[PATCH] D35479: [CodeGen][mips] Support `long_call/far/near` attributes

[Simon Dardis via Phabricator via cfe-commits]

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