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


Shi, Steven <steven.shi@...>
 

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

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