[InstCombine] Eliminate stores to constant memory

If we have a store to a piece of memory which is known constant, then we know the store must be storing back the same value. As a result, the store (or memset, or memmove) must either be down a dead path, or a noop. In either case, it is valid to simply remove the store.

The motivating case for this involves a memmove to a buffer which is constant down a path which is dynamically dead.

Note that I'm choosing to implement the less aggressive of two possible semantics here. We could simply say that the store *is undefined*, and prune the path. Consensus in the review was that the more aggressive form might be a good follow on change at a later date.

Differential Revision: https://reviews.llvm.org/D60659

llvm-svn: 358919

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -120,6 +120,15 @@ Instruction *InstCombiner::SimplifyAnyMemTransfer(AnyMemTransferInst *MI) {
     return MI;
   }
 
+  // If we have a store to a location which is known constant, we can conclude
+  // that the store must be storing the constant value (else the memory
+  // wouldn't be constant), and this must be a noop.
+  if (AA->pointsToConstantMemory(MI->getDest())) {
+    // Set the size of the copy to 0, it will be deleted on the next iteration.
+    MI->setLength(Constant::getNullValue(MI->getLength()->getType()));
+    return MI;
+  }
+
   // If MemCpyInst length is 1/2/4/8 bytes then replace memcpy with
   // load/store.
   ConstantInt *MemOpLength = dyn_cast<ConstantInt>(MI->getLength());
@@ -218,6 +227,15 @@ Instruction *InstCombiner::SimplifyAnyMemSet(AnyMemSetInst *MI) {
     return MI;
   }
 
+  // If we have a store to a location which is known constant, we can conclude
+  // that the store must be storing the constant value (else the memory
+  // wouldn't be constant), and this must be a noop.
+  if (AA->pointsToConstantMemory(MI->getDest())) {
+    // Set the size of the copy to 0, it will be deleted on the next iteration.
+    MI->setLength(Constant::getNullValue(MI->getLength()->getType()));
+    return MI;
+  }
+
   // Extract the length and alignment and fill if they are constant.
   ConstantInt *LenC = dyn_cast<ConstantInt>(MI->getLength());
   ConstantInt *FillC = dyn_cast<ConstantInt>(MI->getValue());

llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

@@ -1438,6 +1438,12 @@ Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
     }
   }
 
+  // If we have a store to a location which is known constant, we can conclude
+  // that the store must be storing the constant value (else the memory
+  // wouldn't be constant), and this must be a noop.
+  if (AA->pointsToConstantMemory(Ptr))
+    return eraseInstFromFunction(SI);
+
   // Do really simple DSE, to catch cases where there are several consecutive
   // stores to the same location, separated by a few arithmetic operations. This
   // situation often occurs with bitfield accesses.

llvm/test/Transforms/InstCombine/memcpy.ll

@@ -40,7 +40,6 @@ define void @test3(i8* %d, i8* %s) {
 
 define void @memcpy_to_constant(i8* %src) {
 ; CHECK-LABEL: @memcpy_to_constant(
-; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 4 bitcast (i128* @UnknownConstant to i8*), i8* align 1 [[SRC:%.*]], i32 16, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   %dest = bitcast i128* @UnknownConstant to i8*

llvm/test/Transforms/InstCombine/memmove.ll

@@ -59,7 +59,6 @@ define void @test4(i8* %a) {
 
 define void @memmove_to_constant(i8* %src) {
 ; CHECK-LABEL: @memmove_to_constant(
-; CHECK-NEXT:    call void @llvm.memmove.p0i8.p0i8.i32(i8* align 4 bitcast (i128* @UnknownConstant to i8*), i8* align 1 [[SRC:%.*]], i32 16, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   %dest = bitcast i128* @UnknownConstant to i8*

llvm/test/Transforms/InstCombine/memset.ll

@@ -26,7 +26,6 @@ define i32 @test([1024 x i8]* %target) {
 
 define void @memset_to_constant() {
 ; CHECK-LABEL: @memset_to_constant(
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* align 4 bitcast (i128* @Unknown to i8*), i8 0, i32 16, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   %p = bitcast i128* @Unknown to i8*

llvm/test/Transforms/InstCombine/store.ll

@@ -295,7 +295,6 @@ define void @write_back7(i32* %p) {
 
 define void @store_to_constant() {
 ; CHECK-LABEL: @store_to_constant(
-; CHECK-NEXT:    store i32 0, i32* @Unknown, align 4
 ; CHECK-NEXT:    ret void
 ;
   store i32 0, i32* @Unknown