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

[Peter Collingbourne via llvm-dev]

On Mon, Oct 10, 2016 at 8:31 PM, Chris Lattner <clattner at apple.com> wrote:

>
> On Oct 10, 2016, at 8:12 PM, Peter Collingbourne via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> 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.
>
>
> Ok, I think I understand the use-case.
>
> 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
>
>
> ConstantSDNode is poorly named, and renaming it to ConstantIntSDNode is
> probably the right thing to do independently of the other changes.
>
> That said, I don’t understand why you’d keep ConstantSDNode around and
> introduce a new derived class of it.  This seems like something that a new
> “imm" immediate matcher would handle: it would match constants in a certain
> range, or a GlobalAddressSDNode known-to-be-small.
>

To begin with: I'm not sure that GlobalAddressSDNode is the right node to
use for these types of immediates. It seems that we have two broad classes
of globals here: those with a fixed-at-link-time address (e.g. regular
non-PIC symbols, absolute symbols) and those where the address needs to be
computed (e.g. PC-relative addresses, TLS variables). To me it seems like
the first class is much more similar to immediates than to the second
class. That suggested to me that there ought to be two separate
representations for global variables, where the former are "morally"
immediates, and the latter are not (i.e. the existing GlobalAddressSDNode).

I went over a couple of approaches for representing "moral" immediates in
my llvm-commits post. The first one seems to be more like what you're
suggesting:

> - Introduce a new opcode for absolute symbol constants. This intuitively
seemed like the least risky approach, as individual instructions could "opt
in" to the new absolute symbol references. However, this seems hard to fit
into the existing SDAG pattern matching engine, as the engine expects each
"variable" to have a specific opcode. I tried adding special support for
"either of the two constant opcodes" to the matcher, but I could not see a
good way to do it without making fundamental changes to how patterns are
matched.
>
> - Use the ISD::Constant opcode for absolute symbol constants, but
introduce a separate class for them. This also seemed problematic, as there
is a strong assumption (both in existing SDAG code and in generated code)
of a many-to-one mapping from opcodes to classes.

We can solve part of the problem with the second approach with a base class
for ISD::Constant. As I worked on that approach, I found that it did turn
out to be a good fit overall: in many cases we're already adhering to a
principle that an unrestricted immediate maps onto potentially relocatable
bytes in the output file. The X86 and ARM backends illustrate this quite
well: the X86 instruction set generally uses power-of-2 wide immediate
forms that neatly map onto instruction bytes, and ARM generally uses
compressed immediate forms (e.g. "mod_imm") which would naturally match
only real constant integers. Using that principle, we can restrict (e.g.)
ImmLeaf to constant integers (see https://reviews.llvm.org/D25355). In
cases where this mapping isn't quite right, we can use more restrictive
matchers.

I'm still a little uneasy about the second approach, and would be
interested in my first approach, but I'm not sure if it would be practical.

Thanks,
-- 
-- 
Peter
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