Excerpts from Daniel Axtens's message of February 26, 2021 10:28 am: > Segher Boessenkool <seg...@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@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
