[llvm-dev] [RFC] New pass: LoopExitValues

[Steve King via llvm-dev]

On Wed, Sep 2, 2015 at 5:36 AM, James Molloy <james at jamesmolloy.co.uk> wrote:
> Hi,
>
> Coremark really isn't a good enough test - have you run the LLVM test suite
> with this patch, and what were the performance differences?

For the test suite single source benches, the 235 tests improved
performance, 2 regressed and 705 were unchanged.  That seems very
optimistic. Comparing consecutive runs with identical setting shows
there is a lot of noise in the performance data.  Tips for stable
results would be appreciated.


> I'm still a bit confused about what pattern exactly this pass is supposed to
> trigger on. I understand the mechanics, but I still can't quite see what
> patterns it would be useful on. You've mentioned matrix multiply - how does
> this pass alter the IR?

Here's before and after IR for the matrix_mul example.  Notice the two
bitcasts %1 and %2 generated in the for.cond.cleanup block.  The L.E.V
pass converts these to scevgep values that already exist.

*** Code after LSR ***

; Function Attrs: nounwind optsize
define void @matrix_mul(i32 %Size, i32* nocapture %Dst, i32* nocapture
readonly %Src, i32 %Val) #0 {
entry:
  %cmp.25 = icmp eq i32 %Size, 0
  br i1 %cmp.25, label %for.cond.cleanup, label %for.body.4.lr.ph.preheader

for.body.4.lr.ph.preheader:                       ; preds = %entry
  %0 = shl i32 %Size, 2
  br label %for.body.4.lr.ph

for.body.4.lr.ph:                                 ; preds =
%for.body.4.lr.ph.preheader, %for.cond.cleanup.3
  %lsr.iv5 = phi i32* [ %Src, %for.body.4.lr.ph.preheader ], [ %2,
%for.cond.cleanup.3 ]
  %lsr.iv1 = phi i32* [ %Dst, %for.body.4.lr.ph.preheader ], [ %1,
%for.cond.cleanup.3 ]
  %Outer.026 = phi i32 [ %inc10, %for.cond.cleanup.3 ], [ 0,
%for.body.4.lr.ph.preheader ]
  %lsr.iv56 = bitcast i32* %lsr.iv5 to i1*
  %lsr.iv12 = bitcast i32* %lsr.iv1 to i1*
  br label %for.body.4

for.cond.cleanup.loopexit:                        ; preds = %for.cond.cleanup.3
  br label %for.cond.cleanup

for.cond.cleanup:                                 ; preds =
%for.cond.cleanup.loopexit, %entry
  ret void

for.cond.cleanup.3:                               ; preds = %for.body.4
  %inc10 = add nuw nsw i32 %Outer.026, 1
  %scevgep = getelementptr i1, i1* %lsr.iv12, i32 %0
  %1 = bitcast i1* %scevgep to i32*
  %scevgep7 = getelementptr i1, i1* %lsr.iv56, i32 %0
  %2 = bitcast i1* %scevgep7 to i32*
  %exitcond27 = icmp eq i32 %inc10, %Size
  br i1 %exitcond27, label %for.cond.cleanup.loopexit, label %for.body.4.lr.ph

for.body.4:                                       ; preds =
%for.body.4, %for.body.4.lr.ph
  %lsr.iv8 = phi i32* [ %scevgep9, %for.body.4 ], [ %lsr.iv5,
%for.body.4.lr.ph ]
  %lsr.iv3 = phi i32* [ %scevgep4, %for.body.4 ], [ %lsr.iv1,
%for.body.4.lr.ph ]
  %lsr.iv = phi i32 [ %lsr.iv.next, %for.body.4 ], [ %Size, %for.body.4.lr.ph ]
  %3 = load i32, i32* %lsr.iv8, align 4, !tbaa !1
  %mul5 = mul i32 %3, %Val
  store i32 %mul5, i32* %lsr.iv3, align 4, !tbaa !1
  %lsr.iv.next = add i32 %lsr.iv, -1
  %scevgep4 = getelementptr i32, i32* %lsr.iv3, i32 1
  %scevgep9 = getelementptr i32, i32* %lsr.iv8, i32 1
  %exitcond = icmp eq i32 %lsr.iv.next, 0
  br i1 %exitcond, label %for.cond.cleanup.3, label %for.body.4
}


*** Code after Loop Exit Values Optimization **

; Function Attrs: nounwind optsize
define void @matrix_mul(i32 %Size, i32* nocapture %Dst, i32* nocapture
readonly %Src, i32 %Val) #0 {
entry:
  %cmp.25 = icmp eq i32 %Size, 0
  br i1 %cmp.25, label %for.cond.cleanup, label %for.body.4.lr.ph.preheader

for.body.4.lr.ph.preheader:                       ; preds = %entry
  br label %for.body.4.lr.ph

for.body.4.lr.ph:                                 ; preds =
%for.body.4.lr.ph.preheader, %for.cond.cleanup.3
  %lsr.iv5 = phi i32* [ %Src, %for.body.4.lr.ph.preheader ], [
%scevgep9, %for.cond.cleanup.3 ]
  %lsr.iv1 = phi i32* [ %Dst, %for.body.4.lr.ph.preheader ], [
%scevgep4, %for.cond.cleanup.3 ]
  %Outer.026 = phi i32 [ %inc10, %for.cond.cleanup.3 ], [ 0,
%for.body.4.lr.ph.preheader ]
  br label %for.body.4

for.cond.cleanup.loopexit:                        ; preds = %for.cond.cleanup.3
  br label %for.cond.cleanup

for.cond.cleanup:                                 ; preds =
%for.cond.cleanup.loopexit, %entry
  ret void

for.cond.cleanup.3:                               ; preds = %for.body.4
  %inc10 = add nuw nsw i32 %Outer.026, 1
  %exitcond27 = icmp eq i32 %inc10, %Size
  br i1 %exitcond27, label %for.cond.cleanup.loopexit, label %for.body.4.lr.ph

for.body.4:                                       ; preds =
%for.body.4, %for.body.4.lr.ph
  %lsr.iv8 = phi i32* [ %scevgep9, %for.body.4 ], [ %lsr.iv5,
%for.body.4.lr.ph ]
  %lsr.iv3 = phi i32* [ %scevgep4, %for.body.4 ], [ %lsr.iv1,
%for.body.4.lr.ph ]
  %lsr.iv = phi i32 [ %lsr.iv.next, %for.body.4 ], [ %Size, %for.body.4.lr.ph ]
  %0 = load i32, i32* %lsr.iv8, align 4, !tbaa !1
  %mul5 = mul i32 %0, %Val
  store i32 %mul5, i32* %lsr.iv3, align 4, !tbaa !1
  %lsr.iv.next = add i32 %lsr.iv, -1
  %scevgep4 = getelementptr i32, i32* %lsr.iv3, i32 1
  %scevgep9 = getelementptr i32, i32* %lsr.iv8, i32 1
  %exitcond = icmp eq i32 %lsr.iv.next, 0
  br i1 %exitcond, label %for.cond.cleanup.3, label %for.body.4


> What value is it avoiding being recomputed?
I'm not precisely sure, but it's residue from LSR.  The pass checks
all computable SCEV values when a loop exits and in this case found
GEPs with the same value.

> How does this pass affect register pressure?
> Also, your example just removes a mov and an add - the push/pops are just
> register allocation (unless your pass in fact *reduces* register pressure?)

Right, the computation eliminated is the mov and add.  Register
savings is a byproduct.

Regards,
-steve