[LLVMdev] LiveInterval Questions

[Evan Cheng]

On Jan 16, 2008, at 11:49 AM, David Greene wrote:

> I had been assuming that give a LiveRange a, a.valno->def, if
> valid, would be the same as a.start.  But this is apparently not
> always the case.  For example:
>
>    Predecessors according to CFG: 0x839d130 (#3) 0x8462780 (#35)
> 308	%reg1051 = MOV64rr %reg1227<kill>
> 312	%reg1052 = MOV64rr %reg1228<kill>
> 316	%reg1053 = MOV64rr %reg1229<kill>
> 320	%reg1054 = MOV64rr %reg1230<kill>
> 324	%reg1055<dead> = LEA64r %reg1047, 1, %reg1053, 0
> 328	%reg1135 = MOVSX64rr32 %reg1025
> 332	%reg1136 = MOV64rr %reg1135<kill>
> 336	%reg1136 = ADD64ri32 %reg1136, -4, %EFLAGS<imp-def,dead>
> 340	TEST64rr %reg1136<kill>, %reg1136, %EFLAGS<imp-def>
> 344	JNS mbb<file solve.f, line 23, in loop at depth 1,  
> bb16,0x83a2c70>,
> %EFLAGS<imp-use,kill>
>
> Here we have the curious case of %reg1055 being defined and then  
> immediately
> killed.  It's a dead assignment.  Why it wasn't removed I don't  
> know, but the
> LiveInterval looks like this:
>
> %reg1055,0 = [308,348:0 [0])  0 at 326-(326 326)
>
> The valno->def is 326, which is what I would expect given the  
> instruction
> numbering above.
>
> So why does the live range extend throughout the entire basic block?
>
> %reg1055 doesn't appear anywhere else in the program so it shouldn't  
> be
> live-in to the block.

It could be a bug. Can you get me a test case?

>
>
> A related question: LiveRange::valno is a single value, correct?  In  
> other
> words it doesn't point to an array or anything, right?

Correct.

>
>
> If so, then isn't LiveInterval::Ranges and LiveInterval::VNInfoList  
> redundant?
> What's in VNInfoList that's not in the valno member of the Ranges  
> elements,
> and vice-versa?

I am not sure if I understand your question. Multiple liveranges can  
be of the same val#. Each VNInfo contains definition (if not  
containing a phi merge, etc.), copy register, and kills that are not  
in the liverange data structure.

Evan
>
>
>                                                 -Dave
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