Hi All,
I am writing a backend for an architecture that has only 16-bit word addressing (No byte addresses ever. All data are always 16-bit).
How can I specify this in the backend?
As an example, consider the following instruction:
%arrayidx = getelementptr [129 x i16]* @flags, i16 0, i16 %i.043
When I generate assembler code, this now results in %i.043 being multiplied by 2 in the address calculation which result in a shift being emitted.
How can I avoid this?
Any help would be greatly appreciated.
Thanks in advance,
Jacques Van Damme.
I am writing a backend for an architecture that has only 16-bit word
addressing (No byte addresses ever. All data are always 16-bit).
How can I specify this in the backend?
In short, you can't. Word-addressable memory is not currently
supported in LLVM (or Clang, for that matter).
As an example, consider the following instruction:
%arrayidx = getelementptr [129 x i16]* @flags, i16 0, i16 %i.043
When I generate assembler code, this now results in %i.043 being multiplied
by 2 in the address calculation which result in a shift being emitted.
How can I avoid this?
You'll need to modify LLVM (and Clang, if that's what you are using as
your front end).
If you are interested, I can send you a patch of the changes that I
made to the 2.8 release for a backend that targets a 24-bit
word-addressable DSP, but it is quite rough and it includes changes in
which you probably aren't interested (support for non-power-of-2
integer sizes and some other bug fixes).
FWIW, I'm working (albeit at a glacial pace) on improving support for
word-addressable memory in Clang, with the plan of eventually working
my way down to LLVM. But I expect it will be a while until it is ready
for production use.
-Ken
Hi Ken,
Thanks for the quick reply.
Since I always emit assembly code, and I own the assembler, I will look into solving the problem there.
Thanks anyway,
Jacques.
I would be interested in non-power-of-two support. I started adding the very basics for i24 in my STM8 repo but there are still a couple places that do getIntegerWidth() * 2 calculations for expansion.
Do you have a public repo and/or plans to merge those features into trunk?
Andreas
Sorry, no public repository.
I hope to someday merge the changes to the trunk, but I can't promise
any definite timeline or that they will retain any semblance of their
current form. Until then, you can find the patch (zipped) attached.
-Ken
non-po2.word-addressable.llvm-clang.patch.zip (45.4 KB)