[llvm-dev] Representations of IR in the output of opt

Sébastien Michelland via llvm-dev llvm-dev at lists.llvm.org
Mon May 27 05:46:29 PDT 2019

Hi Eli,

Unfortunately the differences remain, I do not observe a significant 
change in the output besides the fact that it's random.

I noticed that running opt without options on the random file changes 
the order of references in the predecessors of basic blocks (sample 
below). Further invocations of opt are idempotent.

I don't know of this information is stored in the bytecode file as well.

< ; preds = %CF, %CF80, %CF78
 > ; preds = %CF80, %CF, %CF78

FWIW, the conflicting section of the bytecode file is likely not a 
permutation because the byte patterns don't match (some of the btte 
values of stress-1.bc are not present in stress-2.bc).

Thanks for your help :)
Sébastien Michelland

On 5/24/19 5:32 PM, Eli Friedman wrote:
> Are you passing -preserve-ll-uselistorder when you create the .ll files?  It's off by default because the output tends to be sort of unreadable, but it could explain some of the differences you're seeing.
> -Eli
>> -----Original Message-----
>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Sébastien
>> Michelland via llvm-dev
>> Sent: Friday, May 24, 2019 12:53 PM
>> To: llvm-dev at lists.llvm.org
>> Subject: [EXT] [llvm-dev] Representations of IR in the output of opt
>> Hi LLVM,
>> I'm currently setting up some tools to investigate the influence of the
>> order of optimization passes on the performance of compiled programs
>> -nothing exceptional here.
>> I noticed something inconvenient with opt, namely that splitting a call
>> does not always give the same output:
>> % llvm-stress > stress.ll
>> % opt -dse -verify -dce stress.ll -o stress-1.bc
>> % opt -dse stress.ll | opt -dce -o stress-2.bc
>> % diff stress-{1,2}.bc
>> Binary files stress-1.bc and stress-2.bc differ
>> The difference seems meaningful; it's ~180 bytes out of ~1400 bytes of
>> output in my random case. I can't decode it however, because
>> disassembling the bytecode produces identical text files, even with
>> annotations. (!)
>> I made sure that the sequence for [-dse -verify -dce] is the
>> concatenation of the individual sequences; this falls in place naturally
>> because -dce has no dependencies. The verifier pass helps make two
>> function pass managers, just in case.
>> Now if I do the same thing but staying in text format, I get the same IR
>> (up to module name):
>> % opt -S -dse -verify -dce stress.ll -o stress-1.ll
>> % opt -S -dse stress.ll | opt -S -dce -o stress-2.ll
>> % diff -y --suppress-common-lines stress-{1,2}.ll
>> ; ModuleID = 'stress.ll'	|	; ModuleID = '<stdin>'
>> Is there a specific behavior of opt that could explain this situation?
>> What kind of difference could there be in the bytecode files that is
>> lost in translation to text format ?
>> Cheers,
>> Sébastien Michelland
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