[llvm-dev] target-specific assembly printing

Wed Apr 26 13:27:53 PDT 2017

I have experienced the exact same problem.  The class 'MCAsmStreamer' does
almost everything we need for our custom assembler, but unfortunately this
class is privately implemented totally within 'MCAsmStreamer.cpp'.  Ideally
this class would be advertised in a header because it is so immensely
useful, and then custom assemblers such as yours and mine could simply
specialise it with adaptations of the elements that differ.

A few years ago I did separate this into a header and an implementation
file, but it proved too difficult to track the ongoing changes in the LLVM
head sources and I gave up (one of the downsides of being out-of-tree).  Now
I have a few "hacks" to the implementation of 'MCAsmStreamer' to adapt it to
my needs which goes against the purity of interface versus implementation
that I would prefer.

Writing my own pure custom streamer derived from 'MCStreamer' was massive
overkill when the existing 'MCAsmStreamer' already did 98% of what I needed,
and simply not worth the trouble versus using the hacks when tracking
ongoing LLVM developments was also required.  And because 'MCAsmStreamer' is
in an anonymous namespace in another file, I can't even delegate to it.

Incrementally over the years, we have been altering our native assembler to
be more conformant to the assembly language syntax supported by
'MCAsmStreamer', but this always causes pain to our customers who have to
alter their hand-written assembly files and inline assembly to match the
breaking changes we make.  But these changes have reduced our divergence
from the 'MCAsmStreamer' expectation from about 20 differences to just 5.

Now we have hacked implementations of 'EmitAssignment', 'EmitBytes',
'emitFill' (inconsistent naming), 'EmitValueToAlignment' (for exactly your
point #4 - an unexpected breaking change made between LLVM v3.8 and v3.9)
and 'EmitDwarfLocDirective'.  In each case these are virtual functions, and
a simple override rather than a hack would have been sufficient.

I would be very much in favour of promoting this very useful private class
to 1st class citizen within the LLVM infrastructure, it would resolve the
need for me to make target specific changes to the core implementation, and
would also in great likelihood allow you and others to specialise just those
elements that differ for your particular assembler dialect.

The class 'MCAsmInfo' could also do with some enhancements, a very trivial
one that was relevant to me a while ago, was the assumption that indentation
should use a TAB rather than spaces (this impacted the assembly printer).  I
had to hack/adapt this too, though an extension to 'MCAsmInfo' to allow the
indentation separator to be provided by the target would be way better,
especially for VLIW targets such as ours.

            MartinO

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Davis,
Alan via llvm-dev
Sent: 26 April 2017 19:44
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] target-specific assembly printing

I'm trying to retarget LLVM to a TI processor. We plan to use our existing
assembler, which is not gas-based. I'm finding a lot of inconsistency in the
retargeting hooks available under the AsmPrinter interface. I'll cite four
examples:

1. The TI assembler's global directive is called '.global' rather than
'.globl'. This one's easy: there is a getGlobalDriective() interface in
MCAsmInfo.

2. The TI assembler's common directive is called '.common' rather than
.comm'. Oops, that's hardwired into MCAsmStreamer.

3. The TI assembler's common directive does not imply global linkage; a
separate .global is required. I was able to hack that by overriding
AsmPrinter::EmitGlobalVariable().

4. The TI assembler's align directive is called '.align' rather than
'.p2align' and its argument is absolute bytes, not a power of two. Oops,
that's also hardwired in.

Those are a few of the things I've encountered so far; I'm sure there are
dozens more. I'm wondering how to handle these types of things in general. I
could:

A) Keep extending MCAsmInfo for anything I need. This seems like it agrees
with the intent of that interface, but I'm concerned it could be quite a few
additions . I'm curious why it's implemented as a bunch of scalar properties
with getters rather than a general API. 

B) Override much of the default ASMPrinter in the target version. It seems
like this could end up duplicating much of what is in the base class. Also
much of the implementation is (currently) private, so the target version
cannot access it. Of course, there is 'protected'.

C) Create a new implementation of the MCStreamer interface, similar to
MCAsmStreamer, that would support the TI assembler. This seems overkill, as
the formats are not that different. It also seems like it defeats the
purpose of the MCAsmInfo interface.

There are no plans to upstream any of this in the near future, but it could
happen someday, so I'm looking for the most agreeable way forward. Thoughts?

-Alan

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170426/26ab9a62/attachment.html>