> On 25 Aug 2021, at 18:56, H.J. Lu via Gcc-patches <gcc-patches@gcc.gnu.org>
> wrote:
>
> On Wed, Aug 25, 2021 at 10:51 AM H.J. Lu <hjl.to...@gmail.com> wrote:
>>
>> On Wed, Aug 25, 2021 at 10:42 AM Iain Sandoe <idsan...@googlemail.com> wrote:
>>>
>>> Hi,
>>>
>>>> On 20 Aug 2021, at 11:29, Richard Sandiford <richard.sandif...@arm.com>
>>>> wrote:
>>>>
>>>>>> Maybe it would be easier to have the makefile fragments determine
>>>>>> something like CODE_MODEL_CFLAGS, which can be "-fPIC",
>>>>>> "-mdynamic-no-pic",
>>>>>> etc., and use:
>>>>>>
>>>>>> COMPILER += $(NO_PIE_CFLAGS) $(CODE_MODEL_CFLAGS)
>>>>>
>>>>> OK. I have misgivings about this - the problem is that:
>>>>>
>>>>> -fPIC -fno-PIE != -fno-PIE -fPIC, which is not obvious to many folks -
>>>>> who expect that
>>>>> the “last edition of a flag will be the one in force”.
>>>>>
>>>>> So the PIE-ness and the PIC-ness are decoupled in the configury but they
>>>>> need to be
>>>>> ordered specifically for targets that want PIC code by default (FWIW, I
>>>>> don’t think Darwin
>>>>> is the only default-PIC case here, from discussions on irc).
>>>>
>>>> Yeah, that's what the above was supposed to achieve. In other words,
>>>> if you force non-PIE, you also need to follow that by $(CODE_MODEL_CFLAGS),
>>>> which restates whatever the base code model is.
>>>>
>>>> If it's the decoupling you're worried about, then an alternative would
>>>> be to have:
>>>>
>>>> NO_PIE_CFLAGS="-fno-PIE \$(CODE_MODEL_CFLAGS)”
>>>
>>> I’d like to ask a couple of questions (of HJ who introduced the no-PIE
>>> logic) before implementing this.
>>>
>>> A. We use no-PIE for cc1* because that is needed to handle the PCH
>>> implementation (which relies on the executables being loaded at the same
>>> addresses each time).
>>>
>>> B. It’s certainly not obvious to me why we need to build code to run on
>>> $build to be no-PIE - I don’t see any such dependencies in the generators
>>> etc.
>>>
>>> - So Question1 - HJ what was the motivation for making the XXX_BUILD_XXX
>>> adopt no-PIE?
>>
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71934
>>
>>> ——
>>>
>>> Independently of this we seem to be building the objects for $host thus:
>>>
>>> $(CXX) (NO_PIE_CFLAGS) -c $(ALL_CXXFLAGS) etc.
>>>
>>> but we build for $build thus:
>>>
>>> $(CXX) -c $(ALL_CXXFLAGS) $(GENERATOR_CFLAGS) -DGENERATOR_FILE
>>> $(BUILD_NO_PIE_CFLAGS) $(BUILD_CPPFLAGS)
>>>
>>> which means that code model flags in $ALL_CXXFLAGS are overridden for
>>> $build, but active for $host
>>> ^^ this is actually what causes the Darwin build fail - since on Darwin we
>>> cannot build static linked code for user-space processes.
>>>
>>> in any event that’s inconsistent (unless there’s a reason that it should be
>>> different).
>>>
>>> ----
>>>
>>> below are extracts from gcc/Makefile *on linux* which demonstrates the
>>> different ordering.
>>>
>>> AFAICT,
>>> NO_PIE_CFLAGS_FOR_BUILD, NO_PIE_FLAG_FOR_BUILD are dead variables?
^^ what was the intention for these?
>>> Question 2 : HJ, what was your intention for how a configuration would
>>> request PIC code (for example) for things to run on $build?
>
> We need to fix
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71934
The need for no-PIE for the exectuables that consume PCH is completely clear
(but a target can choose not to use PCH - and configure —disable-libstdcxx-pch).
The PR doesn’t explain:
1. why it’s being enabled for the generators (and other $build code) which do
not use PCH (AFAIK)
2. why the flags ordering is different for $build and $host.
I am completely happy to make the fix Richard suggested - but we seem to be
adding complexity rather than simplifying things; as noted in the PR there are
targets that cannot use no-PIE and therefore have to disable PCH anyway.
thanks
Iain
>
> first.
>
> H.J.
>>> thanks
>>> Iain
>>>
>>> -------
>>>
>>>
>>> ALL_CXXFLAGS = $(T_CFLAGS) $(CFLAGS-$@) $(CXXFLAGS) $(INTERNAL_CFLAGS) \
>>> $(COVERAGE_FLAGS) $(ALIASING_FLAGS) $(NOEXCEPTION_FLAGS) \
>>>
>>> --
>>> # Native compiler for the build machine and its switches.
