[llvm-dev] LLVM Backend for a platform with no (normal) stack

JD Jones via llvm-dev
Fri, 14 Dec 2018 20:09:27 -0000

Thanks, no malloc or free equivalents either (no heap).



So, there are no others (to your knowledge) who have built an LLVM backend for a
platform with no “normal” stack?  I found a presentation about some FPGA work
(using LLVM) but it doesn’t seem to apply to my platform. Perhaps someone else
on the mailing list will have come across this rarity?



Thank you again for your time and responses. If I have inadvertently been rude,
please forgive someone new to LLVM? And if your forgiveness stretches that far,
perhaps you could clue me on just how I was rude so that I can avoid it in the
future?



More thanks,

JD Jones



From: Bruce Hoult [mailto:brucehoult at sifive.com]
Sent: Friday, December 14, 2018 1:29 PM
To: jjones at prc-hsv.com
Cc: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] LLVM Backend for a platform with no (normal) stack



Having your function prologue call malloc() and epilogue call free() (or similar
functions) instead of bumping a stack pointer is not a problem. That stuff is
generated explicitly by ISA-specific code anyway.



On Fri, Dec 14, 2018 at 9:05 AM JD Jones <jjones at prc-hsv.com
<mailto:jjones at prc-hsv.com> > wrote:

Thanks for your response.  Please see below.



From: Bruce Hoult [mailto:brucehoult at sifive.com <mailto:brucehoult at
sifive.com> ]
Sent: Thursday, December 13, 2018 5:58 PM
To: jjones at prc-hsv.com <mailto:jjones at prc-hsv.com>
Cc: LLVM Developers Mailing List <llvm-dev at lists.llvm.org
<mailto:llvm-dev at lists.llvm.org> >
Subject: Re: [llvm-dev] LLVM Backend for a platform with no (normal) stack



Do you have a register that you can store a memory address

>> yes



in and an addressing mode that allows you to add (or subtract) a constant to
that register

>> Sometimes



and use the address calculated to load//store from memory? Do you have enough
registers that you can keep one of them permanently pointed to a particular
memory region?

>> No



The platform intentionally does not allow use of some large chunk of memory for
shared use by function calls.  I can allocate memory (so long as I know the
necessary size before-hand). I can work around this issue, but if someone has
already addressed it, I’d like to learn from their experiences before rolling my
own.



If yes, then you have a stack to exactly the same extent as RISC-V or MIPS do.



On Thu, Dec 13, 2018 at 12:53 PM JD Jones via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > wrote:

Dear Sir or Ma’am;



I have found a wealth of help and information on writing an LLVM backend. And,
my platform has no stack.



Can you point me to any information that would specifically address creating a
backend for this kind of platform?



In previous wanderings, I thought I ran across a phrase “platforms with no stack
such as FPGAs”, but I can’t find that mention, now.



More thanks than I can type,

JD Jones

Software Engineer


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