[LLVMdev] alias analysis on llvm internal globals

[Daniel Berlin]

On Sat, Apr 25, 2015 at 4:32 PM, Xin Tong <trent.tong at gmail.com> wrote:
> Hi
>
> I have this program in which fooBuf  can only take on NULL or the
> address of local_fooBuf, and fooBuf and local_fooBuf have scope of the
> foo function.
>
> Therefore there is no way for the fooPtr argument to alias with
> fooBuf. However, LLVM basicaa and globalsmodref-aa say the 2 pointers
> may alias.
>
> I am thinking whether i should implement a limited form of point-to
> alias on the fooBuf pointer in basicaa, i.e. looking at all stores to
> it and computing what addresses it can take on and then compare with
> fooPtr which can not point to local_fooBuf  as it has a function
> scope.

You don't want to do this in basicaa, you want to make a new pass that oes this.
Additionally, your real problem here is that it needs loop recurrence info.


Basically what you are trying to reproduce is SCEVAA
>
> Also, LLVM global variables do not have seem to have scope
> information. where can i get that information ?
>
> Thanks,
> Xin
>
> int foo(int *fooPtr,int aconst)
> {
>     int i;
>     int sum;
>     int *fooOffsetPtr;
>     static int init = 1;
>     static int *fooBuf = NULL;
>     static int local_fooBuf[512*4] = {};
>
>     if (init == 1) {
>         fooBuf = local_fooBuf;
>         init = 0;
>     }
>     fooOffsetPtr = &fooBuf[aconst/2];
> #pragma nounroll
>     for (i=0;i<aconst;++i) {
>         /// fooPtr can not point to the same memory location as fooOffsetPtr.
>         fooPtr[i] = i;                                      /// <-------  HERE.
>         /// should not this be moved out of the loop. at least, should not
>         /// load fooOffsetPtr[0] be moved out of the loop.  /// <-------  HERE.
>         sum += fooOffsetPtr[0];


>     }
>     return sum;
> }
>
>
> ////////////////////////////////////////// opt -globalsmodref-aa -O3
> ptr.ll -S  //////////////////////////////////////////
> @foo.init = internal unnamed_addr global i1 false
> @foo.fooBuf = internal unnamed_addr global i32* null, align 8
> @foo.local_fooBuf = internal global [2048 x i32] zeroinitializer, align 16
>
> ; Function Attrs: nounwind uwtable
> define i32 @foo(i32* nocapture %fooPtr, i32 %aconst) #0 {
> entry:
>   %.b = load i1, i1* @foo.init, align 1
>   br i1 %.b, label %entry.if.end_crit_edge, label %if.then
>
> entry.if.end_crit_edge:                           ; preds = %entry
>   %.pre = load i32*, i32** @foo.fooBuf, align 8, !tbaa !1
>   br label %if.end
>
> if.then:                                          ; preds = %entry
>   store i32* getelementptr inbounds ([2048 x i32]* @foo.local_fooBuf,
> i64 0, i64 0), i32** @foo.fooBuf, align 8, !tbaa !1
>   store i1 true, i1* @foo.init, align 1
>   br label %if.end
>
> if.end:                                           ; preds =
> %entry.if.end_crit_edge, %if.then
>   %0 = phi i32* [ %.pre, %entry.if.end_crit_edge ], [ getelementptr
> inbounds ([2048 x i32]* @foo.local_fooBuf, i64 0, i64 0), %if.then ]
>   %div = sdiv i32 %aconst, 2
>   %idxprom = sext i32 %div to i64
>   %arrayidx = getelementptr inbounds i32, i32* %0, i64 %idxprom
>   %cmp110 = icmp sgt i32 %aconst, 0
>   br i1 %cmp110, label %for.body.preheader, label %for.end, !llvm.loop !5
>
> for.body.preheader:                               ; preds = %if.end
>   br label %for.body
>
> for.body:                                         ; preds =
> %for.body.preheader, %for.body
>   %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0,
> %for.body.preheader ]
>   %sum.011 = phi i32 [ %add, %for.body ], [ undef, %for.body.preheader ]
>   %arrayidx3 = getelementptr inbounds i32, i32* %fooPtr, i64 %indvars.iv
>   %1 = trunc i64 %indvars.iv to i32
>   store i32 %1, i32* %arrayidx3, align 4, !tbaa !7
>   %2 = load i32, i32* %arrayidx, align 4, !tbaa !7
>   %add = add nsw i32 %2, %sum.011
>   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
>   %lftr.wideiv = trunc i64 %indvars.iv.next to i32
>   %exitcond = icmp eq i32 %lftr.wideiv, %aconst
>   br i1 %exitcond, label %for.end.loopexit, label %for.body, !llvm.loop !5
>
> for.end.loopexit:                                 ; preds = %for.body
>   %add.lcssa = phi i32 [ %add, %for.body ]
>   br label %for.end
>
> for.end:                                          ; preds =
> %for.end.loopexit, %if.end
>   %sum.0.lcssa = phi i32 [ undef, %if.end ], [ %add.lcssa, %for.end.loopexit ]
>   ret i32 %sum.0.lcssa
> }
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