[GVN, EarlyCSE, Missed Opt] Load from memory after invariant.end is undef

[preames]

Bugzilla Link	20811
Version	trunk
OS	Linux
Attachments	Failing test case for GVN
CC	@fhahn,@hfinkel

Extended Description

Currently, GVN has handling to remove a load following a invariant.start with no intervening stores since the memory is undefined and the load can be replaced with undef. This is an analogous case for invariant.end which is not implemented. Once the lifetime of a memory region has ended, the value of the memory becomes undefined. As a result, a load from such memory should be replaced with undef.

Here's the example case (also attached):
define i8 @​test(i8* %P) nounwind {
entry:
call void @​llvm.lifetime.end(i64 32, i8* %P)
%0 = load i8* %P <-- this load is undefined
ret i8 %0
}

The failing test case I've attached is for GVN, but EarlyCSE has the same issue. (In fact, this load is not currently removed by an pass in -O3.) As mentioned in 20809, it would be nice if the logic in question could be refactored to be sharable between the two passes.

[llvmbot]

Updated failing test case for GVN

[llvmbot]

Updated failing test case for GVN
The test case was from before llvm 3.7 and was giving a minor syntax error.
Updated to remove the same.

[llvmbot]

I've tried fixing the issue by doing the following changes:

I added a function to the GVN.cpp file:

static bool isLifetimeEnd(const Instruction Inst) {
if (const IntrinsicInst II = dyn_cast(Inst))
return II->getIntrinsicID() == Intrinsic::lifetime_end;
return false;
}

and modified the following line

if (isa(DepInst) || isMallocLikeFn(DepInst, TLI) ||
isLifetimeStart(DepInst) || isLifetimeEnd(DepInst)) { <-- added isLifetimeEnd()
Res = AvailableValue::get(UndefValue::get(LI->getType()));
return true;
}

But the above doesn't seem to fix the issue. Can someone show me what I have got wrong and point me in the right direction?

I also made changes to the NewGVN.cpp file, but still no progress.

Thanks in advance :)

[fhahn]

FYI, there is a proposed patch https://reviews.llvm.org/D66604