llvm dev - Jul 2007 - [LLVMdev] PassManager Error

On Jul 26, 2007, at 8:01 AM, David Greene wrote:
> On Wednesday 25 July 2007 21:35, David A. Greene wrote:
>
>>> What is Pass and RequiredPass when this assertion hits ?
>>
>> I haven't had a chance to explore too deeply yet (the stack trace  
>> doesn't
>> give any useful information) but I suspect Pass is the register  
>> allocator
>> and RequiredPass is the coalescer.
>
> Turns out I was wrong.  Pass is the register allocator but  
> RequiredPass is
> LiveVariableAnalysis.  Any idea why this would cause a problem?
>
If in register allocator you have requested another higher level pass  
after LiveVariableAnalysis pass then this may happen.
-
Devang

On Thursday 26 July 2007 11:48, Devang Patel wrote:
> If in register allocator you have requested another higher level pass
> after LiveVariableAnalysis pass then this may happen.
Can you explain why this is so?  I would assume that I could declare Pass
dependencies in any order.  The manual certainly doesn't say anything about
restricions like this.  Shouldn't a Pass just be a Pass that can be run?

                                               -Dave

On Jul 26, 2007, at 11:25 AM, David Greene wrote:
> On Thursday 26 July 2007 11:48, Devang Patel wrote:
>
>> If in register allocator you have requested another higher level pass
>> after LiveVariableAnalysis pass then this may happen.
>
> Can you explain why this is so?  I would assume that I could  
> declare Pass
> dependencies in any order.  The manual certainly doesn't say  
> anything about
> restricions like this.  Shouldn't a Pass just be a Pass that can be  
> run?
A pass is not just a pass. It depends on the the level it operates  
on. A function level pass operates on all functions and a module  
level pass works on entire module. For example,

If pass sequence is - ModuleLevelPass A, FunctionLevelPass B,  
FunctionLevelPass C, ModuleLevelPass D then

For each Module
	run ModuleLevelPass A
	for each function in this module
		run FunctionLevelPass B
		run FunctionLevelPass C
	run ModuleLevelPass D
	

Now, if pass sequence is - ModuleLevelPass A, FunctionLevelPass B,   
ModuleLevelPass D, FunctionLevelPass C then

For each Module
	run ModuleLevelPass A
	for each function in this module
		run FunctionLevelPass B
	run ModuleLevelPass D
	for each function in this module
		run FunctionLevelPass C


In this situation, while processing 'foo' FunctionLevelPass C can not  
use analysis info generated by FunctionLevelPass B for 'foo'.

So if FunctionLevelPass C is requiring FunctionLevelPass B and  
ModuleLevelPass D in that order then you may see mentioned  
assertions. I just checked in patch to help investigate such  
failures. Add --debug-pass=Structure on the 'opt' command line to  
understand what is going on.

I welcome patches to update manual to explain this clearly.

-
Devang