64-x86-64
>
>
> Disassembly of section .text:
>
> 0000000000000000 <bar>:
> 0: 55 push %rbp
> 1: 48 89 e5 mov %rsp,%rbp
> 4: 48 8d 05 00 00 00 00 lea 0x0(%rip),%rax # b <bar+0xb>
> 7: R_X86_64_GOTPC32 _GLOBAL_OFFSET_TABLE_-0x4
> b: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # 12 <bar+0x12>
> e: R_X86_64_GOTPCREL a-0x4
> 12: 0f b6 40 02 movzbl 0x2(%rax),%eax
> 16: 0f be d0 movsbl %al,%edx
> 19: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # 20 <bar+0x20>
> 1c: R_X86_64_GOTPCREL b-0x4
> 20: 0f b6 40 10 movzbl 0x10(%rax),%eax
> 24: 0f be c0 movsbl %al,%eax
> 27: 01 c2 add %eax,%edx
> 29: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # 30 <bar+0x30>
> 2c: R_X86_64_GOTPCREL c-0x4
> 30: 0f b6 80 00 01 00 00 movzbl 0x100(%rax),%eax
> 37: 0f be c0 movsbl %al,%eax
> 3a: 01 d0 add %edx,%eax
> 3c: 5d pop %rbp
> 3d: c3 retq
>
> $ gcc pr61599-2a.o pr61599-1a.o
> pr61599-2a.o: In function `bar':
> pr61599-2a.c:(.text+0xe): relocation truncated to fit: R_X86_64_PC32
> against symbol `a' defined in LARGE_COMMON section in pr61599-1a.o
> collect2: error: ld returned 1 exit status
>
> There is no R_X86_64_PC32 in pr61599-2a.o, only X86_64_GOTPCREL, which
> according to assumptions, would reach all entries in the GOT table.
It is a bug in GNU linker:
$ gcc -fuse-ld=gold pr61599-2a.o pr61599-1a.o
$ ./a.out
Uros.
