[llvm-dev] Alias Analysis with inbound GEPs
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Tue Jul 26 05:32:42 PDT 2016
----- Original Message -----
> From: "Elena Demikhovsky" <elena.demikhovsky at intel.com>
> To: "Hal J. Finkel" <hfinkel at anl.gov>, "Eli Friedman"
> <eli.friedman at gmail.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "Richard Smith"
> <richard-llvm at metafoo.co.uk>
> Sent: Tuesday, July 26, 2016 6:49:16 AM
> Subject: RE: [llvm-dev] Alias Analysis with inbound GEPs
> > It seems like we might we able to use TBAA metadata with struct
> > field information to get this then.
> TBAA does not support struct fields yet.
> It shows what type will be stored, but does not have structure info.
No, although, unfortunately, the documentation here seems like it is not up to date. I agree, however, that it might need some extension in this case. For example,
$ cat /tmp/f.c
struct S {
int x;
int y;
};
int foo(struct S *s) {
return s->x + s->y;
};
$ clang -O3 -S -emit-llvm -o - /tmp/f.c
; ModuleID = '/tmp/f.c'
source_filename = "/tmp/f.c"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
%struct.S = type { i32, i32 }
; Function Attrs: norecurse nounwind readonly uwtable
define i32 @foo(%struct.S* nocapture readonly) local_unnamed_addr #0 {
%2 = getelementptr inbounds %struct.S, %struct.S* %0, i64 0, i32 0
%3 = load i32, i32* %2, align 4, !tbaa !1
%4 = getelementptr inbounds %struct.S, %struct.S* %0, i64 0, i32 1
%5 = load i32, i32* %4, align 4, !tbaa !6
%6 = add nsw i32 %5, %3
ret i32 %6
}
attributes #0 = { ... }
!llvm.ident = !{!0}
!0 = !{!"...."}
!1 = !{!2, !3, i64 0}
!2 = !{!"S", !3, i64 0, !3, i64 4}
!3 = !{!"int", !4, i64 0}
!4 = !{!"omnipotent char", !5, i64 0}
!5 = !{!"Simple C/C++ TBAA"}
!6 = !{!2, !3, i64 4}
Note here that the two access to the structure have metadata nodes !1 and !6. If you look at those nodes, you'll see that (third argument), they identify the relevant field offset (0 bytes and 4 bytes in this case).
-Hal
> > See also
> > http://lists.llvm.org/pipermail/llvm-dev/2016-July/102472.html .
> It’s not, probably, the case. I need such “inbounds” for any
> sub-structure.
> - Elena
> From: Finkel, Hal J. [mailto:hfinkel at anl.gov]
> Sent: Tuesday, July 26, 2016 02:03
> To: Eli Friedman <eli.friedman at gmail.com>
> Cc: llvm-dev <llvm-dev at lists.llvm.org>; Demikhovsky, Elena
> <elena.demikhovsky at intel.com>; Richard Smith
> <richard-llvm at metafoo.co.uk>
> Subject: Re: [llvm-dev] Alias Analysis with inbound GEPs
> Sent from my Verizon Wireless 4G LTE DROID
> On Jul 25, 2016 6:10 PM, Eli Friedman < eli.friedman at gmail.com >
> wrote:
> >
> > On Mon, Jul 25, 2016 at 2:06 PM, Hal Finkel via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> >>
> >>
> >> ________________________________
> >>>
> >>> From: "Elena Demikhovsky" < elena.demikhovsky at intel.com >
> >>> To: "Hal Finkel" < hfinkel at anl.gov >
> >>> Cc: "llvm-dev" < llvm-dev at lists.llvm.org >
> >>> Sent: Monday, July 25, 2016 2:46:34 PM
> >>> Subject: RE: [llvm-dev] Alias Analysis with inbound GEPs
> >>>
> >>>
> >>>>
> >>>> I’m checking aliasing of two pointers:
> >>>>
> >>>>
> >>>>
> >>>> %GEP1 = getelementptr inbounds %struct.s, %struct.s* %0, i64 0,
> >>>> i32 1, i64 %indvars.iv41, i64 %indvars.iv39
> >>>>
> >>>> %GEP2 = getelementptr inbounds %struct.s, %struct.s* %0, i64 0,
> >>>> i32 16
> >>>>
> >>>>
> >>>>
> >>>> The result I got is “PartialAlias” because the indices of the
> >>>> GEP1 are variable.
> >>>
> >>> That seems like a bug. PartialAlias should only be returned when
> >>> we can prove a partial overlap. Otherwise, MayAlias should be
> >>> returned.
> >>>
> >>> [Demikhovsky, Elena] There are some comments inside:
> >>>
> >>> // Statically, we can see that the base objects are the same, but
> >>> the
> >>>
> >>> // pointers have dynamic offsets which we can't resolve. And none
> >>> of our
> >>>
> >>> // little tricks above worked.
> >>>
> >>> //
> >>>
> >>> // TODO: Returning PartialAlias instead of MayAlias is a mild
> >>> hack; the
> >>>
> >>> // practical effect of this is protecting TBAA in the case of
> >>> dynamic
> >>>
> >>> // indices into arrays of unions or malloc'd memory.
> >>>
> >>> return PartialAlias;
> >>
> >> Ah, thanks! That, unfortunately, makes sense.
> >>>
> >>>
> >>>>
> >>>> Shouldn’t the “inbounds” keyword mean that the access to
> >>>> sub-array is also in-bounds?
> >>>
> >>> No. inbounds applies only to the whole object.
> >>>>
> >>>> I’m trying to reach “NoAlias” consensus between GEP1 and GEP2.
> >>>
> >>> Did the original code come from C or C++? What are we modeling
> >>> here?
> >>>
> >>> [Demikhovsky, Elena] C-code:
> >>>
> >>> for (m=0; m < params->num; m++) {
> >>>
> >>> params->a[i][m] = expr;
> >>>
> >>> }
> >>>
> >>> %GEP1 is the store for params->a[i][m]
> >>>
> >>> %GEP2 is the load for params->num.
> >>>
> >>> The loop is not vectorized due to a possible collision between
> >>> params->num and params->a[i][m]. If I take loading of
> >>> params->num outside the loop, it is vectorized.
> >>>
> >>> Bounds of array “a” are known at compile time. Limit of “i” and
> >>> “m” are runtime variables.
> >>
> >> The problem is, IIRC, it is not undefined behavior to access one
> >> structure field by over-indexing an earlier array member. C++ has
> >> rules for "safely-derived pointers", and I think they include all
> >> pointer arithmetic on addresses from subobjects. If array access
> >> is just pointer arithmetic, I'm not sure that helps you as much
> >> as you'd like. cc'ing Richard to correct me if necessary.
> >>
> >
> > It is actually undefined behavior. This is explicitly called out in
> > Annex J.2: "An array subscript is out of range, even if an object
> > is apparently accessible with the given subscript (as in the
> > lvalue expression a[1][7] given the declaration int a[4][5]) ". If
> > you break it apart into separate steps, the interesting bit is
> > that the implicit array-to-pointer conversion is not equivalent to
> > a cast; "int* b = (int*)a;" is not equivalent to "int* b = *a;",
> > even though the expressions have the same type and value.
> >
> > There currently isn't any way to model the aliasing behavior of the
> > address-of operator or array-to-pointer decay in LLVM IR. See also
> > http://lists.llvm.org/pipermail/llvm-dev/2016-July/102472.html .
> It seems like we might we able to use TBAA metadata with struct field
> information to get this then.
> -Hal
> >
> > -Eli
> ---------------------------------------------------------------------
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--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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