Reviewed-by: Liming Gao <[email protected]>
>-----Original Message----- >From: Leif Lindholm [mailto:[email protected]] >Sent: Wednesday, June 21, 2017 4:00 AM >To: Ard Biesheuvel <[email protected]> >Cc: [email protected]; Gao, Liming <[email protected]>; Zhu, >Yonghong <[email protected]>; [email protected] >Subject: Re: [PATCH 2/2] BaseTools/tools_def: AARCH64: disable LTO type >mismatch warnings > >On Tue, Jun 20, 2017 at 08:43:54PM +0200, Ard Biesheuvel wrote: >> On AARCH64, any code that may execute with the MMU off needs to be >built >> with -mstrict-align, given that unaligned accesses are not allowed unless >> the MMU is enabled. This does not only affect SEC and PEI modules, but >> also static libraries of the BASE type, which may be linked into such >> modules, as well as into modules of other types. As it turns out, the >> presence of -mstrict-align is reflected in the internal representations >> of the types defined in those libraries. >> >> When -fstrict-aliasing is passed to GCC, it assumes that pointers to >> objects of different types cannot refer to the same memory location, and >> attempts to exploit this fact when optimizing the code. Since such >> assumptions are only valid under very strict conditions which are not >> guaranteed to be met in EDK2, we disable this optimization by passing >> -fno-strict-aliasing by default. > >Just a comment - you don't necessarily have to update this excellent >commit mesage, but: >-fstrict-aliasing is GCC default, because it is a restriction in the C >language. Because it's a bit non-obvious, things can go hilariously >wrong in very non-obvious ways, and the potential optimization gains >are ulikely to be generally relevant, -fno-strict-aliasing is a >sensible thing to always have set (like we do). > >> When LTO is in effect, this applies equally to the code generation that >> may occur at link time, which is why the linker warns about unexpected >> differences in type definitions between the intermediate representations >> that are present in the object files being linked. This may result in >> warnings such as the one below, even if -fno-strict-aliasing is used: >> >> MdePkg/Include/Library/BaseLib.h:1712:1: >> warning: type of 'StrToGuid' does not match original declaration >> [-Wlto-type-mismatch] >> StrToGuid ( >> ^ >> MdePkg/Library/BaseLib/SafeString.c:1506:1: >> note: 'StrToGuid' was previously declared here >> StrToGuid ( >> ^ >> MdePkg/Library/BaseLib/SafeString.c:1506:1: >> note: code may be misoptimized unless -fno-strict-aliasing is used >> >> This warning is inadvertently triggered when linking BASE libraries built >> with -mstrict-align into modules of types other than SEC or PEI, since the >> types are subtly different, even though the use of code that maintains >> strict alignment in a module that does not care about this is unlikely to >> cause problems. And even if it did, it would still only affect code built >> with -fstrict-aliasing enabled, which we disable unconditionally. So let's >> just silence the warning by passing -Wno-lto-type-mismatch. >> >> Contributed-under: TianoCore Contribution Agreement 1.0 >> Signed-off-by: Ard Biesheuvel <[email protected]> > >Reviewed-by: Leif Lindholm <[email protected]> > >> --- >> BaseTools/Conf/tools_def.template | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/BaseTools/Conf/tools_def.template >b/BaseTools/Conf/tools_def.template >> index 7a58ce365ed2..9a3ba9defb12 100755 >> --- a/BaseTools/Conf/tools_def.template >> +++ b/BaseTools/Conf/tools_def.template >> @@ -5407,7 +5407,7 @@ RELEASE_GCC5_ARM_DLINK_FLAGS = >DEF(GCC5_ARM_DLINK_FLAGS) -flto -Os -L$(WORKS >> DEBUG_GCC5_AARCH64_DLINK_XIPFLAGS = -z common-page-size=0x20 >> >> RELEASE_GCC5_AARCH64_CC_FLAGS = DEF(GCC5_AARCH64_CC_FLAGS) - >flto -Wno-unused-but-set-variable -mcmodel=tiny -fomit-frame-pointer >> -RELEASE_GCC5_AARCH64_DLINK_FLAGS = >DEF(GCC5_AARCH64_DLINK_FLAGS) -flto -Os - >L$(WORKSPACE)/ArmPkg/Library/GccLto -llto-aarch64 -Wl,-plugin-opt=-pass- >through=-llto-aarch64 >> +RELEASE_GCC5_AARCH64_DLINK_FLAGS = >DEF(GCC5_AARCH64_DLINK_FLAGS) -flto -Os - >L$(WORKSPACE)/ArmPkg/Library/GccLto -llto-aarch64 -Wl,-plugin-opt=-pass- >through=-llto-aarch64 -Wno-lto-type-mismatch >> >> NOOPT_GCC5_AARCH64_CC_FLAGS = DEF(GCC5_AARCH64_CC_FLAGS) - >O0 -mcmodel=small >> NOOPT_GCC5_AARCH64_DLINK_FLAGS = >DEF(GCC5_AARCH64_DLINK_FLAGS) -z common-page-size=0x1000 -O0 >> -- >> 2.7.4 >> _______________________________________________ edk2-devel mailing list [email protected] https://lists.01.org/mailman/listinfo/edk2-devel
