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

[Hal Finkel]

----- 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
> 
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> 

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