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[ConstHoist] Do not rebase single (or few) dependent constant
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If an instance (InsertionPoint or IP) of Base constant A has only one or few
rebased constants depending on it, do NOT rebase. One extra ADD instruction is
required to materialize each rebased constant, assuming A and the rebased have
the same materialization cost.

Differential Revision: https://reviews.llvm.org/D52243

llvm-svn: 342994
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Zhaoshi Zheng committed Sep 25, 2018
1 parent ef2ae74 commit 2c1a091
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Showing 2 changed files with 237 additions and 22 deletions.
68 changes: 46 additions & 22 deletions llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
View file
Expand Up @@ -86,6 +86,12 @@ static cl::opt<bool> ConstHoistGEP(
"consthoist-gep", cl::init(false), cl::Hidden,
cl::desc("Try hoisting constant gep expressions"));

static cl::opt<unsigned>
MinNumOfDependentToRebase("consthoist-min-num-to-rebase",
cl::desc("Do not rebase if number of dependent constants of a Base is less "
"than this number."),
cl::init(2), cl::Hidden);

namespace {

/// The constant hoisting pass.
Expand Down Expand Up @@ -822,7 +828,34 @@ bool ConstantHoistingPass::emitBaseConstants(GlobalVariable *BaseGV) {

unsigned UsesNum = 0;
unsigned ReBasesNum = 0;
unsigned NotRebasedNum = 0;
for (Instruction *IP : IPSet) {
// First, collect constants depending on this IP of the base.
unsigned Uses = 0;
using RebasedUse = std::tuple<Constant *, Type *, ConstantUser>;
SmallVector<RebasedUse, 4> ToBeRebased;
for (auto const &RCI : ConstInfo.RebasedConstants) {
for (auto const &U : RCI.Uses) {
Uses++;
BasicBlock *OrigMatInsertBB =
findMatInsertPt(U.Inst, U.OpndIdx)->getParent();
// If Base constant is to be inserted in multiple places,
// generate rebase for U using the Base dominating U.
if (IPSet.size() == 1 ||
DT->dominates(IP->getParent(), OrigMatInsertBB))
ToBeRebased.push_back(RebasedUse(RCI.Offset, RCI.Ty, U));
}
}
UsesNum = Uses;

// If only few constants depend on this IP of base, skip rebasing,
// assuming the base and the rebased have the same materialization cost.
if (ToBeRebased.size() < MinNumOfDependentToRebase) {
NotRebasedNum += ToBeRebased.size();
continue;
}

// Emit an instance of the base at this IP.
Instruction *Base = nullptr;
// Hoist and hide the base constant behind a bitcast.
if (ConstInfo.BaseExpr) {
Expand All @@ -840,36 +873,27 @@ bool ConstantHoistingPass::emitBaseConstants(GlobalVariable *BaseGV) {
<< ") to BB " << IP->getParent()->getName() << '\n'
<< *Base << '\n');

// Emit materialization code for all rebased constants.
unsigned Uses = 0;
for (auto const &RCI : ConstInfo.RebasedConstants) {
for (auto const &U : RCI.Uses) {
Uses++;
BasicBlock *OrigMatInsertBB =
findMatInsertPt(U.Inst, U.OpndIdx)->getParent();
// If Base constant is to be inserted in multiple places,
// generate rebase for U using the Base dominating U.
if (IPSet.size() == 1 ||
DT->dominates(Base->getParent(), OrigMatInsertBB)) {
emitBaseConstants(Base, RCI.Offset, RCI.Ty, U);
ReBasesNum++;
}

Base->setDebugLoc(DILocation::getMergedLocation(
Base->getDebugLoc(), U.Inst->getDebugLoc()));
}
// Emit materialization code for rebased constants depending on this IP.
for (auto const &R : ToBeRebased) {
Constant *Off = std::get<0>(R);
Type *Ty = std::get<1>(R);
ConstantUser U = std::get<2>(R);
emitBaseConstants(Base, Off, Ty, U);
ReBasesNum++;
// Use the same debug location as the last user of the constant.
Base->setDebugLoc(DILocation::getMergedLocation(
Base->getDebugLoc(), U.Inst->getDebugLoc()));
}
UsesNum = Uses;

