[LLVMdev] loop metdata instruction

[Redmond, Paul]

Hi,

On 2013-02-25, at 5:11 PM, Krzysztof Parzyszek wrote:

> On 2/25/2013 2:08 PM, Redmond, Paul wrote:
>> 
>> I've been looking through past threads looking for an answer to why the loop metadata is attached to the loop latch branch. What is the reason for putting the metadata inside the loop rather than outside (for example on the branch into the loop header.)
> 
> Latch is a branch to the header.  What branch in particular do you have in mind?

On the branch into the loop:

define void @foo(i32 %n, float* %a, float* %b) #0 {
entry:
  [...]
  br label %for.cond, !llvm.loop.parallel !0
for.cond:                                         ; preds = %for.inc, %entry
  [...]
  br i1 %cmp, label %for.body, label %for.end
for.body:                                         ; preds = %for.cond
  [...]
  br label %for.inc
for.inc:                                          ; preds = %for.body
  [...]
  br label %for.cond
for.end:                                          ; preds = %for.cond
  ret void
}

Which optimizes to the following (with a small change to LoopRotation.cpp)

define void @foo(i32 %n, float* nocapture %a, float* nocapture %b) #0 {
entry:
  %cmp6 = icmp sgt i32 %n, 0
  br i1 %cmp6, label %for.body, label %for.end, !llvm.loop.parallel !0

for.body:                                         ; preds = %entry, %for.body
  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
  %arrayidx = getelementptr inbounds float* %b, i64 %indvars.iv
  %0 = load float* %arrayidx, align 4, !tbaa !1
  %arrayidx2 = getelementptr inbounds float* %a, i64 %indvars.iv
  store float %0, float* %arrayidx2, align 4, !tbaa !1
  %indvars.iv.next = add i64 %indvars.iv, 1
  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
  %exitcond = icmp eq i32 %lftr.wideiv, %n
  br i1 %exitcond, label %for.end, label %for.body

for.end:                                          ; preds = %for.body, %entry
  ret void
}

paul

> 
> Loop latch identifies the loop.  Loops with multiple latches are not very amenable to optimizations.
> 
> -Krzysztof
> 
> 
> -- 
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