sdardis added inline comments.
================ Comment at: include/clang/Basic/AttrDocs.td:1336 +if code compiled using ``-mlong-calls`` switch, it forces compiler to use +the ``jal`` instruction to call the function. + }]; ---------------- rjmccall wrote: > sdardis wrote: > > rjmccall wrote: > > > I suggest the following wording: > > > > > > Clang supports the ``__attribute__((long_call))``, > > > ``__attribute__((far))``, and > > > ``__attribute__((near))`` attributes on MIPS targets. These attributes > > > may only be > > > added to function declarations and change the code generated by the > > > compiler when > > > directly calling the function. The ``near`` attribute allows calls to > > > the function to > > > be made using the ``jal`` instruction, which requires the function to be > > > defined in the > > > same 256MB segment as the caller. The ``long_call`` and ``far`` > > > attributes are > > > synonyms and require the use of a different call sequence that works > > > regardless of > > > the distance between the functions. > > > > > > These attributes take priority over command line switches such as > > > ``-mlong-calls``. > > > requires the function to be defined > > > > I'd change the "defined" to be "located". > > > > > in the same 256MB segment as the caller. > > > > I'd change this to: "in the same naturally aligned 256MB segment as the > > caller." > > > > This also needs a note saying that it has no effect for code compiled with > > -fpic. > Oh, yes, if these statements are true then it's absolutely important to > include them. I was going off of the raw ISA specification, since jal just > takes a relative offset, but if the toolchain has more specific requirements > then those are what should be documented. > > This whole feature is strange to me; do MIPS linkers just never introduce > things like branch islands and lazy-binding functions? > since jal just takes a relative offset, jal doesn't take an offset, the instruction takes an 26-bit immediate (called the instruction index) shifts it left 2 bits and combines it with the remaining upper bits of the address of the instruction in the delay slot of jal to form the new $pc--so it's possible to jump from the bottom of a 256MB segment to the top. (My apologies for the long answer here, jal is one of the odder control transfer instruction for MIPS.) > This whole feature is strange to me; do MIPS linkers just never introduce > things like branch islands and lazy-binding functions? This feature is orthogonal to lazy-binding functions. The usage of 'long-call' and 'near' is to support bare-metal / static relocation model environments where the caller and callee might be in different memory segments (e.g. KSEG0 and KSEG1) or have a custom memory layout for their program's sections. To the best of my knowledge, the GNU linker for MIPS only inserts stub functions / branch islands when compiling static code that calls PIC code. Repository: rL LLVM https://reviews.llvm.org/D35479 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
