Re: [PATCH 2/4] BaseTools-Conf:Introduce CLANG38 new toolchain for x86

["Shi, Steven" <steven.shi@intel.com>]

Hello Ard,

> 
> Hello Steven,
> 
> On 3 August 2016 at 08:48, Shi, Steven <steven.shi@intel.com> wrote:
> > This adds support for LLVM 3.8.x in LTO mode for IA32 and X64.
> > CLANG38 enable LLVM Link Time Optimization (LTO) and code size
> > optimization flag (-Oz) by default for aggressive code size
> > improvement. CLANG38 X64 code is small code model + PIE.
> >
> > CLANG LTO needs PIE in link flags to generate PIE code correctly,
> > otherwise the PIE is not really enabled. (e.g. OvmfPkgX64 will
> > hang in 64bits SEC at high address because of small model code
> > displacement overflow).
> >
> 
> This is probably caused by the same issue I am addressing with the
> series I sent out yesterday, to pass the CC flags to the DLINK
> command.
> 
> The reason is that code is generated by the link pass, so it needs to
> see the same -fpie -mcmodel=small options that we passed to the
> compiler as wel.
> 
> Could you check whether replacing '-Wl,-pie' with -fpie does the trick
> as well? As I mentioned before, creating a PIE executable at link time
> is not the same as generatic position independent code at compile time
> (whether it is via $(CC) or via $(DLINK)). The PIE executable will
> contain a .rela section that partially overlaps with other absolute
> relocations, so it is best to avoid it.

[Steven]: I just tried it. No, replacing '-Wl,-pie' with -fpie cannot works for clang38. With -fpie in link flags, the OvmfPkgX64 still hang in 64bits SEC at high address.


> 
> Some more comments below
> 
> > +*_CLANG38_IA32_OBJCOPY_PATH         = DEF(GCC5_IA32_PREFIX)objcopy
> 
> Why are you using the GCC5 prefix here? A clang user may not set GCC5_BIN
> 
[Steven]: OK, I will remove the GCC5 prefix.


> > +*_CLANG38_IA32_ASLCC_FLAGS          = -x c -Os -m32
> DEF(CLANG38_IA32_TARGET) -flto
> 
> Does LTO make any sense for ACPI tables?
> 
[Steven]: OK, I will follow GCC5 to disable the LTO for ASLCC flags.


Thanks
Steven