// Use the same debug location as the last user of the constant.
assert(!Base->use_empty() && "The use list is empty!?");
assert(isa<Instruction>(Base->user_back()) &&
"All uses should be instructions.");
}
(void)UsesNum;
(void)ReBasesNum;
(void)NotRebasedNum;
// Expect all uses are rebased after rebase is done.
assert(UsesNum == ReBasesNum && "Not all uses are rebased");
assert(UsesNum == (ReBasesNum + NotRebasedNum) &&
"Not all uses are rebased");

NumConstantsHoisted++;

Expand Down
191 changes: 191 additions & 0 deletions llvm/test/CodeGen/Thumb/consthoist-few-dependents.ll
View file
@@ -0,0 +1,191 @@
; RUN: opt -consthoist -S %s -o - | FileCheck %s --check-prefix=OPT
; RUN: opt -consthoist -S -consthoist-min-num-to-rebase=1 %s -o - | FileCheck %s --check-prefix=OPT --check-prefix=OPT-1
; RUN: opt -consthoist -S -consthoist-min-num-to-rebase=2 %s -o - | FileCheck %s --check-prefix=OPT --check-prefix=OPT-2
; RUN: opt -consthoist -S -consthoist-min-num-to-rebase=3 %s -o - | FileCheck %s --check-prefix=OPT --check-prefix=OPT-3

; RUN: llc %s -o - | FileCheck %s --check-prefix=LLC

target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
target triple = "thumbv6m-none-unknown-musleabi"

; Test that constant 0 and 1 of i1 type is NOT hoisted due low
; materializing cost.

; OPT-LABEL: avalon
; OPT: bb1:
; OPT: store i1 true
; OPT: bb2:
; OPT: store i1 false
; OPT: bb3:
; OPT: store i1 false
; OPT: store i1 false
; OPT-NOT: add

; LLC-LABEL: avalon
; LLC-DAG: movs r{{[0-9]+}}, #0
; LLC-DAG: movs r{{[0-9]+}}, #0
; LLC-DAG: movs r{{[0-9]+}}, #1
; LLC-NOT: add

@global = local_unnamed_addr global i1 undef, align 1
@global.0 = local_unnamed_addr global i1 undef, align 1

define void @avalon() #0 {
bb:
switch i8 undef, label %bb5 [
i8 0, label %bb1
i8 -1, label %bb2
i8 1, label %bb3
]

bb1:
store i1 1, i1* @global, align 1
unreachable

bb2:
store i1 0, i1* @global, align 1
unreachable

bb3:
store i1 0, i1* @global.0, align 1
store i1 0, i1* @global, align 1
unreachable

bb5:
ret void
}

; Test that for i8 type, constant -1 is not rebased since it's the only
; dependent of base constant -2.
; This test is also covered by r342898, see
; test/CodeGen/Thumb/consthoist-imm8-costs-1.ll

; OPT-LABEL: barney
; OPT: bb1:
; OPT: store i8 -1
; OPT: bb2:
; OPT: store i8 -2
; OPT: bb3:
; OPT: store i8 -2
; OPT: store i8 -2
; OPT-NOT: add

; LLC-LABEL: barney
; LLC-DAG: movs r{{[0-9]+}}, #254
; LLC-DAG: movs r{{[0-9]+}}, #255
; LLC-NOT: mvn
; LLC-NOT: add

