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

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

OK, I get it. Adding "-pie" link option can force 64bits relocation
address (e.g. EM_X86_64), instead of the 32bits one (e.g. R_X86_64_32S). I only
used -fpic and -fpie in clang LTO compile option, and forgot add the
"-pie" link option in ld. So my clang + ld LTO executable was still
not position independent.

Thank you all!

Steven Shi
Intel\SSG\STO\UEFI Firmware

Tel: +86 021-61166522
iNet: 821-6522
> -----Original Message-----
> From: Rafael Espíndola [mailto:rafael.espindola at gmail.com]
> Sent: Tuesday, May 31, 2016 9:21 PM
> To: Shi, Steven <steven.shi at intel.com>
> Cc: Mehdi Amini <mehdi.amini at apple.com>; Umesh Kalappa
> <umesh.kalappa0 at gmail.com>; eliben at gmail.com; llvm-dev
<llvm-
> dev at lists.llvm.org>; cfe-dev at lists.llvm.org; afish at apple.com
> Subject: Re: [cfe-dev] [llvm-dev] How to debug if LTO generate wrong code?
> 
> 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

Hi Rafael,
I finally enable the clang LTO build with small code model and  PIE, and my
clang LTO Uefi firmware works now. Thank you! But I have one more issue on the
clang normal build (without LTO) now. I find the small code model +
"-fpie" build option will let clang generate some R_X86_64_GOTPCREL
type relocation entries in my firmware image, which not happen in the clang LTO
build. I wish I could enforce the clang normal build not to use the
R_X86_64_GOTPCREL relocation type but to use R_X86_64_PC32 or R_X86_64_PLT32
instead. How could I do it?


Steven Shi
Intel\SSG\STO\UEFI Firmware

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

> -----Original Message-----
> From: Shi, Steven
> Sent: Tuesday, May 31, 2016 10:24 PM
> To: Rafael Espíndola <rafael.espindola at gmail.com>
> Cc: Mehdi Amini <mehdi.amini at apple.com>; Umesh Kalappa
> <umesh.kalappa0 at gmail.com>; eliben at gmail.com; llvm-dev
<llvm-
> dev at lists.llvm.org>; cfe-dev at lists.llvm.org; afish at apple.com
> Subject: RE: [cfe-dev] [llvm-dev] How to debug if LTO generate wrong code?
> 
> OK, I get it. Adding "-pie" link option can force 64bits
relocation address (e.g.
> EM_X86_64), instead of the 32bits one (e.g. R_X86_64_32S). I only used
-fpic
> and -fpie in clang LTO compile option, and forgot add the "-pie"
link option in
> ld. So my clang + ld LTO executable was still not position independent.
> 
> Thank you all!
> 
> Steven Shi
> Intel\SSG\STO\UEFI Firmware
> 
> Tel: +86 021-61166522
> iNet: 821-6522
> 
> > -----Original Message-----
> > From: Rafael Espíndola [mailto:rafael.espindola at gmail.com]
> > Sent: Tuesday, May 31, 2016 9:21 PM
> > To: Shi, Steven <steven.shi at intel.com>
> > Cc: Mehdi Amini <mehdi.amini at apple.com>; Umesh Kalappa
> > <umesh.kalappa0 at gmail.com>; eliben at gmail.com; llvm-dev
<llvm-
> > dev at lists.llvm.org>; cfe-dev at lists.llvm.org; afish at
apple.com
> > Subject: Re: [cfe-dev] [llvm-dev] How to debug if LTO generate wrong
code?
> >
> > 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