>>> CC_FOR_BUILD = $(CC)
>>> CXX_FOR_BUILD = $(CXX)
>>> NO_PIE_CFLAGS_FOR_BUILD =
>>> NO_PIE_FLAG_FOR_BUILD =
>>> BUILD_CFLAGS= $(ALL_CFLAGS) $(GENERATOR_CFLAGS) -DGENERATOR_FILE
>>> BUILD_CXXFLAGS = $(ALL_CXXFLAGS) $(GENERATOR_CFLAGS) -DGENERATOR_FILE
>>> BUILD_NO_PIE_CFLAGS = $(NO_PIE_CFLAGS)
>>> BUILD_CFLAGS += $(BUILD_NO_PIE_CFLAGS)
>>> BUILD_CXXFLAGS += $(BUILD_NO_PIE_CFLAGS)
>>>
>>> # Native compiler that we use. This may be C++ some day.
>>> COMPILER_FOR_BUILD = $(CXX_FOR_BUILD)
>>> BUILD_COMPILERFLAGS = $(BUILD_CXXFLAGS)
>>>
>>> # Native linker that we use.
>>> LINKER_FOR_BUILD = $(CXX_FOR_BUILD)
>>> BUILD_LINKERFLAGS = $(BUILD_CXXFLAGS)
>>>
>>> # Native linker and preprocessor flags. For x-fragment overrides.
>>> BUILD_LDFLAGS=$(LDFLAGS)
>>> BUILD_NO_PIE_FLAG = $(NO_PIE_FLAG)
>>> BUILD_LDFLAGS += $(BUILD_NO_PIE_FLAG)
>>> BUILD_CPPFLAGS= -I. -I$(@D) -I$(srcdir) -I$(srcdir)/$(@D) \
>>> -I$(srcdir)/../include $(CPPINC) $(CPPFLAGS)
>>> --
>>> # This is the variable actually used when we compile. If you change this,
>>> # you probably want to update BUILD_CFLAGS in configure.ac
>>> ALL_CFLAGS = $(T_CFLAGS) $(CFLAGS-$@) \
>>>
>>> build/%.o : # dependencies provided by explicit rule later
>>> $(COMPILER_FOR_BUILD) -c $(BUILD_COMPILERFLAGS) $(BUILD_CPPFLAGS) \
>>> -o $@ $<
>>>
>>> ^^^^ this has
>>> $(CXX) -c $(ALL_CXXFLAGS) $(GENERATOR_CFLAGS) -DGENERATOR_FILE
>>> $(BUILD_NO_PIE_CFLAGS) $(BUILD_CPPFLAGS)
>>> NO_PIE_CFLAGS_FOR_BUILD is apparently ignored
>>>
>>> # Rule for the generator programs:
>>> $(genprog:%=build/gen%$(build_exeext)): build/gen%$(build_exeext):
>>> build/gen%.o $(BUILD_LIBDEPS)
>>> +$(LINKER_FOR_BUILD) $(BUILD_LINKERFLAGS) $(BUILD_LDFLAGS) -o $@ \
>>> $(filter-out $(BUILD_LIBDEPS), $^) $(BUILD_LIBS)
>>>
>>> --
>>> build/genversion$(build_exeext): build/genversion.o
>>> +$(LINKER_FOR_BUILD) $(BUILD_LINKERFLAGS) $(BUILD_LDFLAGS) \
>>> build/genversion.o -o $@
>>> =========
>>>
>>> # The name of the compiler to use.
>>> COMPILER = $(CXX)
>>> COMPILER_FLAGS = $(CXXFLAGS)
>>> # If HOST_LIBS is set, then the user is controlling the libraries to
>>> --
>>> CET_HOST_FLAGS = -fcf-protection
>>> COMPILER += $(CET_HOST_FLAGS)
>>>
>>> --
>>> # We don't want to compile the compilers with -fPIE, it make PCH fail.
>>> COMPILER += $(NO_PIE_CFLAGS)
>>>
>>> --
>>> # A list of all the language-specific executables.
>>> COMPILERS = gnat1$(exeext) cc1$(exeext) cc1plus$(exeext) d21$(exeext)
>>> f951$(exeext) go1$(exeext) lto1$(exeext) cc1obj$(exeext)
>>> cc1objplus$(exeext)
>>>
>>> --
>>> # Native compiler that we use. This may be C++ some day.
>>> COMPILER_FOR_BUILD = $(CXX_FOR_BUILD)
>>> BUILD_COMPILERFLAGS = $(BUILD_CXXFLAGS)
>>>
>>> --
>>>
>>> # This is the variable to use when using $(COMPILER).
>>> ALL_COMPILERFLAGS = $(ALL_CXXFLAGS)
>>>
>>> --
>>>
>>> COMPILE.base = $(COMPILER) -c $(ALL_COMPILERFLAGS) $(ALL_CPPFLAGS) -o $@
>>>
>>> This is :
>>> $(CXX) (NO_PIE_CFLAGS) -c $(ALL_CXXFLAGS) etc.
>>>
>>> COMPILE = $(COMPILE.base) -MT $@ -MMD -MP -MF $(@D)/$(DEPDIR)/$(*F).TPo
>>> POSTCOMPILE = @mv $(@D)/$(DEPDIR)/$(*F).TPo $(@D)/$(DEPDIR)/$(*F).Po
>>>
>>> .cc.o .c.o:
>>> $(COMPILE) $<
>>> $(POSTCOMPILE)
>>>
>>
>>
>> --
>> H.J.
>
>
>
> --
> H.J.
