[DSE] Bugfix to avoid PartialStoreMerging involving non byte-sized stores

Summary:
The DeadStoreElimination pass now skips doing
PartialStoreMerging when stores overlap according to
OW_PartialEarlierWithFullLater and at least one of
the stores is having a store size that is different
from the size of the type being stored.

This solves problems seen in
  https://bugs.llvm.org/show_bug.cgi?id=41949
for which we in the past could end up with
mis-compiles or assertions.

The content and location of the padding bits is not
formally described (or undefined) in the LangRef
at the moment. So the solution is chosen based on
that we cannot assume anything about the padding bits
when having a store that clobbers more memory than
indicated by the type of the value that is stored
(such as storing an i6 using an 8-bit store instruction).

Fixes: https://bugs.llvm.org/show_bug.cgi?id=41949

Reviewers: spatel, efriedma, fhahn

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62250

llvm-svn: 361605

include/llvm/IR/DataLayout.h

@@ -453,6 +453,14 @@ public:
     return 8 * getTypeStoreSize(Ty);
   }
 
+  /// Returns true if no extra padding bits are needed when storing the
+  /// specified type.
+  ///
+  /// For example, returns false for i19 that has a 24-bit store size.
+  bool typeSizeEqualsStoreSize(Type *Ty) const {
+    return getTypeSizeInBits(Ty) == getTypeStoreSizeInBits(Ty);
+  }
+
   /// Returns the offset in bytes between successive objects of the
   /// specified type, including alignment padding.
   ///

lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -1211,12 +1211,17 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
           auto *Earlier = dyn_cast<StoreInst>(DepWrite);
           auto *Later = dyn_cast<StoreInst>(Inst);
           if (Earlier && isa<ConstantInt>(Earlier->getValueOperand()) &&
+              DL.typeSizeEqualsStoreSize(
+                  Earlier->getValueOperand()->getType()) &&
               Later && isa<ConstantInt>(Later->getValueOperand()) &&
+              DL.typeSizeEqualsStoreSize(
+                  Later->getValueOperand()->getType()) &&
               memoryIsNotModifiedBetween(Earlier, Later, AA)) {
             // If the store we find is:
             //   a) partially overwritten by the store to 'Loc'
             //   b) the later store is fully contained in the earlier one and
             //   c) they both have a constant value
+            //   d) none of the two stores need padding
             // Merge the two stores, replacing the earlier store's value with a
             // merge of both values.
             // TODO: Deal with other constant types (vectors, etc), and probably

test/Transforms/DeadStoreElimination/PartialStore2.ll

@@ -0,0 +1,55 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s --data-layout "e" -dse -enable-dse-partial-store-merging=true -S | FileCheck --check-prefix CHECK --check-prefix CHECK-LE %s
+; RUN: opt < %s --data-layout "E" -dse -enable-dse-partial-store-merging=true -S | FileCheck --check-prefix CHECK --check-prefix CHECK-BE %s
+
+; This test used to hit an assertion (see PR41949).
+;
+; Better safe than sorry, do not assume anything about the padding for the
+; i28 store that has 32 bits as store size.
+define void @test1(i32* %p) {
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:    [[A:%.*]] = alloca i32
+; CHECK-NEXT:    [[B:%.*]] = bitcast i32* [[A]] to i28*
+; CHECK-NEXT:    [[C:%.*]] = bitcast i32* [[A]] to { i16, i16 }*
+; CHECK-NEXT:    [[C1:%.*]] = getelementptr inbounds { i16, i16 }, { i16, i16 }* [[C]], i32 0, i32 1
+; CHECK-NEXT:    store i28 10, i28* [[B]]
+; CHECK-NEXT:    store i16 20, i16* [[C1]]
+; CHECK-NEXT:    call void @test1(i32* [[A]])
+; CHECK-NEXT:    ret void
+;
+  %a = alloca i32
+  %b = bitcast i32* %a to i28*
+  %c = bitcast i32* %a to { i16, i16 }*
+  %c1 = getelementptr inbounds { i16, i16 }, { i16, i16 }* %c, i32 0, i32 1
+  store i28 10, i28* %b
+  store i16 20, i16* %c1
+
+  call void @test1(i32* %a)
+  ret void
+}
+
+
+; This test used to mis-compile (see PR41949).
+;
+; Better safe than sorry, do not assume anything about the padding for the
+; i12 store that has 16 bits as store size.
+define void @test2(i32* %p) {
+; CHECK-LABEL: @test2(
+; CHECK-NEXT:    [[U:%.*]] = alloca i32
+; CHECK-NEXT:    [[A:%.*]] = bitcast i32* [[U]] to i32*
+; CHECK-NEXT:    [[B:%.*]] = bitcast i32* [[U]] to i12*
+; CHECK-NEXT:    store i32 -1, i32* [[A]]
+; CHECK-NEXT:    store i12 20, i12* [[B]]
+; CHECK-NEXT:    call void @test2(i32* [[U]])
+; CHECK-NEXT:    ret void
+;
+  %u = alloca i32
+  %a = bitcast i32* %u to i32*
+  %b = bitcast i32* %u to i12*
+  store i32 -1, i32* %a
+  store i12 20, i12* %b
+
+  call void @test2(i32* %u)
+  ret void
+}
+