llvm dev - Jan 2012 - [LLVMdev] How to identify the first BB in a loop of non-constant trip count?

Hi,

The compilation of the following code

void f(int *a, int b[], int n)
{
  int i;
  for(i=0; i< n; ++i)
    a[i] = b[i];
}

gives the IR:

define void @f(i32* nocapture %a, i32* nocapture %b, i32 %n) nounwind {
  %1 = icmp sgt i32 %n, 0
  br i1 %1, label %.lr.ph, label %._crit_edge

.lr.ph:                                           ; preds = %0
  %tmp = zext i32 %n to i64
  br label %2

; <label>:2                                       ; preds = %2, %.lr.ph
  %indvar = phi i64 [ 0, %.lr.ph ], [ %indvar.next, %2 ]
  %scevgep = getelementptr i32* %a, i64 %indvar
  %scevgep2 = getelementptr i32* %b, i64 %indvar
  %3 = load i32* %scevgep2, align 4, !tbaa !0
  store i32 %3, i32* %scevgep, align 4, !tbaa !0
  %indvar.next = add i64 %indvar, 1
  %exitcond = icmp eq i64 %indvar.next, %tmp
  br i1 %exitcond, label %._crit_edge, label %2

._crit_edge:                                      ; preds = %2, %0
  ret void
}

My question is how can I identify the first BB in a loop of non-constant
trip count? In this example, it is the first BB of the function, which
decides whether going to the loop body or not. What's the name of such BBs?
For example, some BBs have the name preheader or header, ...

Bo
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Hi,

you can use LoopInfo for this:

LoopInfo& LI = getAnalysis<LoopInfo>();

You can ask LoopInfo for all kinds of information (e.g. which loop a 
block belongs to or iterate over the loop tree). It can return an object 
of type Loop which can be queried for the loop header (first block to be 
executed), preheader (unique block outside the loop from which the 
header can be reached), latch (block from which a back edge leads back 
to the header, exit blocks, etc.

Cheers,
Ralf

P.S. Be sure to add AU.addRequired<LoopInfo>() to getAnalysisUsage() and 
use the INITIALIZE_PASS_DEPENDENCY macro. Have a look at other passes in 
the LLVM source that use loop info.

On 1/23/12 4:37 PM, Bo Wu wrote:
> Hi,
>
> The compilation of the following code
>
> void f(int *a, int b[], int n)
> {
>    int i;
>    for(i=0; i< n; ++i)
>      a[i] = b[i];
> }
>
> gives the IR:
>
> define void @f(i32* nocapture %a, i32* nocapture %b, i32 %n) nounwind {
>    %1 = icmp sgt i32 %n, 0
>    br i1 %1, label %.lr.ph <http://lr.ph>, label %._crit_edge
> .lr.ph <http://lr.ph>:                                           ;
preds
> = %0
>    %tmp = zext i32 %n to i64
>    br label %2
>
> ; <label>:2                                       ; preds = %2,
%.lr.ph
> <http://lr.ph>
>    %indvar = phi i64 [ 0, %.lr.ph <http://lr.ph> ], [ %indvar.next,
%2 ]
>    %scevgep = getelementptr i32* %a, i64 %indvar
>    %scevgep2 = getelementptr i32* %b, i64 %indvar
>    %3 = load i32* %scevgep2, align 4, !tbaa !0
>    store i32 %3, i32* %scevgep, align 4, !tbaa !0
>    %indvar.next = add i64 %indvar, 1
>    %exitcond = icmp eq i64 %indvar.next, %tmp
>    br i1 %exitcond, label %._crit_edge, label %2
> ._crit_edge:                                      ; preds = %2, %0
>    ret void
> }
>
> My question is how can I identify the first BB in a loop of non-constant
> trip count? In this example, it is the first BB of the function, which
> decides whether going to the loop body or not. What's the name of such
> BBs? For example, some BBs have the name preheader or header, ...
>
> Bo
>
>
>
>
> _______________________________________________
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