[LLVMdev] Bug in optimization pass related to strcmp and bigendian back-ends

[Stripf, Timo]

Alright thank you all for your help and information and sry for describing it as a bug.

For a "fast" workaround I simple use llvm-gcc with -O0, modify the endian information within the ll file and use opt to optimize the code. That way also the debugging information is not removed and everything works atm fine for a non-trivial application. Later I'll also modify the front-end to support the back-end but atm I think this is easier.

Just out of curiosity, are there any plans/ideas to increase the platform independence of optimization steps or the LLVM IR? It is clear that for C/C++ front-ends it is useless but maybe it is useful for other platform independent front-ends like java. Maybe add some kind of is_big_endian function to express "is_big_endian ? 30 : 30<<8" within the IR that is replaced within the back-end.

-Timo

-----Ursprüngliche Nachricht-----
Von: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] Im Auftrag von Nick Lewycky
Gesendet: Mittwoch, 12. August 2009 04:35
An: LLVM Developers Mailing List
Betreff: Re: [LLVMdev] Bug in optimization pass related to strcmp and bigendian back-ends

Stripf, Timo wrote:
> I thought the LLVM IR is target independent

Yes.

  and that "llvm-gcc -c -emit-llvm -O2" produces target independent code.

No.

> I'm working on a back-end and use llvm-gcc to first generate the bc file. Afterwards I use llc including the new back-end to produce the assembler file.

LLVM IR contains a target-information line but is otherwise target 
independent. This does *not* mean that you can convert C to LLVM IR in a 
target independent way.

C code may contain "#ifdef __ppc__". Now what? Or how about "switch (x) 
{ case sizeof(int): ... }". This question is a FAQ: 
http://llvm.org/docs/FAQ.html#platformindependent

LLVM IR is portable in the sense that it will run the same on any 
platform. C is portable in the sense that you can detect things about 
the platform so you may correct for them. It turns out that these are 
two fundamentally incompatible paradigms.

Some LLVM optimizations take advantage of the information in the target 
info line which could change the behaviour of the program if your target 
system doesn't match the one described in the info line.

Nick

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