[LLVMdev] A potential bug

[Zeng Bin]

So capture means if the function returns a pointer calculated with the
passed pointer parameter or store somewhere a pointer calculated from the
passed pointer? Thanks a lot, Eli.

On Thu, Oct 6, 2011 at 2:28 PM, Eli Friedman <eli.friedman at gmail.com> wrote:

> If int_guard_load returns a pointer based on the passed-in pointer, it
> captures it (at least according to the definition of "capture" which
> NoCapture uses).
>
> -Eli
>
> On Thu, Oct 6, 2011 at 2:26 PM, Zeng Bin <ezengbin at gmail.com> wrote:
> > It does not do anything. It is an abstract function which transforms a
> > pointer and returns another pointer of the same type. It does not visit
> > memory or capture the pointer parameter.
> >
> > On Thu, Oct 6, 2011 at 2:22 PM, Eli Friedman <eli.friedman at gmail.com>
> wrote:
> >>
> >> On Thu, Oct 6, 2011 at 2:20 PM, Eli Friedman <eli.friedman at gmail.com>
> >> wrote:
> >> > On Thu, Oct 6, 2011 at 2:12 PM, Zeng Bin <ezengbin at gmail.com> wrote:
> >> >> Hi all,
> >> >>
> >> >> There might be a bug in DeadStoreElimination.cpp. This pass
> eliminates
> >> >> stores backwards aggressively in an end BB. It does not check
> >> >> dependencies
> >> >> on stores in an end BB though. For example, in this code snippet:
> >> >>   ...
> >> >> 1.  %sum.safe_r47.pre-phi = phi i64* [ %sum.safe_r47.pre,
> >> >> %entry.for.end_crit_edge ], [ %sum.safe_r42, %for.body ]
> >> >> 2.  %call9 = call i32 @gettimeofday(%struct.timeval* %end,
> >> >> %struct.timeval*
> >> >> null) nounwind
> >> >> 3.  %0 = bitcast %struct.timeval* %start to i64*           //
> >> >> eliminated by
> >> >> HandleEndBlock in DeadStoreElimination.cpp
> >> >> 4.  %1 = bitcast %struct.timeval* %agg.tmp to i64*     // eliminated
> >> >> ...
> >> >> 5.  %tmp49 = load i64* %0, align 8                             //
> >> >> eliminated
> >> >> ...
> >> >> 6.  store i64 %tmp49, i64* %1, align 8                         //
> >> >> eliminated
> >> >> ...
> >> >> 7.  %2 = bitcast %struct.timeval* %end to i64*             //
> >> >> eliminated ...
> >> >> 8.  %3 = bitcast %struct.timeval* %agg.tmp12 to i64*   // eliminated
> >> >> ...
> >> >> 9.  %tmp50 = load i64* %2, align 8                               //
> >> >> eliminated ...
> >> >> 10.  store i64 %tmp50, i64* %3, align 8                           //
> >> >> eliminated ...
> >> >> 11.  %tv_sec = getelementptr inbounds %struct.timeval* %agg.tmp, i32
> 0,
> >> >> i32
> >> >> 0
> >> >> 12.  %tv_sec.safe_r = call i32* @llvm.guard.load.p0i32(i32* %tv_sec)
> >> >> //
> >> >> intrinsic  function call inserted by me
> >> >> 13.  %tmp15 = load i32* %tv_sec.safe_r, align 4, !tbaa
> >> >> !4                     // this loads the value stored at line 6
> >> >> 14.  %tv_usec = getelementptr inbounds %struct.timeval* %agg.tmp, i32
> >> >> 0, i32
> >> >> 1
> >> >> 15.  %tv_usec.safe_r = call i32* @llvm.guard.load.p0i32(i32*
> %tv_usec)
> >> >> //
> >> >> intrinsic function call ...
> >> >> 16.  %tmp16 = load i32* %tv_usec.safe_r, align 4, !tbaa !4
> >> >> 17.  %tv_sec17 = getelementptr inbounds %struct.timeval* %agg.tmp12,
> >> >> i32 0,
> >> >> i32 0
> >> >> 18.  %tv_sec17.safe_r = call i32* @llvm.guard.load.p0i32(i32*
> >> >> %tv_sec17)  //
> >> >> intrinsic function call ...
> >> >> 19.  %tmp18 = load i32* %tv_sec17.safe_r, align 4, !tbaa !4
> >> >> 20.  %tv_usec19 = getelementptr inbounds %struct.timeval* %agg.tmp12,
> >> >> i32 0,
> >> >> i32 1
> >> >> 21.  %tv_usec19.safe_r = call i32* @llvm.guard.load.p0i32(i32*
> >> >> %tv_usec19)
> >> >> // intrinsic function call
> >> >> 22.  %tmp20 = load i32* %tv_usec19.safe_r, align 4, !tbaa !4
> >> >> 23.  %call21 = call i32 @delta(i32 %tmp15, i32 %tmp16, i32 %tmp18,
> i32
> >> >> %tmp20)
> >> >>   ...
> >> >>
> >> >> It is compiled by clang 2.9. This BB is an end block in a function.
> >> >> Intrinsic function llvm.guard.load.p0i32 is defined as follows:
> >> >> let Properties = [IntrNoMem, NoCapture<0>] in {
> >> >>     def int_guard_load   : Intrinsic<[llvm_anyptr_ty],
> >> >> [LLVMMatchType<0>]>
> >> >> }
> >> >>
> >> >> Thanks a lot.
> >> >
> >> > IntrNoMem means that your intrinsic doesn't access memory... I don't
> >> > think that is what you want.
> >>
> >> Well, either that or the NoCapture marking is wrong.  What exactly is
> >> your int_guard_load supposed to do?
> >>
> >> -Eli
> >
> >
>
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