@global.1 = local_unnamed_addr global i8 undef, align 1
@global.2 = local_unnamed_addr global i8 undef, align 1

define void @barney() #0 {
bb:
switch i8 undef, label %bb5 [
i8 0, label %bb1
i8 -1, label %bb2
i8 1, label %bb3
]

bb1: ; preds = %bb
store i8 -1, i8* @global.1, align 1
unreachable

bb2: ; preds = %bb
store i8 -2, i8* @global.1, align 1
unreachable

bb3: ; preds = %bb
store i8 -2, i8* @global.2, align 1
store i8 -2, i8* @global.1, align 1
unreachable

bb5: ; preds = %bb
ret void
}

; Test that for i16 type constant 65532 is not rebased if it's the only
; dependent of base constant 65531. Cost would be the same if rebased.
; If rebased, 3 two-byte instructions:
; movs r0, #4
; mvns r0, r0
; adds r0, r0, #1
; If NOT rebased, 1 two-byte instruction plus 1 four-byte CP entry:
; ldr r1, .LCPI2_3
; ...
; .LCPI2_3:
; .long 65532

; OPT-LABEL: carla

; -consthoist-min-num-to-rebase=1, check that 65532 and single use of 65531
; in bb2 is rebased
; OPT-1: bb1:
; OPT-1: %[[C1:const[0-9]?]] = bitcast i16 -5 to i16
; OPT-1-NEXT: %const_mat = add i16 %[[C1]], 1
; OPT-1-NEXT: store i16 %const_mat, i16* @global.3, align 1
; OPT-1: bb2:
; OPT-1-NEXT: %[[C2:const[0-9]?]] = bitcast i16 -5 to i16
; OPT-1-NEXT: store i16 %[[C2]], i16* @global.3, align 1
; OPT-1: bb3:
; OPT-1-NEXT: %[[C3:const[0-9]?]] = bitcast i16 -5 to i16
; OPT-1-NEXT: store i16 %[[C3]], i16* @global.4, align 1
; OPT-1-NEXT: store i16 %[[C3]], i16* @global.3, align 1

; -consthoist-min-num-to-rebase=2, check that 65532 and single use of 65531
; in bb2 is not rebased
; OPT-2: bb1:
; OPT-2-NEXT: store i16 -4, i16* @global.3, align 1
; OPT-2: bb2:
; OPT-2-NEXT: store i16 -5, i16* @global.3, align 1
; OPT-2: bb3:
; OPT-2-NEXT: %[[C4:const[0-9]?]] = bitcast i16 -5 to i16
; OPT-2-NEXT: store i16 %[[C4]], i16* @global.4, align 1
; OPT-2-NEXT: store i16 %[[C4]], i16* @global.3, align 1
; OPT-2-NOT: add

; -consthoist-min-num-to-rebase=3, check that dual uses of 65531 in bb3 are
; not rebase
; OPT-3: bb1:
; OPT-3-NEXT: store i16 -4, i16* @global.3, align 1
; OPT-3: bb2:
; OPT-3-NEXT: store i16 -5, i16* @global.3, align 1
; OPT-3: bb3:
; OPT-3-NEXT: store i16 -5, i16* @global.4, align 1
; OPT-3-NEXT: store i16 -5, i16* @global.3, align 1
; OPT-3-NOT: add
; OPT-3-NOT: bitcast

; LLC-LABEL: carla
; LLC-DAG: ldr r{{[0-9]+}}, .LCPI2_1
; LLC-DAG: ldr r{{[0-9]+}}, .LCPI2_3
; LLC-NOT: mvn
; LLC-NOT: add

@global.3 = local_unnamed_addr global i16 undef, align 2
@global.4 = local_unnamed_addr global i16 undef, align 2

define void @carla() {
bb:
switch i8 undef, label %bb5 [
i8 0, label %bb1
i8 -1, label %bb2
i8 1, label %bb3
]

bb1: ; preds = %bb
store i16 65532, i16* @global.3, align 1
unreachable

bb2: ; preds = %bb
store i16 65531, i16* @global.3, align 1
unreachable

bb3: ; preds = %bb
store i16 65531, i16* @global.4, align 1
store i16 65531, i16* @global.3, align 1
unreachable

bb5: ; preds = %bb
ret void
}

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