[LLVMdev] A potential bug

[Eli Friedman]

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