[InstCombine] try to narrow a truncated load

trunc (load X) --> load (bitcast X to narrow type)

We have this transform in DAGCombiner::ReduceLoadWidth(), but the truncated
load pattern can interfere with other instcombine transforms, so I'd like to
allow the fold sooner.

Example:
https://bugs.llvm.org/show_bug.cgi?id=16739
...in that report, we have bitcasts bracketing these ops, so those could get
eliminated too.

We've generally ruled out widening of loads early in IR ( LoadCombine -
http://lists.llvm.org/pipermail/llvm-dev/2016-September/105291.html ), but
that reasoning may not apply to narrowing if we can preserve information
such as the dereferenceable range.

Differential Revision: https://reviews.llvm.org/D64432

llvm-svn: 367011

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 2c9ba20..08a7a06 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -681,6 +681,42 @@ static Instruction *shrinkInsertElt(CastInst &Trunc,
   return nullptr;
 }
 
+static Instruction *narrowLoad(TruncInst &Trunc,
+                               InstCombiner::BuilderTy &Builder,
+                               const DataLayout &DL) {
+  // Check the layout to ensure we are not creating an unsupported operation.
+  // TODO: Create a GEP to offset the load?
+  if (!DL.isLittleEndian())
+    return nullptr;
+  unsigned NarrowBitWidth = Trunc.getDestTy()->getPrimitiveSizeInBits();
+  if (!DL.isLegalInteger(NarrowBitWidth))
+    return nullptr;
+
+  // Match a truncated load with no other uses.
+  Value *X;
+  if (!match(Trunc.getOperand(0), m_OneUse(m_Load(m_Value(X)))))
+    return nullptr;
+  LoadInst *WideLoad = cast<LoadInst>(Trunc.getOperand(0));
+  if (!WideLoad->isSimple())
+    return nullptr;
+
+  // Don't narrow this load if we would lose information about the
+  // dereferenceable range.
+  bool CanBeNull;
+  uint64_t DerefBits = X->getPointerDereferenceableBytes(DL, CanBeNull) * 8;
+  if (DerefBits < WideLoad->getType()->getPrimitiveSizeInBits())
+    return nullptr;
+
+  // trunc (load X) --> load (bitcast X)
+  PointerType *PtrTy = PointerType::get(Trunc.getDestTy(),
+                                        WideLoad->getPointerAddressSpace());
+  Value *Bitcast = Builder.CreatePointerCast(X, PtrTy);
+  LoadInst *NarrowLoad = new LoadInst(Trunc.getDestTy(), Bitcast);
+  NarrowLoad->setAlignment(WideLoad->getAlignment());
+  copyMetadataForLoad(*NarrowLoad, *WideLoad);
+  return NarrowLoad;
+}
+
 Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
   if (Instruction *Result = commonCastTransforms(CI))
     return Result;
@@ -840,6 +876,9 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
   if (Instruction *I = foldVecTruncToExtElt(CI, *this))
     return I;
 
+  if (Instruction *NewLoad = narrowLoad(CI, Builder, DL))
+    return NewLoad;
+
   return nullptr;
 }
 

diff --git a/llvm/test/Transforms/InstCombine/trunc-load.ll b/llvm/test/Transforms/InstCombine/trunc-load.ll
index e17e99b..c85c035 100644 (file)
--- a/llvm/test/Transforms/InstCombine/trunc-load.ll
+++ b/llvm/test/Transforms/InstCombine/trunc-load.ll
@@ -29,10 +29,15 @@ define i32 @truncload_small_deref(i64* dereferenceable(7) %ptr) {
 ; On little-endian, we can narrow the load without an offset.
 
