[LLVMdev] opt -O2 leads to incorrect operation (possibly a bug in the DSE) (original) (raw)

Hal Finkel hfinkel at anl.gov
Fri Oct 17 05:12:12 PDT 2014

----- Original Message -----

From: "Aliaksei Zasenka" <listhex at gmail.com> To: "Hal Finkel" <hfinkel at anl.gov> Cc: LLVMdev at cs.uiuc.edu Sent: Friday, October 17, 2014 6:57:14 AM Subject: Re: [LLVMdev] opt -O2 leads to incorrect operation (possibly a bug in the DSE)

Hal, Thanks for your answer. Should I report a bug?

Generally speaking, yes. In this case, don't bother; I removed the offending logic in r220035. Thanks for isolating the problem!

-Hal

Best regards, Alexey

2014-10-17 14:30 GMT+03:00 Hal Finkel < hfinkel at anl.gov > :

----- Original Message ----- > From: "Aliaksei Zasenka" < listhex at gmail.com > > To: LLVMdev at cs.uiuc.edu > Sent: Friday, October 17, 2014 4:24:27 AM > Subject: [LLVMdev] opt -O2 leads to incorrect operation (possibly a > bug in the DSE) > > Hi all, > > Consider the following example: > > define void @fn(i8* %buf) #0 { > entry: > %arrayidx = getelementptr i8* %buf, i64 18 > tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %arrayidx, i8* %buf, > i64 18, i32 1, i1 false) > %arrayidx1 = getelementptr i8* %buf, i64 18 > store i8 1, i8* %arrayidx1, align 1 > tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %buf, i8* %arrayidx, > i64 18, i32 1, i1 false) > ret void > } > > > > I ran opt -O2 ex.ll -S, and got: > > define void @fn(i8* nocapture %buf) #0 { > entry: > %arrayidx = getelementptr i8* %buf, i64 18 > store i8 1, i8* %arrayidx, align 1 > tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %buf, i8* %arrayidx, > i64 18, i32 1, i1 false) > ret void > } > > > In that case previous contents of bytes 0-17 are overwritten by > bytes > 18-35 (pre-opt code doesn't do that). > > > > Another point, if I change the code to: > > define void @fn(i8* %buf) #0 { > entry: > %arrayidx = getelementptr i8* %buf, i64 18 > tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %arrayidx, i8* %buf, > i64 18, i32 1, i1 false) > > ;changing 19th byte, not 18th: > > %arrayidx1 = getelementptr i8* %buf, i64 19 > store i8 1, i8* %arrayidx1, align 1 > tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %buf, i8* %arrayidx, > i64 18, i32 1, i1 false) > ret void > } > > > > I get correct piece of code: > > define void @fn(i8* nocapture %buf) #0 { > entry: > %arrayidx = getelementptr i8* %buf, i64 18 > tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %arrayidx, i8* %buf, > i64 18, i32 1, i1 false) > %arrayidx1 = getelementptr i8* %buf, i64 19 > store i8 1, i8* %arrayidx1, align 1 > tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %buf, i8* %arrayidx, > i64 18, i32 1, i1 false) > ret void > } > > Adding some datalayout to module also solves the issue. > > -print-after-all shows that the 'Dead Store Elimination' pass > erases > first call to memcpy. > > I found that the following code in DeadStoreElimination.cpp leads > to > such behavior: > if (DL == nullptr && Later.Ptr->getType() == > Earlier.Ptr->getType()) > return OverwriteComplete; > > > Is this issue a bug in DSE or maybe I'm doing some wrong? > Looks like a bug in DSE, you're not doing anything wrong. We don't test much without data layouts anymore. This code might pre-date DSE's handling of memcpy intrinsics (just a guess). In any case, we'll fix it. -Hal > > > > Best regards, > Alexey > _> ________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > -- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory

-- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory

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