[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands

[Peter Collingbourne via llvm-dev]

Hi all,

I wanted to summarise some discussion on llvm-commits [0,1] as an RFC, as I
felt it demanded wider circulation.

Our support for references to absolute symbols is not very good. The symbol
will be resolved accurately in non-PIC code, but suboptimally: the symbol
reference cannot currently appear as the immediate operand of an
instruction, and the code generator cannot make any assumptions about the
value of the symbol (so for example, it could not use a R_X86_64_8
relocation if the value is known to be in the range 0..255).

In PIC mode, if the reference is not known to be DSO-local, the value is
loaded from the GOT (or a synthetic GOT entry), which again means
suboptimal code. If the reference is known to be DSO-local, the symbol will
be referenced with a PC relative relocation and therefore cannot be
resolved properly to an absolute value (c.f. https://reviews.llvm.org/D19844).
The latter case in particular would seem to indicate that a
representational change is required for correctness to distinguish
references to absolute symbols from references to regular symbols.

The specific change I have in mind is to allow !range metadata on
GlobalObjects. This would
be similar to existing !range metadata, but it would apply to the "address"
of the attached GlobalObject, rather than any value loaded from it. Its
presence on a GlobalObject would also imply that the address of the
GlobalObject is "fixed" at link time. Alongside !range we could potentially
use other sources of information, such as the relocation model, code model
and visibility, to identify "fixed" globals, although that can be done
separately.

I have been experimenting with a number of approaches to representation in
SDAG, and I have found one that seems to work best, and would be the least
intrusive (unfortunately most approaches to this problem are somewhat
intrusive).

Specifically, I want to:
1) move most of the body of ConstantSDNode to a new class,
ConstantIntSDNode, which would derives from ConstantSDNode. ConstantSDNode
would act as the base class for immediates-post-static-linking. Change most
references to ConstantSDNode in C++ code to refer to ConstantIntSDNode.
However, "imm" in tblgen code would continue to match ConstantSDNode.
2) introduce a new derived class of ConstantSDNode for references to
globals with !range metadata, and teach SDAG to use this new derived class
for fixed address references

I will shortly be sending out a patch that implements 1.

Thanks,
-- 
-- 
Peter

[0]
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160926/394194.html
[1]
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20161003/394983.html
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