llvm dev - Apr 2007 - [LLVMdev] Live intervals and aliasing registers problem

On Mar 27, 2007, at 3:25 PM, Evan Cheng wrote:
>
> On Mar 25, 2007, at 7:12 AM, Christopher Lamb wrote:
>
>> While beginning to add vector registers to a back end I came  
>> across the following problem: as soon as I define two sets of  
>> registers that have a many-to-one mapping the live interval pass  
>> appears to double-kill the mapped-onto register. I have the  
>> following excerpts from my RegisterInfo.td.
>>
>> def V4R0   : R4v<0 , "V4R0 ", []>,
DwarfRegNum<0>;
>>
>> def R0   : Rg<0 , "R0", [V4R0]>, DwarfRegNum<0>;
>> def R1   : Rg<1 , "R1", [V4R0]>, DwarfRegNum<1>;
>
> How are R4v and Rg defined?
class Rg<bits<6> num, string n, list<Register> aliases> :
MyReg<n>
{
   let Num = num;
   let Aliases = aliases;
}

class R4v<bits<6> num, string n, list<Register> aliases> :
MyReg<n> {
   let Num = num;
   let Aliases = aliases;
}

>>
>> when trying to compile:
>>
>> define void @_Z3fooii(i32 %a, i32 %b) {
>> entry:
>>         %retval = select i1 false, i32 %a, i32 %b               ;  
>> <i32> [#uses=0]
>>         ret void
>> }
>>
>> I get this error:
>>
>> entry (0x8503b90, LLVM BB @0x8501b00, ID#0):
>>         %reg1024 = ORI %R0, 0
>>         %reg1025 = ORI %R1, 0
>>         RETL
>> Machine Function
>> ********** REWRITING TWO-ADDR INSTRS **********
>> ********** Function: _Z3fooff
>>
>> ********** COMPUTING LIVE INTERVALS **********
>> ********** Function: _Z3fooii
>> entry:
>>                 livein register: R0 killed +[0,2:0)
>>                 livein register: V4R0  killed +[0,2:0)  <=== this  
>> is bad
>>                 livein register: R1 killed +[0,6:0)
>>                 livein register: V4R0  killed +[0,2:1)
>> lib/CodeGen/LiveInterval.cpp:189: failed assertion `B->end <=  
>> Start && "Cannot overlap two LiveRanges with differing
ValID's"
>> " (did you def the same reg twice in a MachineInstr?)"'
>
> V4R0 should not have been killed twice. The second ORI instruction  
> is the last use of V4R0. Are you adding the correct livein's (just  
> R0 and R1) to the function?
Yes, only R0 and R1 are being added.
> Can you dump out the machine basic block? It should have an  
> implicit use of V4R0 at first ORI but it should not be marked kill.  
> If it is marked kill, then you need to walk LiveVariables.cpp to  
> find out why.
Here is the beginning of the BB dump.

entry (0x8503c80, LLVM BB @0x8501af0, ID#0):
Live Ins: %R0 %R1
	%reg1024 = ORI %R0<kill>, 0
	%reg1025 = ORI %R1<kill>, 0

V4R0 is getting killed because handleLiveInRegister() is called on  
all results of getAliasSet() for each of the liveins (this is in  
LiveIntervals::computeIntervals() ).

handleRegisterDef() does a similar thing where calls  
handlePhysicalRegisterDef() on all members of getAliasSet() returned  
for the def, which also triggers this problem.

Is it calling handle*() on the alias set of a register thats the  
culprit, or is it some mishandling in KillsRegister()?

Thanks
--
Christopher Lamb

On Apr 3, 2007, at 3:45 PM, Christopher Lamb wrote:
>
>> Can you dump out the machine basic block? It should have an
>> implicit use of V4R0 at first ORI but it should not be marked kill.
>> If it is marked kill, then you need to walk LiveVariables.cpp to
>> find out why.
>
> Here is the beginning of the BB dump.
>
> entry (0x8503c80, LLVM BB @0x8501af0, ID#0):
> Live Ins: %R0 %R1
> 	%reg1024 = ORI %R0<kill>, 0
> 	%reg1025 = ORI %R1<kill>, 0
>
> V4R0 is getting killed because handleLiveInRegister() is called on
> all results of getAliasSet() for each of the liveins (this is in
> LiveIntervals::computeIntervals() ).
>
> handleRegisterDef() does a similar thing where calls
> handlePhysicalRegisterDef() on all members of getAliasSet() returned
> for the def, which also triggers this problem.
>
> Is it calling handle*() on the alias set of a register thats the
> culprit, or is it some mishandling in KillsRegister()?
This is a pretty serious bug. LiveVariables::KillsRegister should not  
kill aliases that are "larger". The correct way to fix this is to  
explicitly list registers that are defined, used, and killed. So your  
example should look like:

entry (0x8503c80, LLVM BB @0x8501af0, ID#0):
Live Ins: %R0 %R1
	%reg1024 = ORI %R0<kill>, 0, %V4R0<imp-use>
	%reg1025 = ORI %R1<kill>, 0, %V4R0<imp-use,kill>

KillsRegister should check for exact match rather than regsOverlap.  
There are probably other similar bugs in LiveVariables.

Please file a bug, I'll look at after I get back from vacation in a  
week.

Evan
>
> Thanks
> --
> Christopher Lamb
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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On Apr 4, 2007, at 1:38 PM, Evan Cheng wrote:
>
> On Apr 3, 2007, at 3:45 PM, Christopher Lamb wrote:
>
>>
>>> Can you dump out the machine basic block? It should have an
>>> implicit use of V4R0 at first ORI but it should not be marked kill.
>>> If it is marked kill, then you need to walk LiveVariables.cpp to
>>> find out why.
>>
>> Here is the beginning of the BB dump.
>>
>> entry (0x8503c80, LLVM BB @0x8501af0, ID#0):
>> Live Ins: %R0 %R1
>> 	%reg1024 = ORI %R0<kill>, 0
>> 	%reg1025 = ORI %R1<kill>, 0
>>
>> V4R0 is getting killed because handleLiveInRegister() is called on
>> all results of getAliasSet() for each of the liveins (this is in
>> LiveIntervals::computeIntervals() ).
>>
>> handleRegisterDef() does a similar thing where calls
>> handlePhysicalRegisterDef() on all members of getAliasSet() returned
>> for the def, which also triggers this problem.
>>
>> Is it calling handle*() on the alias set of a register thats the
>> culprit, or is it some mishandling in KillsRegister()?
>
> This is a pretty serious bug. LiveVariables::KillsRegister should  
> not kill aliases that are "larger". The correct way to fix this
is
> to explicitly list registers that are defined, used, and killed. So  
> your example should look like:
>
> entry (0x8503c80, LLVM BB @0x8501af0, ID#0):
> Live Ins: %R0 %R1
> 	%reg1024 = ORI %R0<kill>, 0, %V4R0<imp-use>
> 	%reg1025 = ORI %R1<kill>, 0, %V4R0<imp-use,kill>
>
> KillsRegister should check for exact match rather than regsOverlap.  
> There are probably other similar bugs in LiveVariables.
>
> Please file a bug, I'll look at after I get back from vacation in a  
> week.
I've filed it as PR1306. Thanks for looking into this.

--
Christopher Lamb



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