 define i32 @truncload_deref(i64* dereferenceable(8) %ptr) {
-; CHECK-LABEL: @truncload_deref(
-; CHECK-NEXT:    [[X:%.*]] = load i64, i64* [[PTR:%.*]], align 4
-; CHECK-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i32
-; CHECK-NEXT:    ret i32 [[R]]
+; LE-LABEL: @truncload_deref(
+; LE-NEXT:    [[TMP1:%.*]] = bitcast i64* [[PTR:%.*]] to i32*
+; LE-NEXT:    [[R:%.*]] = load i32, i32* [[TMP1]], align 4
+; LE-NEXT:    ret i32 [[R]]
+;
+; BE-LABEL: @truncload_deref(
+; BE-NEXT:    [[X:%.*]] = load i64, i64* [[PTR:%.*]], align 4
+; BE-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i32
+; BE-NEXT:    ret i32 [[R]]
 ;
   %x = load i64, i64* %ptr
   %r = trunc i64 %x to i32
@@ -42,10 +47,15 @@ define i32 @truncload_deref(i64* dereferenceable(8) %ptr) {
 ; Preserve alignment.
 
 define i16 @truncload_align(i32* dereferenceable(14) %ptr) {
-; CHECK-LABEL: @truncload_align(
-; CHECK-NEXT:    [[X:%.*]] = load i32, i32* [[PTR:%.*]], align 16
-; CHECK-NEXT:    [[R:%.*]] = trunc i32 [[X]] to i16
-; CHECK-NEXT:    ret i16 [[R]]
+; LE-LABEL: @truncload_align(
+; LE-NEXT:    [[TMP1:%.*]] = bitcast i32* [[PTR:%.*]] to i16*
+; LE-NEXT:    [[R:%.*]] = load i16, i16* [[TMP1]], align 16
+; LE-NEXT:    ret i16 [[R]]
+;
+; BE-LABEL: @truncload_align(
+; BE-NEXT:    [[X:%.*]] = load i32, i32* [[PTR:%.*]], align 16
+; BE-NEXT:    [[R:%.*]] = trunc i32 [[X]] to i16
+; BE-NEXT:    ret i16 [[R]]
 ;
   %x = load i32, i32* %ptr, align 16
   %r = trunc i32 %x to i16
@@ -98,12 +108,40 @@ define i32 @truncload_volatile(i64* dereferenceable(8) %ptr) {
 ; Preserve address space.
 
 define i32 @truncload_address_space(i64 addrspace(1)* dereferenceable(8) %ptr) {
-; CHECK-LABEL: @truncload_address_space(
-; CHECK-NEXT:    [[X:%.*]] = load i64, i64 addrspace(1)* [[PTR:%.*]], align 4
-; CHECK-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i32
-; CHECK-NEXT:    ret i32 [[R]]
+; LE-LABEL: @truncload_address_space(
+; LE-NEXT:    [[TMP1:%.*]] = bitcast i64 addrspace(1)* [[PTR:%.*]] to i32 addrspace(1)*
+; LE-NEXT:    [[R:%.*]] = load i32, i32 addrspace(1)* [[TMP1]], align 4
+; LE-NEXT:    ret i32 [[R]]
+;
+; BE-LABEL: @truncload_address_space(
+; BE-NEXT:    [[X:%.*]] = load i64, i64 addrspace(1)* [[PTR:%.*]], align 4
+; BE-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i32
+; BE-NEXT:    ret i32 [[R]]
 ;
   %x = load i64, i64 addrspace(1)* %ptr, align 4
   %r = trunc i64 %x to i32
   ret i32 %r
 }
+
+; Most metadata should be transferred to the narrow load.
+; TODO: We lost the range.
+
+define i32 @truncload_metadata(i64* dereferenceable(8) %ptr) {
+; LE-LABEL: @truncload_metadata(
+; LE-NEXT:    [[TMP1:%.*]] = bitcast i64* [[PTR:%.*]] to i32*
+; LE-NEXT:    [[R:%.*]] = load i32, i32* [[TMP1]], align 4, !invariant.load !0, !nontemporal !1
+; LE-NEXT:    ret i32 [[R]]
+;
+; BE-LABEL: @truncload_metadata(
+; BE-NEXT:    [[X:%.*]] = load i64, i64* [[PTR:%.*]], align 4, !range !0, !invariant.load !1, !nontemporal !2
+; BE-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i32
+; BE-NEXT:    ret i32 [[R]]
+;
+  %x = load i64, i64* %ptr, align 4, !invariant.load !0, !nontemporal !1, !range !2
+  %r = trunc i64 %x to i32
+  ret i32 %r
+}
+
+!0 = !{}
+!1 = !{i32 1}
+!2 = !{i64 0, i64 2}