llvm dev - Feb 2013 - [LLVMdev] loop metdata instruction

Hi,

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.) Note that I'm asking about llvm.loop.parallel not
llvm.mem.parallel_loop_access which obviously must be inside the loop.

It seems that if the metadata is attached outside of the loop it is less likely
that the metadata will be lost during loop transformations.

paul

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?

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

-Krzysztof


-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation

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
> 
> 
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
by The Linux Foundation
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