[RFC PATCH 8/8] powerpc/64/asm: don't reassign labels

[Nicholas Piggin]

Excerpts from Daniel Axtens's message of February 26, 2021 10:28 am:
> Segher Boessenkool <segher at kernel.crashing.org> writes:
> 
>> On Thu, Feb 25, 2021 at 02:10:06PM +1100, Daniel Axtens wrote:
>>> The assembler really does not like us reassigning things to the same
>>> label:
>>> 
>>> <instantiation>:7:9: error: invalid reassignment of non-absolute variable 'fs_label'
>>> 
>>> This happens across a bunch of platforms:
>>> https://github.com/ClangBuiltLinux/linux/issues/1043
>>> https://github.com/ClangBuiltLinux/linux/issues/1008
>>> https://github.com/ClangBuiltLinux/linux/issues/920
>>> https://github.com/ClangBuiltLinux/linux/issues/1050
>>> 
>>> There is no hope of getting this fixed in LLVM, so if we want to build
>>> with LLVM_IAS, we need to hack around it ourselves.
>>> 
>>> For us the big problem comes from this:
>>> 
>>> \#define USE_FIXED_SECTION(sname)				\
>>> 	fs_label = start_##sname;				\
>>> 	fs_start = sname##_start;				\
>>> 	use_ftsec sname;
>>> 
>>> \#define USE_TEXT_SECTION()
>>> 	fs_label = start_text;					\
>>> 	fs_start = text_start;					\
>>> 	.text
>>> 
>>> and in particular fs_label.
>>
>> The "Setting Symbols" super short chapter reads:
>>
>> "A symbol can be given an arbitrary value by writing a symbol, followed
>> by an equals sign '=', followed by an expression.  This is equivalent
>> to using the '.set' directive."
>>
>> And ".set" has
>>
>> "Set the value of SYMBOL to EXPRESSION.  This changes SYMBOL's value and
>> type to conform to EXPRESSION.  If SYMBOL was flagged as external, it
>> remains flagged.
>>
>> You may '.set' a symbol many times in the same assembly provided that
>> the values given to the symbol are constants.  Values that are based on
>> expressions involving other symbols are allowed, but some targets may
>> restrict this to only being done once per assembly.  This is because
>> those targets do not set the addresses of symbols at assembly time, but
>> rather delay the assignment until a final link is performed.  This
>> allows the linker a chance to change the code in the files, changing the
>> location of, and the relative distance between, various different
>> symbols.
>>
>> If you '.set' a global symbol, the value stored in the object file is
>> the last value stored into it."
>>
>> So this really should be fixed in clang: it is basic assembler syntax.
> 
> No doubt I have explained this poorly.
> 
> LLVM does allow some things, this builds fine for example:
> 
> .set foo, 8192
> addi %r3, %r3, foo
> .set foo, 1234
> addi %r3, %r3, foo
> 
> However, this does not:
> 
> a:
> .set foo, a
> addi %r3, %r3, foo at l
> b:
> .set foo, b
> addi %r3, %r3, foo-a
> 
> clang -target ppc64le -integrated-as  foo.s -o foo.o -c
> foo.s:5:11: error: invalid reassignment of non-absolute variable 'foo' in '.set' directive
> .set foo, b
>           ^

So that does seem to be allowed by the specification.

I don't have a huge problem with the patch actually, doesn't seem too 
bad.

Thanks,
Nick