[llvm] r214897 - Remove dead zero store to calloc initialized memory
Philip Reames
listmail at philipreames.com
Wed Aug 6 15:43:45 PDT 2014
I think I just spotted the error in this patch. I was checking that the
defining instruction was a calloc, but I was not checking that the
dependent instruction (the store) was actually storing to that calloc.
Oops.
I'll confirm this, prepare another version, and send it out for review.
Are there easy instructions somewhere for replicating the msam build
that failed?
Philip
On 08/06/2014 12:28 PM, Rui Ueyama wrote:
> We found that this patch seems to miscompile and confuse msan. I'm
> going to roll it back. I think the follow up comments about the issue
> will come from the people who know more on this.
>
>
> On Tue, Aug 5, 2014 at 10:48 AM, Philip Reames
> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>
> Author: reames
> Date: Tue Aug 5 12:48:20 2014
> New Revision: 214897
>
> URL: http://llvm.org/viewvc/llvm-project?rev=214897&view=rev
> Log:
> Remove dead zero store to calloc initialized memory
>
> Optimize the following IR:
>
> %1 = tail call noalias i8* @calloc(i64 1, i64 4)
> %2 = bitcast i8* %1 to i32*
> ; This store is dead and should be removed
> store i32 0, i32* %2, align 4
>
> Memory returned by calloc is guaranteed to be zero initialized. If
> the value being stored is the constant zero (and the store is not
> otherwise observable across threads), we can delete the store. If
> the store is to an out of bounds address, it is undefined and thus
> also removable.
>
> Reviewed By: nicholas
>
> Differential Revision: http://reviews.llvm.org/D3942
>
>
>
> Added:
> llvm/trunk/test/Transforms/DeadStoreElimination/calloc-store.ll
> Modified:
> llvm/trunk/lib/Transforms/Scalar/DeadStoreElimination.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/DeadStoreElimination.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/DeadStoreElimination.cpp?rev=214897&r1=214896&r2=214897&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/DeadStoreElimination.cpp
> (original)
> +++ llvm/trunk/lib/Transforms/Scalar/DeadStoreElimination.cpp Tue
> Aug 5 12:48:20 2014
> @@ -514,30 +514,50 @@ bool DSE::runOnBasicBlock(BasicBlock &BB
> if (!InstDep.isDef() && !InstDep.isClobber())
> continue;
>
> - // If we're storing the same value back to a pointer that we just
> - // loaded from, then the store can be removed.
> + // Check for cases where the store is redundant.
> if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
> + bool DeleteStore = false;
> + // If we're storing the same value back to a pointer that
> we just
> + // loaded from, then the store can be removed.
> if (LoadInst *DepLoad =
> dyn_cast<LoadInst>(InstDep.getInst())) {
> if (SI->getPointerOperand() ==
> DepLoad->getPointerOperand() &&
> SI->getOperand(0) == DepLoad && isRemovable(SI)) {
> DEBUG(dbgs() << "DSE: Remove Store Of Load from same
> pointer:\n "
> << "LOAD: " << *DepLoad << "\n STORE: "
> << *SI << '\n');
> + DeleteStore = true;
> + }
> + }
>
> - // DeleteDeadInstruction can delete the current
> instruction. Save BBI
> - // in case we need it.
> - WeakVH NextInst(BBI);
> -
> - DeleteDeadInstruction(SI, *MD, TLI);
> -
> - if (!NextInst) // Next instruction deleted.
> - BBI = BB.begin();
> - else if (BBI != BB.begin()) // Revisit this
> instruction if possible.
> - --BBI;
> - ++NumFastStores;
> - MadeChange = true;
> - continue;
> + // If we find a store to memory which was defined by calloc
> + // we can remove the store if the value being stored is a
> + // constant zero (since calloc initialized the memory to
> + // that same value) or the store is undefined (if out of
> + // bounds).
> + if (isCallocLikeFn(InstDep.getInst(), TLI) &&
> isRemovable(SI)) {
> + Value *V = SI->getValueOperand();
> + if (isa<Constant>(V) && cast<Constant>(V)->isNullValue()) {
> + DEBUG(dbgs() << "DSE: Remove Store Of Zero to Calloc:\n "
> + << "CALLOC: " << *InstDep.getInst() << "\n"
> + << "STORE: " << *SI << '\n');
> + DeleteStore = true;
> }
> }
> +
> + if (DeleteStore) {
> + // DeleteDeadInstruction can delete the current
> instruction. Save BBI
> + // in case we need it.
> + WeakVH NextInst(BBI);
> +
> + DeleteDeadInstruction(SI, *MD, TLI);
> +
> + if (!NextInst) // Next instruction deleted.
> + BBI = BB.begin();
> + else if (BBI != BB.begin()) // Revisit this instruction
> if possible.
> + --BBI;
> + ++NumFastStores;
> + MadeChange = true;
> + continue;
> + }
> }
>
> // Figure out what location is being stored to.
>
> Added: llvm/trunk/test/Transforms/DeadStoreElimination/calloc-store.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/DeadStoreElimination/calloc-store.ll?rev=214897&view=auto
> ==============================================================================
> ---
> llvm/trunk/test/Transforms/DeadStoreElimination/calloc-store.ll
> (added)
> +++
> llvm/trunk/test/Transforms/DeadStoreElimination/calloc-store.ll
> Tue Aug 5 12:48:20 2014
> @@ -0,0 +1,17 @@
> +; RUN: opt < %s -basicaa -dse -S | FileCheck %s
> +
> +target datalayout =
> "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> +
> +; Function Attrs: nounwind
> +declare noalias i8* @calloc(i64, i64)
> +
> +; Function Attrs: nounwind uwtable
> +define noalias i32* @test_store() {
> +; CHECK-LABEL: test_store
> + %1 = tail call noalias i8* @calloc(i64 1, i64 4)
> + %2 = bitcast i8* %1 to i32*
> + ; This store is dead and should be removed
> + store i32 0, i32* %2, align 4
> +; CHECK-NOT: store i32 0, i32* %2, align 4
> + ret i32* %2
> +}
>
>
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