llvm dev - May 2016 - [cfe-dev] How to debug if LTO generate wrong code?

On 30 May 2016 at 19:55, Mehdi Amini <mehdi.amini at apple.com>
wrote:
>
>
> Sent from my iPhone
>
>> On May 30, 2016, at 4:52 PM, Rafael Espíndola <rafael.espindola at
gmail.com> wrote:
>>
>>> On 30 May 2016 at 16:56, Mehdi Amini <mehdi.amini at
apple.com> wrote:
>>>
>>>
>>> On 05/30/16 01:34 PM, Rafael Espíndola <rafael.espindola at
gmail.com> wrote:
>>>
>>> We don't use cl::opt in gold, instead we parse the -plugin-opts
that
>>> gold passes the plugin (see process_plugin_option).
>>>
>>> What about that:
>>>
>>> $ grep ParseCommandLineOptions tools/gold/gold-plugin.cpp
>>>
>>>      // ParseCommandLineOptions() expects argv[0] to be program
name.
>>> Lazily
>>>
>>>    cl::ParseCommandLineOptions(NumOpts, &options::extra[0]);
>>
>>
>> That is for the options that the gold plugin itself doesn't
understand
>> and just passes to llvm. This allows you to do things like
>> --plugin-opt=-debug-pass=Arguments.
>
> This is what I expected, so my cl:opt should work, right? I don't
really get your original point?
Just that the gold plugin itself never defines a cl::opt. It just
forwards the llvm ones.

Cheers,
Rafael

Hi Mehdi,
What's the default code model for x86_64 Mac OS X App?  Andrew showed me
some example code of Mac OS X App as below, which looks to use the small code
model but can run at  >4GB high address.

For example if you read a global like this the compiler will generate this code.
int constant = 0;

int get_constant(void)
{
	return constant;
}

(lldb) dis -n get_constant -b 
a.out`get_constant:
a.out[0x100000f8c] <+0>:  55                           pushq  %rbp
a.out[0x100000f8d] <+1>:  48 89 e5                 movq   %rsp, %rbp
a.out[0x100000f90] <+4>:  8b 05 6a 00 00 00  movl   0x6a(%rip), %eax
a.out[0x100000f96] <+10>: 5d                          popq   %rbp
a.out[0x100000f97] <+11>: c3                          retq   


Steven Shi
Intel\SSG\STO\UEFI Firmware

Tel: +86 021-61166522
iNet: 821-6522

On 31 May 2016 at 01:08, Shi, Steven <steven.shi at intel.com>
wrote:
> Hi Mehdi,
> What's the default code model for x86_64 Mac OS X App?  Andrew showed
me some example code of Mac OS X App as below, which looks to use the small code
model but can run at  >4GB high address.
Small, but PIC.


> For example if you read a global like this the compiler will generate this
code.
> int constant = 0;
>
> int get_constant(void)
> {
>         return constant;
> }

Compiling for ELF with -FPIE -Os I get

get_constant:                           # @get_constant
# BB#0:                                 # %entry
        movl    constant(%rip), %eax
        retq

Which should also be able to run at any address.

Cheers,
Rafael