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james woodyatt

2008-May-06 16:46 UTC

[LLVMdev] ARM TargetLowering broken?

everyone--

My build of LLVM-GCC from top-of-trunk sources is failing with the  
assertion in ARMISelTargetLowering at line 667, in LowerRET, where it  
says "Do not know how to return this many arguments!"  This happens  
when the cross-compiler attempts to build _muldc3 for libgcc2.

The more I have investigated this problem, them more convinced I have  
become that this isn't a stupid newbie bug, but something more  
troubling in the integration of ARM code generation with LLVM-GCC.

For reasons I haven't been able to understand yet, there are nine  
operands to the ISD::RET node when this happens, which has me a little  
confused because _muldc3 is not really a very complex function (please  
pardon the pun).  It has one return point, where it returns a CTYPE  
value, which I'm expecting is a represented internally as a pair of  
floating point numbers.

What's the more likely explanation for the error I'm seeing?

+ The ARM TargetLowering code for ISD::RET is busted for GCC's complex  
number types.

+ The LLVM backed in GCC is busted for complex types.

I don't know, but I suspect the former.  I'm still grinding on this  
problem in my copious spare time, and I'll be grateful for any tips  
from LLVM experts that come my way.  My next step: try to coax my LLVM- 
GCC variant to emit the LLVM IR for what it's doing when it compiles  
_muldc3 in libgcc2.

If you've read this far, then thank you for your attention.


-- 
j h woodyatt <jhw at conjury.org>

james woodyatt

2008-May-07 01:26 UTC

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[LLVMdev] ARM TargetLowering broken?

<html><body class="ApplePlainTextBody" style="word-wrap:
break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;
">On May 6, 2008, at 09:46, james woodyatt
wrote:<br><blockquote
type="cite"><br></blockquote><blockquote
type="cite">My build of LLVM-GCC from top-of-trunk sources is
failing with the<br></blockquote><blockquote
type="cite">assertion in ARMISelTargetLowering at line 667, in
LowerRET, where it<br></blockquote><blockquote
type="cite">says "Do not know how to return this many
arguments!"  This happens<br></blockquote><blockquote
type="cite">when the cross-compiler attempts to build _muldc3 for
libgcc2.<br></blockquote><br>The good news (for me) is
that...<br><br><span class="Apple-tab-span"
style="white-space:pre"> </span>#define ARM_ABI_DEFAULT
ARM_ABI_AAPCS<br><br>...makes this problem go
away.<br><br>Why?  Because, without it, the LLVM-GCC code generator
seems to want to bitcast the CDType complex number structure into an i128 return
value, and the ARM target in LLVM doesn't like that at all.  It turns out
that neither do most of the other targets.  The only target that doesn't
seem to complain about this is the x86-64 target.  Too bad I don't have
x86-64 hardware to play with...<br><br>In any case, it looks like
either A) the ARM backend, as well as the MIPS and PowerPC backends, could use
to be improved to support lowering i128 return values in the ISD::RET nodes into
instructions that makes sense, or B) the LLVM-GCC code generator ought not to be
generating returns for data types that the underlying target architecture
doesn't like.  I'd try to tackle the former approach, but I'd
probably make a hash of it because I'm a total noob.  I'm not sure I
think the latter approach is the right choice.  So, I'm just going to drop
this problem and move on to the next interesting
thing.<br><br>There's nothing about any of this in
$LLVM/lib/Target/ARM/README.txt, and that made me a little sad.  I would have
liked to see something in there about it, and I would write something there
myself, except I can't.  This kvetch here will have to
do.<br><br><blockquote type="cite">If you've
read this far, then thank you for your
attention.<br></blockquote><br>I now have a toolchain building
for my crazy new ARM target that doesn't really exist yet.  If you have
questions about anything above, please ask on the
list.<br><br><br>—<br>j h woodyatt
<jhw@conjury.org><br>http://jhw.vox.com/<br><br></body></html>

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