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

[Sanjoy Das via llvm-dev]

Sanjoy Das wrote:
> Hi Steve,
>
> Do you primarily find this to help for nested loops? If so, that
> could be because LSR explicitly bails out of processing them:
>
> // Skip nested loops until we can model them better with formulae.
> if (!L->empty()) {
> DEBUG(dbgs() << "LSR skipping outer loop " << *L << "n");
> return;
> }
>
> I don't know how much time you're willing to commit to this, but
> perhaps a more principled fix is to change LSR to actually work with
> nested loops?
>
> If I comment out this change, after LSR the matric_mul routine does

I meant if I comment out the `if (L->empty()) return;` check in LSR, not "change".

> not actually look any better (possibly even worse):
>
> define void @matrix_mul(i32 %Size, i32* nocapture %Dst, i32* nocapture readonly %Src, i32 %Val) {
> entry:
> %Src12 = bitcast i32* %Src to i8*
> %Dst14 = bitcast i32* %Dst to i8*
> %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.iv17 = phi i32 [ %Size, %for.body.4.lr.ph.preheader ], [ %lsr.iv.next18, %for.cond.cleanup.3 ]
> %lsr.iv10 = phi i32 [ 0, %for.body.4.lr.ph.preheader ], [ %lsr.iv.next11, %for.cond.cleanup.3 ]
> %uglygep = getelementptr i8, i8* %Src12, i32 %lsr.iv10
> %uglygep13 = bitcast i8* %uglygep to i32*
> %uglygep15 = getelementptr i8, i8* %Dst14, i32 %lsr.iv10
> %uglygep1516 = bitcast i8* %uglygep15 to i32*
> br label %for.body.4
>
> for.body.4: ; preds = %for.body.4, %for.body.4.lr.ph
> %lsr.iv8 = phi i32* [ %scevgep9, %for.body.4 ], [ %uglygep13, %for.body.4.lr.ph ]
> %lsr.iv3 = phi i32* [ %scevgep4, %for.body.4 ], [ %uglygep1516, %for.body.4.lr.ph ]
> %lsr.iv = phi i32 [ %lsr.iv.next, %for.body.4 ], [ %Size, %for.body.4.lr.ph ]
> %1 = load i32, i32* %lsr.iv8, align 4, !tbaa !0
> %mul5 = mul i32 %1, %Val
> store i32 %mul5, i32* %lsr.iv3, align 4, !tbaa !0
> %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
>
> for.cond.cleanup.3: ; preds = %for.body.4
> %lsr.iv.next11 = add i32 %lsr.iv10, %0
> %lsr.iv.next18 = add i32 %lsr.iv17, -1
> %exitcond27 = icmp eq i32 %lsr.iv.next18, 0
> br i1 %exitcond27, label %for.cond.cleanup.loopexit, label %for.body.4.lr.ph
>
> 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
> }
>
>
> -- Sanjoy