[InstCombine] try to narrow a truncated load

[spatel]

Summary

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 I'm not sure if that reasoning applies to narrowing.

There's another request for narrowing in IR here, but it's a different pattern:
https://bugs.llvm.org/show_bug.cgi?id=42424

[reames]

I'm not sure that doing this at the IR level is the best idea. The problem is that when we narrow, we loose the dereferenceable fact about part of the memory access. This can in turn limit other transforms which would have been profitable. As an example:
a = load <2 x i8>* p
b = load <2 x i8>* (p+1)
sum = a[0] + a[1] + b[1]

Narrowing the b load to i8 looses the fact that the memory location corresponding to b[0] is dereferenceable, which would prevent transforms such as:
a = load <4 x i8>* p
a[2] = 0;
sum = horizontal_sum(a);

(Note: I'm not saying this alternate transform is always profitable. I'm just making a point about lost opportunity.)

[spatel]

In D64432#1582980, @reames wrote:

I'm not sure that doing this at the IR level is the best idea. The problem is that when we narrow, we loose the dereferenceable fact about part of the memory access. This can in turn limit other transforms which would have been profitable. As an example:
a = load <2 x i8>* p
b = load <2 x i8>* (p+1)
sum = a[0] + a[1] + b[1]

Narrowing the b load to i8 looses the fact that the memory location corresponding to b[0] is dereferenceable, which would prevent transforms such as:
a = load <4 x i8>* p
a[2] = 0;
sum = horizontal_sum(a);

(Note: I'm not saying this alternate transform is always profitable. I'm just making a point about lost opportunity.)

Yes, I agree that we can lose information by narrowing. I was hoping to avoid that conflict with D64258, but we need to refine our definition of 'dereferenceable'. Potentially, we could have instcombine preserve the dereferenceable range in this transform?

I don't know if it makes a difference, but my intent is to not allow narrowing for vectors in this patch by using the data layout legality check. (We could make that vector bailout explicit.) So I don't think the given example with a vector type is at risk.

define i8 @narrowload(<2 x i8>* %p) {
  %a = load <2 x i8>, <2 x i8>* %p
  %p1 = getelementptr <2 x i8>, <2 x i8>* %p, i64 1
  %b = load <2 x i8>, <2 x i8>* %p1
  %a0 = extractelement <2 x i8> %a, i64 0
  %a1 = extractelement <2 x i8> %a, i64 1
  %b1 = extractelement <2 x i8> %b, i64 1
  %add1 = add i8 %a0, %a1
  %add2 = add i8 %add1, %b1
  ret i8 %add2
}
;sum = a[0] + a[1] + b[1]

[jdoerfert]

In D64432#1582980, @reames wrote:

I'm not sure that doing this at the IR level is the best idea. The problem is that when we narrow, we loose the dereferenceable fact about part of the memory access. This can in turn limit other transforms which would have been profitable. As an example:
a = load <2 x i8>* p
b = load <2 x i8>* (p+1)
sum = a[0] + a[1] + b[1]

Narrowing the b load to i8 looses the fact that the memory location corresponding to b[0] is dereferenceable, which would prevent transforms such as:
a = load <4 x i8>* p
a[2] = 0;
sum = horizontal_sum(a);

(Note: I'm not saying this alternate transform is always profitable. I'm just making a point about lost opportunity.)

Could we check here if the base pointer has dereferenceable annotation and use that as a condition for this transformation? (It's more complicated to be completely lossless but this seems to be an easy to test starting point).

[spatel]

In D64432#1584782, @jdoerfert wrote:

Could we check here if the base pointer has dereferenceable annotation and use that as a condition for this transformation? (It's more complicated to be completely lossless but this seems to be an easy to test starting point).

That seems like a reasonable way to overcome the potential loss, so I'll update this patch with that change.
I'm expecting that will significantly reduce the opportunities for the transform though...until Attributor is working at full strength.

[spatel]

Patch updated:

1. Add limitation based on dereferenceable attribute to prevent information loss.
2. Add/adjust tests to include dereferenceable attributes.

[jdoerfert]

I think you are missing the negative test without dereferenceable.

In D64432#1588103, @spatel wrote:

I'm expecting that will significantly reduce the opportunities for the transform though...until Attributor is working at full strength.

Agreed. Though, we already have dereferenceabl arguments. @uenoku Is now working on dereferenceable deduction for the Attributor :)

[spatel]

Patch updated:
Add a test with no 'dereferenceable' attribute on the pointer argument.

[spatel]

In D64432#1589539, @jdoerfert wrote:

I think you are missing the negative test without dereferenceable.

Yes - updated.

In D64432#1588103, @spatel wrote:

I'm expecting that will significantly reduce the opportunities for the transform though...until Attributor is working at full strength.

Agreed. Though, we already have dereferenceabl arguments. @uenoku Is now working on dereferenceable deduction for the Attributor :)

Great!

[reames]

Looks like you guys came to a completely reasonable short term solution with the dereferenceable limitation. I agree we need a better answer here long term, but being able to make progress on short term items without blocking is good to see.

Amusingly, I just stumbled across an example locally where narrowing a vector load was the right answer performance wise. :)

[spatel]

Ping.

[lebedev.ri]

LGTM with dereferenceable() restriction, but maybe wait for one more review.

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

714

This one is deprecated, let's use the proper one

LoadInst *NarrowLoad = new LoadInst(PtrTy, Bitcast);

llvm/test/Transforms/InstCombine/trunc-load.ll

8

Please precommit the tests.

[spatel]

llvm/test/Transforms/InstCombine/trunc-load.ll

8

Yes - the tests were evolving with the code changes here, but now that we are settled on the constraints, I'll push those with baseline results as a preliminary step.

Thanks for reviewing!

[spatel]

Patch updated:

1. Use new API for creating load that takes element type and pointer.
2. Rebased after adding baseline tests (rL366901).

[spatel]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

714

This isn't obvious (because there are apparently no header comments for any of these constructors), but the type specified by that 1st parameter is the element type of the loaded value, not the pointer type.

[jdoerfert]

One final request, otherwise LGTM.

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

715

I think you need to copy over more information (probably there is code for that somewhere):

• !nontemporal !<index>
• !invariant.load !<index>
• !invariant.group !<index>

[spatel]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

715

The closest match that I see is here:
rL366949

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

if (isa<UndefValue>(VecOp)) {
     UndefValue *NarrowUndef = UndefValue::get(DestTy);
     Value *NarrowOp = Builder.CreateCast(Opcode, ScalarOp, DestScalarTy);
     return InsertElementInst::Create(NarrowUndef, NarrowOp, Index);
   }
 
   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.CreateBitCast(X, PtrTy);
+  LoadInst *NarrowLoad = new LoadInst(Bitcast);
+  NarrowLoad->setAlignment(WideLoad->getAlignment());
+  return NarrowLoad;
+}
+
 Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
   if (Instruction *Result = commonCastTransforms(CI))
     return Result;
 
   Value *Src = CI.getOperand(0);
   Type *DestTy = CI.getType(), *SrcTy = Src->getType();
 
   // Attempt to truncate the entire input expression tree to the destination
if (match(Src, m_Shl(m_Value(A), m_ConstantInt(Cst))) &&
           ConstantInt::get(DestTy, Cst->getValue().trunc(DestSize)));
       }
     }
   }
 
   if (Instruction *I = foldVecTruncToExtElt(CI, *this))
     return I;
 
+  if (Instruction *NewLoad = narrowLoad(CI, Builder, DL))
+    return NewLoad;
+
   return nullptr;
 }
 
 Instruction *InstCombiner::transformZExtICmp(ICmpInst *ICI, ZExtInst &CI,
                                              bool DoTransform) {
   // If we are just checking for a icmp eq of a single bit and zext'ing it
   // to an integer, then shift the bit to the appropriate place and then
   // cast to integer to avoid the comparison.

llvm/test/Transforms/InstCombine/trunc-load.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -instcombine -S -data-layout="e-n16:32:64" | FileCheck %s --check-prefixes=CHECK,LE
 ; RUN: opt < %s -instcombine -S -data-layout="E-n16:32:64" | FileCheck %s --check-prefixes=CHECK,BE
 
-define i32 @truncload(i64* %ptr) {
+; Don't narrow if it would lose information about the dereferenceable range of the pointer.
+
+define i32 @truncload(i64* dereferenceable(7) %ptr) {
 ; CHECK-LABEL: @truncload(
 ; CHECK-NEXT:    [[X:%.*]] = load i64, i64* [[PTR:%.*]], align 4
 ; CHECK-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i32
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %x = load i64, i64* %ptr
   %r = trunc i64 %x to i32
   ret i32 %r
 }
 
-define i16 @truncload_align(i32* %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]]
+; On little-endian, we can narrow the load without an offset.
+
+define i32 @truncload_deref(i64* dereferenceable(8) %ptr) {
+; 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
+  ret i32 %r
+}
+
+; Preserve alignment.
+
+define i16 @truncload_align(i32* dereferenceable(14) %ptr) {
+; 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
   ret i16 %r
 }
 
+; Negative test - extra use means we would not eliminate the original load.
+
 declare void @use(i64)
 
-define i32 @truncload_extra_use(i64* %ptr) {
+define i32 @truncload_extra_use(i64* dereferenceable(100) %ptr) {
 ; CHECK-LABEL: @truncload_extra_use(
 ; CHECK-NEXT:    [[X:%.*]] = load i64, i64* [[PTR:%.*]], align 2
 ; CHECK-NEXT:    call void @use(i64 [[X]])
 ; CHECK-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i32
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %x = load i64, i64* %ptr, align 2
   call void @use(i64 %x)
   %r = trunc i64 %x to i32
   ret i32 %r
 }
 
-define i8 @truncload_type(i64* %ptr) {
+; Negative test - don't create a load if the type is not allowed by the data-layout.
+
+define i8 @truncload_type(i64* dereferenceable(9) %ptr) {
 ; CHECK-LABEL: @truncload_type(
 ; CHECK-NEXT:    [[X:%.*]] = load i64, i64* [[PTR:%.*]], align 2
 ; CHECK-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i8
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %x = load i64, i64* %ptr, align 2
   %r = trunc i64 %x to i8
   ret i8 %r
 }
 
-define i32 @truncload_volatile(i64* %ptr) {
+; Negative test - don't transform volatiles.
+
+define i32 @truncload_volatile(i64* dereferenceable(8) %ptr) {
 ; CHECK-LABEL: @truncload_volatile(
 ; CHECK-NEXT:    [[X:%.*]] = load volatile i64, i64* [[PTR:%.*]], align 8
 ; CHECK-NEXT:    [[R:%.*]] = trunc i64 [[X]] to i32
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %x = load volatile i64, i64* %ptr, align 8
   %r = trunc i64 %x to i32
   ret i32 %r
 }
 
-define i32 @truncload_address_space(i64 addrspace(1)* %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]]
+; Preserve address space.
+
+define i32 @truncload_address_space(i64 addrspace(1)* dereferenceable(8) %ptr) {
+; 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
 }