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[Loads] Forward constant vector store to load of first element

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InstCombine performs simple forwarding from stores to loads, but
currently only handles the case where the load and store have the
same size. This extends it to also handle a store of a constant
with a larger size followed by a load with a smaller size.

This is implemented through ConstantFoldLoadThroughBitcast() which
is fairly primitive (e.g. does not allow storing a large integer
and then loading a small one), but at least can forward the first
element of a vector store. Unfortunately it seems that we currently
don't have a generic helper for "read a constant value as a different
type", it's all tangled up with other logic in either
ConstantFolding or VNCoercion.

Differential Revision: https://reviews.llvm.org/D98114
This commit is contained in:
Nikita Popov 2021-03-06 16:10:21 +01:00
parent 108a3b9085
commit 725bd76263
2 changed files with 21 additions and 6 deletions
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11
lib/Analysis/Loads.cpp
View File

@ -495,12 +495,15 @@ static Value *getAvailableLoadStore(Instruction *Inst, const Value *Ptr,
if (!AreEquivalentAddressValues(StorePtr, Ptr))
return nullptr;
if (IsLoadCSE)
*IsLoadCSE = false;
Value *Val = SI->getValueOperand();
if (CastInst::isBitOrNoopPointerCastable(Val->getType(), AccessTy, DL)) {
if (IsLoadCSE)
*IsLoadCSE = false;
if (CastInst::isBitOrNoopPointerCastable(Val->getType(), AccessTy, DL))
return Val;
}
if (auto *C = dyn_cast<Constant>(Val))
return ConstantFoldLoadThroughBitcast(C, AccessTy, DL);
}
return nullptr;

16
test/Transforms/InstCombine/load-store-forward.ll
View File

@ -22,8 +22,7 @@ define i32 @vec_store_load_first(i32* %p) {
; CHECK-LABEL: @vec_store_load_first(
; CHECK-NEXT: [[P2:%.*]] = bitcast i32* [[P:%.*]] to <2 x i32>*
; CHECK-NEXT: store <2 x i32> <i32 1, i32 2>, <2 x i32>* [[P2]], align 8
; CHECK-NEXT: [[LOAD:%.*]] = load i32, i32* [[P]], align 4
; CHECK-NEXT: ret i32 [[LOAD]]
; CHECK-NEXT: ret i32 1
;
%p2 = bitcast i32* %p to <2 x i32>*
store <2 x i32> <i32 1, i32 2>, <2 x i32>* %p2
@ -31,6 +30,19 @@ define i32 @vec_store_load_first(i32* %p) {
ret i32 %load
}
define i32 @vec_store_load_first_constexpr(i32* %p) {
; CHECK-LABEL: @vec_store_load_first_constexpr(
; CHECK-NEXT: [[P2:%.*]] = bitcast i32* [[P:%.*]] to <2 x i32>*
; CHECK-NEXT: store <2 x i32> bitcast (i64 ptrtoint (i32 (i32*)* @vec_store_load_first to i64) to <2 x i32>), <2 x i32>* [[P2]], align 8
; CHECK-NEXT: [[LOAD:%.*]] = load i32, i32* [[P]], align 4
; CHECK-NEXT: ret i32 [[LOAD]]
;
%p2 = bitcast i32* %p to <2 x i32>*
store <2 x i32> bitcast (i64 ptrtoint (i32 (i32*)* @vec_store_load_first to i64) to <2 x i32>), <2 x i32>* %p2, align 8
%load = load i32, i32* %p, align 4
ret i32 %load
}
define i32 @vec_store_load_second(i32* %p) {
; CHECK-LABEL: @vec_store_load_second(
; CHECK-NEXT: [[P2:%.*]] = bitcast i32* [[P:%.*]] to <2 x i32>*