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 <ard.biesheu...@linaro.org> Reviewed-by: Leif Lindholm <leif.lindh...@linaro.org> > --- > 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 edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel
